ALBERT

Description: Uncertainties in calculating precipitation climatology in East Asia Hydrology and Earth System Sciences Discussions, 12, 7765-7783, 2015 Author(s): J. Kim and S. K. Park This study examines the uncertainty in calculating the fundamental climatological characteristics of precipitation in the East Asia region from multiple fine-resolution gridded analysis datasets based on in-situ rain gauge observations. Five observation-based gridded precipitation datasets are used to derive the long-term means, standard deviations in lieu of interannual variability and linear trends over the 28-year period from 1980 to 2007. Both the annual and summer (June–July–August) mean precipitation is examined. The agreement amongst these precipitation datasets are examined using multiple metrics including the signal-to-noise ratio (SNR) defined as the ratio between long-term means and the corresponding standard deviations, and Taylor diagrams which allows examinations of the pattern correlation, the standard deviation, and the centered root mean square error. It is found that the five gauge-based precipitation analysis datasets agree well in the long-term mean and interannual variability in most of the East Asia region including eastern China, Manchuria, South Korea, and Japan, which are densely populated and have fairly high density observation networks. The regions of large inter-dataset variations include Tibetan Plateau, Mongolia, northern Indo-China, and North Korea. The regions of large uncertainties are typically lightly populated and are characterized by severe terrain and/or extreme high elevations. Unlike the long-term mean and interannual variability, agreements between datasets in the linear trend is weak, both for the annual and summer mean values. In most of the East Asia region, the SNR for the linear trend is below 0.5, i.e., the inter-dataset variability exceeds the multi-data ensemble mean. The uncertainty in the spatial distribution of long-term means among these datasets occurs both in the spatial pattern and variability, but the uncertainty for the interannual variability and time trend is much larger in the variability than in the pattern correlation. Thus, care must be taken in using long-term trends calculated from gridded precipitation analysis data for climate studies over the East Asia region.

Description: Nonlinear effects of locally heterogeneous hydraulic conductivity fields on regional stream–aquifer exchanges Hydrology and Earth System Sciences Discussions, 12, 7727-7764, 2015 Author(s): J. Zhu, C. L. Winter, and Z. Wang Computational experiments are performed to evaluate the effects of locally heterogeneous conductivity fields on regional exchanges of water between stream and aquifer systems in the Middle Heihe River Basin (MHRB) of northwestern China. The effects are found to be nonlinear in the sense that simulated discharges from aquifers to streams are systematically lower than discharges produced by a base model parameterized with relatively coarse effective conductivity. A similar, but weaker, effect is observed for stream leakage. The study is organized around three hypotheses: (H1) small-scale spatial variations of conductivity significantly affect regional exchanges of water between streams and aquifers in river basins, (H2) aggregating small-scale heterogeneities into regional effective parameters systematically biases estimates of stream–aquifer exchanges, and (H3) the biases result from slow-paths in groundwater flow that emerge due to small-scale heterogeneities. The hypotheses are evaluated by comparing stream–aquifer fluxes produced by the base model to fluxes simulated using realizations of the MHRB characterized by local (grid-scale) heterogeneity. Levels of local heterogeneity are manipulated as control variables by adjusting coefficients of variation. All models are implemented using the MODFLOW simulation environment, and the PEST tool is used to calibrate effective conductivities defined over 16 zones within the MHRB. The effective parameters are also used as expected values to develop log-normally distributed conductivity ( K ) fields on local grid scales. Stream-aquifer exchanges are simulated with K fields at both scales and then compared. Results show that the effects of small-scale heterogeneities significantly influence exchanges with simulations based on local-scale heterogeneities always producing discharges that are less than those produced by the base model. Although aquifer heterogeneities are uncorrelated at local scales, they appear to induce coherent slow-paths in groundwater fluxes that in turn reduce aquifer–stream exchanges. Since surface water–groundwater exchanges are critical hydrologic processes in basin-scale water budgets, these results also have implications for water resources management.

Description: Near–surface air temperature and snow skin temperature comparison from CREST-SAFE station data with MODIS land surface temperature data Hydrology and Earth System Sciences Discussions, 12, 7665-7687, 2015 Author(s): C. L. Pérez Díaz, T. Lakhankar, P. Romanov, J. Muñoz, R. Khanbilvardi, and Y. Yu Land Surface Temperature (LST) is a key variable (commonly studied to understand the hydrological cycle) that helps drive the energy balance and water exchange between the Earth's surface and its atmosphere. One observable constituent of much importance in the land surface water balance model is snow. Snow cover plays a critical role in the regional to global scale hydrological cycle because rain-on-snow with warm air temperatures accelerates rapid snow-melt, which is responsible for the majority of the spring floods. Accurate information on near-surface air temperature ( T -air) and snow skin temperature ( T -skin) helps us comprehend the energy and water balances in the Earth's hydrological cycle. T -skin is critical in estimating latent and sensible heat fluxes over snow covered areas because incoming and outgoing radiation fluxes from the snow mass and the air temperature above make it different from the average snowpack temperature. This study investigates the correlation between MODerate resolution Imaging Spectroradiometer (MODIS) LST data and observed T -air and T -skin data from NOAA-CREST-Snow Analysis and Field Experiment (CREST-SAFE) for the winters of 2013 and 2014. LST satellite validation is imperative because high-latitude regions are significantly affected by climate warming and there is a need to aid existing meteorological station networks with the spatially continuous measurements provided by satellites. Results indicate that near-surface air temperature correlates better than snow skin temperature with MODIS LST data. Additional findings show that there is a negative trend demonstrating that the air minus snow skin temperature difference is inversely proportional to cloud cover. To a lesser extent, it will be examined whether the surface properties at the site are representative for the LST properties within the instrument field of view.

Description: Does drought alter hydrological functions in forest soils? An infiltration experiment Hydrology and Earth System Sciences Discussions, 12, 7689-7725, 2015 Author(s): K. F. Gimbel, H. Puhlmann, and M. Weiler The water cycle is expected to change in future and severely affect precipitation patterns across central Europe and in other parts of the world, leading to more frequent and severe droughts. Usually, it is assumed that system properties, like soil properties, remain stable and will not be affected by drought events. To study if this assumption is appropriate, we address the effects of drought on the infiltration behavior of forest soils using dye tracer experiments on six sites in three regions across Germany, which were forced into drought conditions. The sites cover clayey, loamy and sandy textured soils. In each region, we compared a deciduous and a coniferous forest stand to address differences between the main tree species. The results of the dye tracer experiments show clear evidence for changes in infiltration behavior at the sites. The infiltration changed at the clayey plots from regular and homogeneous flow to fast preferential flow. Similar behavior was observed at the loamy plots, where large areas in the upper layers remained dry, displaying signs of strong water repellency. This was confirmed by WDPT tests, which revealed, in all except one plot, moderate to severe water repellency. Water repellency was also accountable for the change of regular infiltration to fingered flow in the sandy soils. The results of this study suggest that the "drought-history" or generally the climatic conditions in the past of a soil are more important than the actual antecedent soil moisture status regarding hydrophobicity and infiltration behavior; and also, that drought effects on infiltration need to be considered in hydrological models to obtain realistic predictions concerning water quality and quantity in runoff and groundwater recharge.

Description: Does the Budyko curve reflect a maximum power state of hydrological systems? A backward analysis Hydrology and Earth System Sciences Discussions, 12, 7821-7842, 2015 Author(s): M. Westhoff, E. Zehe, P. Archambeau, and B. Dewals Almost all catchments plot within a small envelope around the Budyko curve. This apparent behaviour suggests that organizing principles may play a role in the evolution of catchments. In this paper we applied the thermodynamic principle of maximum power as the organizing principle. In a top-down approach we derived mathematical formulations of the relation between relative wetness and gradients driving runoff and evaporation for a simple one-box model. We did this in such a way that when the conductances are optimized with the maximum power principle, the steady state behaviour of the model leads exactly to a point on the Budyko curve. Subsequently we derived gradients that, under constant forcing, resulted in a Budyko curve following the asymptotes closely. With these gradients we explored the sensitivity of dry spells and dynamics in actual evaporation. Despite the simplicity of the model, catchment observations compare reasonably well with the Budyko curves derived with dynamics in rainfall and evaporation. This indicates that the maximum power principle may be used (i) to derive the Budyko curve and (ii) to move away from the empiricism in free parameters present in many Budyko functions. Future work should focus on better representing the boundary conditions of real catchments and eventually adding more complexity to the model.

Description: Impacts of land use change and climate variations on annual inflow into Miyun Reservoir, Beijing, China Hydrology and Earth System Sciences Discussions, 12, 7785-7819, 2015 Author(s): J. K. Zheng, G. Sun, W. H. Li, X. X. Yu, C. Zhang, Y. B. Gong, and L. H. Tu Miyun reservoir, the only surface water source for Beijing city, has experienced water supply decline in recent decades. Previous studies suggest that both land use change and climate contributes the changes of water supply in this critical watershed. However, the specific causes of the decline in Miyun reservoir are debatable in a non-stationary climate in the past four decades. The central objective of this study was to quantify the separate and collective contributions of land use change and climate variability to the decreasing inflow into Miyun reservoir during 1961–2008. Different from previous studies, this work objectively identified breakpoints by analyzing the long-term historical hydrometeorology and land cover records. To effectively study the different impacts of the climate variation and land cover change during different sub-periods, annual water balance model (AWB), climate elasticity model (CEM), and rainfall–runoff model (RRM) were employed to conduct attribution analysis synthetically. We found a significant decrease in annual streamflow ( p 〈 0.01), a significant positive trend in annual potential evapotranspiration ( p 〈 0.01), and an insignificant negative trend in annual precipitation ( p 〉 0.1) during 1961–2008. Combined with historical records, we identified two breakpoints as in 1983 and 1999 for the period 1961–2008 by the sequential Mann–Kendall Test and Double Mass Curve. Climate variability alone did not explain the decrease in inflow to Miyun reservoir. Reduction of water yield was closely related to increase in evapotranspiration rates due to the expansion of forestlands and reduction in cropland and grassland, and was likely exacerbated by increased water consumption for domestic and industrial uses in the basin. Our study found that the contribution to the observed streamflow decline from land use change fell from 64–92 % during 1984–1999 to 36–58 % during 2000–2008, whereas the contribution from climate variation climbed from 8–36 % during the 1984–1999 to 42–64 % during 2000–2008. Model uncertainty analysis further demonstrated that climate warming played a dominant role in streamflow reduction in the 2000s. We conclude that future climate change and variability will further challenge the goal of water supply of Miyun reservoir to meet water demand. A comprehensive watershed management strategy needs to consider the climate variations besides vegetation management.

Description: Future changes in flash flood frequency and intensity of the Tha Di River (Thailand) based on rainfall–runoff modeling and advanced delta change scaling Hydrology and Earth System Sciences Discussions, 12, 7327-7352, 2015 Author(s): S. Hilgert, A. Wagner, and S. Fuchs As a consequence of climate change, extreme and flood-causing precipitation events are expected to increase in magnitude and frequency, especially in today's high-precipitation areas. During the north-east monsoon seasons, Nakhon Si Thammarat in southern Thailand is flash-flooded every 2.22 years on average. This study investigates frequency and intensity of harmful discharges of the Tha Di River regarding the IPCC emission scenarios A2 and B2. The regional climate model (RCM) PRECIS was transformed using the advanced delta change (ADC) method. The hydrologic response model HBV-Light was calibrated to the catchment and supplied with ADC-scaled daily precipitation and temperature data for 2010–2089. Under the A2 (B2) scenario, the flood threshold exceedance frequency on average increases by 133 % (decreases by 10 %), average flood intensity increases by 3 % (decreases by 2 %) and the annual top five discharge peaks intensities increase by 46 % (decrease by 5 %). Yearly precipitation sums increase by 30 % (10 %) towards the end of the century. The A2 scenario predicts a precipitation increase during the rainy season, which intensifies flood events; while increases projected exclusively for the dry season are not expected to cause floods. Retention volume demand of past events was calculated to be up to 12 × 10 6 m 3 . Flood risks are staying at high levels under the B2 scenario or increase dramatically under the A2 scenario. Results show that the RCM scaling process is inflicted with systematic biases but is crucial to investigate small, mountainous catchments. Improvement of scaling techniques should therefore accompany the development towards high-resolution climate models.

Description: Multiscale evaluation of the standardized precipitation index as a groundwater drought indicator Hydrology and Earth System Sciences Discussions, 12, 7405-7436, 2015 Author(s): R. Kumar, J. L. Musuuza, A. F. Van Loon, A. J. Teuling, R. Barthel, J. Ten Broek, J. Mai, L. Samaniego, and S. Attinger The lack of comprehensive groundwater observations at regional and global scales has promoted the use of alternative proxies and indices to quantify and predict groundwater droughts. Among them, the Standardized Precipitation Index (SPI) is commonly used to characterize droughts in different compartments of the hydro-meteorological system. In this study, we explore the suitability of the SPI to characterize local and regional scale groundwater droughts using observations at more than 2000 groundwater wells in geologically different areas in Germany and the Netherlands. A multiscale evaluation of the SPI is performed using the station data and their corresponding 0.5° gridded estimates to analyze the local and regional behavior of groundwater droughts, respectively. The standardized anomalies in the groundwater heads (SGI) were correlated against SPIs obtained using different accumulation periods. The accumulation periods to achieve maximum correlation exhibited high spatial variability (ranges 3 to 36 months) at both scales, leading to the conclusion that an a priori selection of the accumulation period (for computing the SPI) would result in inadequate characterization of groundwater droughts. The application of the uniform accumulation periods over the entire domain significantly reduced the correlation between SPI and SGI (≈ 21–66 %) indicating the limited applicability of SPI as a proxy for groundwater droughts even at long accumulation times. Furthermore, the low scores of the hit rate (0.3–0.6) and high false alarm ratio (0.4–0.7) at the majority of the wells and grid cells demonstrated the low reliability of groundwater drought predictions using the SPI. The findings of this study highlight the pitfalls of using the SPI as a groundwater drought indicator at both local and regional scales, and stress the need for more groundwater observations and accounting for regional hydrogeological characteristics in groundwater drought monitoring.

Description: Comparing the Ensemble and Extended Kalman Filters for in situ soil moisture assimilation with contrasting soil conditions Hydrology and Earth System Sciences Discussions, 12, 7353-7403, 2015 Author(s): D. Fairbairn, A. L. Barbu, J.-F. Mahfouf, J.-C. Calvet, and E. Gelati Two data assimilation methods are compared for their ability to produce a deterministic soil moisture analysis on the Météo-France land surface model: (i) SEKF, a Simplified Extended Kalman Filter, which uses a climatological background-error covariance, (ii) EnSRF, the Ensemble Square Root Filter, which uses an ensemble background-error covariance and approximates random forcing errors stochastically. The accuracy of the deterministic analysis is measured on 12 sites with in situ observations and various soil textures in Southwest France (SMOSMANIA network). In the experiments with real observations, the two methods perform similarly and improve on the open loop. Both methods suffer from incorrect linear assumptions which are particularly degrading to the analysis during water-stressed conditions: the EnSRF by a dry bias and the SEKF by an over-sensitivity of the model Jacobian between the surface and the root zone layers. These problems are less severe for sandy soils than clay soils because sandy soils are less sensitive to perturbations in the initial conditions. A simple bias correction technique is tested on the EnSRF. Although this reduces the bias, it also suppresses the ensemble spread, which degrades the analysis performance. However, the EnSRF flow-dependent background-error covariance evidently captures seasonal variability in the soil moisture errors and should exploit planned improvements in the model physics. Synthetic experiments demonstrate that when there is only a random component in the precipitation forcing errors, the correct stochastic representation of these errors enables the EnSRF to perform better than the SEKF. But in the real experiments the same rainfall error specification does not improve the EnSRF analysis. It is likely that the actual rainfall errors are underestimated and that other sources of errors could limit the usefulness of this information. More comprehensive ways of representing the rainfall errors are suggested, which might improve the EnSRF performance.

Description: Morphological dynamics of an englacial channel Hydrology and Earth System Sciences Discussions, 12, 7615-7664, 2015 Author(s): G. Vatne and T. D. L. Irvine-Fynn Despite an interest in the hydraulic functioning of supraglacial and englacial channels over the last four decades, the processes and forms of such ice-bounded streams have remained poorly documented. Recent glaciological research has demonstrated the potential significance of so-called "cut and closure" streams, where englacial or subglacial flowpaths are created from the long-term incision of supraglacial channels. These flowpaths are reported to exhibit step-pool morphology, comprising knickpoints and/or knickzones, albeit exaggerated in dimensions in comparison to their supraglacial channel counterparts. However, little is known of the development of such channels' morphology. Here, we examine the spatial organization of step-pools and the upstream migration of steps, many of which form knickzones, with repeated surveys over a 10 year period in an englacial conduit in cold-based Austre Brøggerbreen, Svalbard. The observations show upstream knickpoint recession to be the dominant process for channel evolution. This is paralleled by an increase in average step height and conduit gradient over time. Characteristic channel reach types and step-riser forms are consistently observed in each of the morphological surveys reported. We suggest that the formation of steps has a hydrodynamic origin, where step-pool geometry is more efficient for energy dissipation than meanders, and that the englacial channel system is one in rapid transition rather than in dynamic equilibrium. The evolution and recession of knickzones reported here result in the formation of a 37 m moulin, suggesting over time the englacial channel may evolve towards a stable end-point characterised by a singular vertical descent to the local hydraulic base level. In light of this, our observations highlight the need to further examine the adjustment processes in cut-and-closure channels to better understand their coupling to supraglacial meltwater sources and role and potential significance in cold-based glacier hydrology and ice dynamics.

Description: The impact of near-surface soil moisture assimilation at subseasonal, seasonal, and inter-annual time scales Hydrology and Earth System Sciences Discussions, 12, 7971-8004, 2015 Author(s): C. Draper and R. Reichle Nine years of Advanced Microwave Scanning Radiometer – Earth Observing System (AMSR-E) soil moisture retrievals are assimilated into the Catchment land surface model at four locations in the US. The assimilation is evaluated using the unbiased Mean Square Error (ubMSE) relative to watershed-scale in situ observations, with the ubMSE separated into contributions from the subseasonal (SM short ), mean seasonal (SM seas ) and inter-annual (SM long ) soil moisture dynamics. For near-surface soil moisture, the average ubMSE for Catchment without assimilation was (1.8 × 10 −3 m 3 m −3 ) 2 , of which 19 % was in SM long , 26 % in SM seas , and 55 % in SM short . The AMSR-E assimilation significantly reduced the total ubMSE at every site, with an average reduction of 33 %. Of this ubMSE reduction, 37 % occurred in SM long , 24 % in SM seas , and 38 % in SM short . For root-zone soil moisture, in situ observations were available at one site only, and the near-surface and root-zone results were very similar at this site. These results suggest that, in addition to the well-reported improvements in SM short , assimilating a sufficiently long soil moisture data record can also improve the model representation of important long term events, such as droughts. The improved agreement between the modeled and in situ SM seas is harder to interpret, given that mean seasonal cycle errors are systematic, and systematic errors are not typically targeted by (bias-blind) data assimilation. Finally, the use of one year subsets of the AMSR-E and Catchment soil moisture for estimating the observation-bias correction (rescaling) parameters is investigated. It is concluded that when only one year of data is available, the associated uncertainty in the rescaling parameters should not greatly reduce the average benefit gained from data assimilation, but locally and in extreme years there is a risk of increased errors.

Description: A rainfall design method for spatial flood risk assessment: considering multiple flood sources Hydrology and Earth System Sciences Discussions, 12, 8005-8033, 2015 Author(s): X. Jiang and H. Tatano Information about the spatial distribution of flood risk is important for integrated urban flood risk management. Focusing on urban areas, spatial flood risk assessment must reflect all risk information derived from multiple flood sources: rivers, drainage, coastal flooding etc. that may affect the area. However, conventional flood risk assessment deals with each flood source independently, which leads to an underestimation of flood risk in the floodplain. Even in floodplains that have no risk from coastal flooding, flooding from river channels and inundation caused by insufficient drainage capacity should be considered simultaneously. For integrated flood risk management, it is necessary to establish a methodology to estimate flood risk distribution across a floodplain. In this paper, a rainfall design method for spatial flood risk assessment, which considers the joint effects of multiple flood sources, is proposed. The concept of critical rainfall duration determined by the concentration time of flooding is introduced to connect response characteristics of different flood sources with rainfall. A copula method is then adopted to capture the correlation of rainfall amount with different critical rainfall durations. Rainfall events are designed taking advantage of the copula structure of correlation and marginal distribution of rainfall amounts within different critical rainfall durations. A case study in the Otsu River Basin, Osaka prefecture, Japan was conducted to demonstrate this methodology.

Description: Technical Note: Testing an improved index for analysing storm nutrient hysteresis Hydrology and Earth System Sciences Discussions, 12, 7875-7892, 2015 Author(s): C. E. M. Lloyd, J. E. Freer, P. J. Johnes, and A. L. Collins Analysis of hydrochemical behaviour in extreme flow events can provide new insights into the process controls on nutrient transport in catchments. The examination of storm behaviours using hysteresis analysis has increased in recent years, partly due to the increased availability of high temporal resolution datasets for discharge and nutrient parameters. A number of these analyses involve the use of an index to describe the characteristics of a hysteresis loop in order to compare different storm behaviours both within and between catchments. This technical note reviews the methods for calculation of the hysteresis index (HI) and explores a new more effective methodology. Each method is systematically tested and the impact of the chosen calculation on the results is examined. Recommendations are made regarding the most effective method of calculating a HI which can be used for comparing data between storms and between different parameters and catchments.

Description: Soil storage influences climate–evapotranspiration interactions in three western United States catchments Hydrology and Earth System Sciences Discussions, 12, 7893-7931, 2015 Author(s): E. S. Garcia and C. L. Tague In the winter-wet, summer-dry forests of the western United States, total annual evapotranspiration (ET) varies with precipitation and temperature. Geologically mediated drainage and storage properties, however, may strongly influence these relationships between climate and ET. We use a physically based process model to evaluate how soil available water capacity (AWC) and rates of drainage influence model estimates of ET-climate relationships for three snow-dominated, mountainous catchments with differing precipitation regimes. Model estimates show that total annual precipitation is a primary control on inter-annual variation in ET across all catchments and that the timing of recharge is a second order control. Low soil AWC, however, increases the sensitivity of annual ET to these climate drivers by three to five times in our two study basins with drier summers. ET–climate relationships in our Colorado basin receiving summer precipitation are more stable across subsurface drainage and storage characteristics. Climate driver-ET relationships are most sensitive to soil AWC and soil drainage parameters related to lateral redistribution in the relatively dry Sierra site that receives little summer precipitation. Our results demonstrate that uncertainty in geophysically mediated storage and drainage properties can strongly influence model estimates of watershed scale ET responses to climate variation and climate change. This sensitivity to uncertainty in geophysical properties is particularly true for sites receiving little summer precipitation. A parallel interpretation of this parameter sensitivity is that spatial variation in soil properties are likely to lead to substantial within-watershed plot scale differences in forest water use and drought stress.

Description: Sub-daily runoff simulations with parameters inferred at the daily time scale Hydrology and Earth System Sciences Discussions, 12, 7437-7467, 2015 Author(s): J. E. Reynolds, S. Halldin, C. Y. Xu, J. Seibert, and A. Kauffeldt Concentration times in small and medium-sized watersheds (~ 100–1000 km 2 ) are commonly less than 24 h. Flood-forecasting models then require data at sub-daily time scales, but time-series of input and runoff data with sufficient lengths are often only available at the daily time scale, especially in developing countries. This has led to a search for time-scale relationships to infer parameter values at the time scales where they are needed from the time scales where they are available. In this study, time-scale dependencies in the HBV-light conceptual hydrological model were assessed within the generalized likelihood uncertainty estimation (GLUE) approach. It was hypothesised that the existence of such dependencies is a result of the numerical method or time-stepping scheme used in the models rather than a real time-scale-data dependence. Parameter values inferred showed a clear dependence on time scale when the explicit Euler method was used for modelling at the same time steps as the time scale of the input data (1–24 h). However, the dependence almost fully disappeared when the explicit Euler method was used for modelling in 1 h time steps internally irrespectively of the time scale of the input data. In other words, it was found that when an adequate time-stepping scheme was implemented, parameter sets inferred at one time scale (e.g., daily) could be used directly for runoff simulations at other time scales (e.g., 3 or 6 h) without any time scaling and this approach only resulted in a small (if any) model performance decrease, in terms of Nash–Sutcliffe and volume-error efficiencies. The overall results of this study indicated that as soon as sub-daily driving data can be secured, flood forecasting in watersheds with sub-daily concentration times is possible with model-parameter values inferred from long time series of daily data, as long as an appropriate numerical method is used.

Description: Analysis of three-dimensional groundwater flow toward a radial collector well in a finite-extent unconfined aquifer Hydrology and Earth System Sciences Discussions, 12, 7503-7540, 2015 Author(s): C.-S. Huang, J.-J. Chen, and H.-D. Yeh This study develops a three-dimensional mathematical model for describing transient hydraulic head distributions due to pumping at a radial collector well (RCW) in a rectangular confined or unconfined aquifer bounded by two parallel streams and no-flow boundaries. The governing equation with a point-sink term is employed. A first-order free surface equation delineating the water table decline induced by the well is considered. The head solution for the point sink is derived by applying the methods of double-integral transform and Laplace transform. The head solution for a RCW is obtained by integrating the point-sink solution along the laterals of the RCW and then dividing the integration result by the sum of lateral lengths. On the basis of Darcy's law and head distributions along the streams, the solution for the stream depletion rate (SDR) can also be developed. With the aid of the head and SDR solutions, the sensitivity analysis can then be performed to explore the response of the hydraulic head to the change in a specific parameter such as the horizontal and vertical hydraulic conductivities, streambed permeability, specific storage, specific yield, lateral length and well depth. Spatial head distributions subject to the anisotropy of aquifer hydraulic conductivities are analyzed. A quantitative criterion is provided to identify whether groundwater flow at a specific region is 3-D or 2-D without the vertical component. In addition, another criterion is also given to allow the neglect of vertical flow effect on SDR. Conventional 2-D flow models can be used to provide accurate head and SDR predictions if satisfying these two criteria.

Description: Effects of cultivation and reforestation on suspended sediment concentrations: a case study in a mountainous catchment in China Hydrology and Earth System Sciences Discussions, 12, 7583-7614, 2015 Author(s): N. F. Fang, F. X. Chen, H. Y. Zhang, Y. X. Wang, and Z. H. Shi Understanding how sediment concentrations vary with land use/cover is critical for evaluating the current and future impacts of human activities on river systems. This paper presents suspended sediment concentration (SSC) dynamics and the relationship between SSC and discharge ( Q ) in the 8973 km 2 Du catchment and its sub-catchment (4635 km 2 ). In the Du catchment and its sub-catchment, 4235 and 3980 paired Q -SSC samples, respectively, were collected over 30 years. Under the influence of the "Household Contract Responsibility System" and Grain-for-Green projects in China, three periods were designated, the original period (1980s), cultivation period (1990s), and reforestation period (2000s). The results of a Mann–Kendall test showed that rainfall slightly increased during the study years; however, the annual discharge and sediment load significantly decreased. The annual suspended sediment yield of the Du catchment varied between 4 and 332 kg s −1 , and that of the sub-catchment varied between 2 and 135 kg s −1 . The SSCs in the catchment and sub-catchment fluctuated between 1 and 22 400 g m −3 and between 1 and 31 800 g m −3 , respectively. The mean SSC of the Du catchment was relatively stable during the three periods (±83 g m −3 ). ANOVA indicated that the SSC did not significantly change under cultivation for low and moderate flows, but was significantly different under high flow during reforestation of the Du catchment. The SSC in the sub-catchment was more variable, and the mean-SSC in the sub-catchment varied from 1058 g m −3 in the 1980s to 1256 g m −3 in the 1990s and 891 g m −3 in the 2000s. Reforestation significantly decreased the SSCs during low and moderate flows, whereas cultivation increased the SSCs during high flow. The sediment rating curves showed a stable relationship between the SSC and Q in the Du catchment during the three periods. However, the SSC- Q of the sub-catchment exhibited scattered relationships during the original and cultivation periods and a more linear relationship during the reforestation period.

Description: Identification of spatiotemporal patterns of biophysical droughts in semi-arid region – a case study of the Karkheh river basin in Iran Hydrology and Earth System Sciences Discussions, 12, 5187-5217, 2015 Author(s): B. Kamali, K. C. Abbaspour, A. Lehmann, B. Wehrli, and H. Yang This study aims at identifying historical patterns of meteorological, hydrological, and agricultural (inclusively biophysical) droughts in the Karkheh River Basin (KRB), one of the nine benchmark watersheds of the CGIAR Challenge Program on Water and Food. Standardized precipitation index (SPI), standardized runoff index (SRI), and soil moisture deficit index (SMDI) were used to represent the above three types of droughts, respectively. The three drought indices were compared across temporal and spatial dimensions. Variables required for calculating the indices were obtained from the Soil and Water Assessment Tool (SWAT) constructed for the region. The model was calibrated based on monthly runoff and yearly wheat yield using the Sequential Uncertainty Fitting (SUFI-2) algorithm. Five meteorological drought events were identified in the studied period (1980–2004), of which four corresponded with the hydrological droughts with 1–3 month lag. The meteorological droughts corresponded well with the agricultural droughts during dry months (May–August), while the latter lasted for a longer period of time. Analysis of drought patterns showed that southern parts of the catchment were more prone to agricultural drought, while less influenced by hydrological drought. Our analyses highlighted the necessity for monitoring all three aspects of drought for a more effective watershed management. The analysis on different types of droughts in this study provides a framework for assessing their possible impacts under future climate change in semi-arid areas.

Description: Long-term effects of climate and land cover change on freshwater provision in the tropical Andes Hydrology and Earth System Sciences Discussions, 12, 5219-5250, 2015 Author(s): A. Molina, V. Vanacker, E. Brisson, D. Mora, and V. Balthazar Andean headwater catchments play a pivotal role to supply fresh water for downstream water users. However, few long-term studies exist on the relative importance of climate change and direct anthropogenic perturbations on flow regimes. In this paper, we assess multi-decadal change in freshwater provision based on long time series (1974–2008) of hydrometeorological data and land cover reconstructions for a 282 km 2 catchment located in the tropical Andes. Three main land cover change trajectories can be distinguished: (1) rapid decline of native vegetation in montane forest and páramo ecosystems in ~1/5 or 20% of the catchment area, (2) expansion of agricultural land by 14% of the catchment area, (3) afforestation of 12% of native páramo grasslands with exotic tree species in recent years. Given the strong temporal variability of precipitation and streamflow data related to El Niño–Southern Oscillation, we use empirical mode decomposition techniques to detrend the time series. The long-term increasing trend in rainfall is remarkably different from the observed changes in streamflow that exhibit a decreasing trend. Hence, observed changes in streamflow are not the result of long-term climate change but very likely result from direct anthropogenic disturbances after land cover change. Partial water budgets for montane cloud forest and páramo ecosystems suggest that the strongest changes in evaporative water losses are observed in páramo ecosystems, where progressive colonization and afforestation of high alpine grasslands leads to a strong increase in transpiration losses.

Description: Estimating evapotranspiration with thermal UAV data and two source energy balance models Hydrology and Earth System Sciences Discussions, 12, 7469-7502, 2015 Author(s): H. Hoffmann, H. Nieto, R. Jensen, R. Guzinski, P. J. Zarco-Tejada, and T. Friborg Estimating evapotranspiration is important when managing water resources and cultivating crops. Evapotranspiration can be estimated using land surface heat flux models and remotely sensed land surface temperatures (LST) which recently have become obtainable in very high resolution using Unmanned Aerial Vehicles (UAVs). Very high resolution LST can give insight into e.g. distributed crop conditions within cultivated fields. In this study evapotranspiration is estimated using LST retrieved with a UAV and the physically-based, two source energy balance models: the Priestley–Taylor TSEB (TSEB-PT) and the Dual-Temperature-Difference (DTD). A fixed-wing UAV was flown over a barley field in western Denmark during the spring and summer in 2014 and retrieved images of LST is successfully processed into thermal mosaics which serve as model input for both TSEB-PT and DTD. The aim is to assess whether a lightweight thermal camera mounted on a UAV is able to provide data of sufficient quality to obtain high spatial and temporal resolution surface energy heat fluxes. Furthermore, this study evaluates the performance of the two source energy balance (TSEB) model scheme during cloudy and overcast weather conditions. This is feasible due to the low data retrieval altitude compared to satellite thermal data that are only available during clear skies and sunny conditions. Flux estimates from TSEB-PT and DTD are compared and validated against field data collected using an eddy covariance system located at same site at which the UAV flights were conducted. Furthermore, spatially distributed evapotranspiration patterns are evaluated using known irrigation patterns. Evapotranspiration is well estimated by both TSEB-PT and DTD with DTD as the best predictor. The DTD model provides results comparable to studies estimating evapotranspiration with satellite retrieved LST and physical land-surface models. This study shows that the UAV platform and the lightweight thermal camera provide high spatial and temporal resolution data valid for model input and for other potential applications requiring high resolution and consistent LST. Lastly, this study explicates thermal UAV data processing and the mosaicking of images into ortho-photos suited for model input.

Description: Crop-specific seasonal estimates of irrigation water demand in South Asia Hydrology and Earth System Sciences Discussions, 12, 7843-7873, 2015 Author(s): H. Biemans, C. Siderius, A. Mishra, and B. Ahmad Especially in the Himalayan headwaters of the main rivers in South Asia, shifts in runoff are expected as a result of a rapidly changing climate. In recent years, our insight in these shifts and their impact on water availability has increased. However, a similar detailed understanding of the seasonal pattern in water demand is surprisingly absent. This hampers a proper assessment of water stress and ways to cope and adapt. In this study, the seasonal pattern of irrigation water demand resulting from the typical practice of multiple-cropping in South Asia was accounted for by introducing double-cropping with monsoon-dependent planting dates in a hydrology and vegetation model. Crop yields were calibrated to the latest subnational statistics of India, Pakistan, Bangladesh and Nepal. The representation of seasonal land use and more accurate cropping periods lead to lower estimates of irrigation water demand compared to previous model-based studies, despite the net irrigated area being higher. Crop irrigation water demand differs sharply between seasons and regions; in Pakistan, winter (Rabi) and summer (Kharif) irrigation demands are almost equal, whereas in Bangladesh the Rabi demand is ~ 100 times higher. Moreover, the relative importance of irrigation supply vs. rain decreases sharply from west to east. Given the size and importance of South Asia, improved regional estimates of food production and its irrigation water demand will also affect global estimates. In models used for global water resources and food-security assessments, processes like multiple-cropping and monsoon-dependent planting dates should not be ignored.

Description: Evaluation of global fine-resolution precipitation products and their uncertainty quantification in ensemble discharge simulations Hydrology and Earth System Sciences Discussions, 12, 9337-9391, 2015 Author(s): W. Qi, C. Zhang, G. T. Fu, C. Sweetapple, and H. C. Zhou The applicability of six fine-resolution precipitation products, including precipitation radar, infrared, microwave and gauge-based products using different precipitation computation recipes, is comprehensively evaluated using statistical and hydrological methods in a usually-neglected area (northeastern China), and a framework quantifying uncertainty contributions of precipitation products, hydrological models and their interactions to uncertainties in ensemble discharges is proposed. The investigated precipitation products include TRMM3B42, TRMM3B42RT, GLDAS/Noah, APHRODITE, PERSIANN and GSMAP-MVK+. Two hydrological models of different complexities, i.e., a water and energy budget-based distributed hydrological model and a physically-based semi-distributed hydrological model, are employed to investigate the influence of hydrological models on simulated discharges. Results show APHRODITE has high accuracy at a monthly scale compared with other products, and the cloud motion vectors used by GSMAP-MVK+ show huge advantage. These findings could be very useful for validation, refinement and future development of satellite-based products (e.g., NASA Global Precipitation Measurement). Although significant uncertainty exists in heavy precipitation, hydrological models contribute most of the uncertainty in extreme discharges. Interactions between precipitation products and hydrological models contribute significantly to uncertainty in discharge simulations and a better precipitation product does not guarantee a better discharge simulation because of interactions. It is also found that a good discharge simulation depends on a good coalition of a hydrological model and a precipitation product, suggesting that, although the satellite-based precipitation products are not as accurate as the gauge-based product, they could have better performance in discharge simulations when appropriately combined with hydrological models. This information is revealed for the first time and very beneficial for precipitation product applications.

Description: Technical Note: Application of artificial neural networks in groundwater table forecasting – a case study in Singapore swamp forest Hydrology and Earth System Sciences Discussions, 12, 9317-9336, 2015 Author(s): Y. Sun, D. Wendi, D. E. Kim, and S.-Y. Liong Accurate prediction of groundwater table is important for the efficient management of groundwater resources. Despite being the most widely used tools for depicting the hydrological regime, numerical models suffer from formidable constraints, such as extensive data demanding, high computational cost and inevitable parameter uncertainty. Artificial neural networks (ANNs), in contrast, can make predictions on the basis of more easily accessible variables, rather than requiring explicit characterization of the physical systems and prior knowledge of the physical parameters. This study applies ANN to predict the groundwater table in a swamp forest of Singapore. A standard multilayer perceptron (MLP) is selected, trained with the Levenberg–Marquardt (LM) algorithm. The inputs to the network are solely the surrounding reservoir levels and rainfall. The results reveal that ANN is able to produce accurate forecast with a leading time up to 7 days, whereas the performance slightly decreases when leading time increases.

Description: Using geochemical tracers to distinguish groundwater and parafluvial inflows in rivers (the Avon Catchment, SE Australia) Hydrology and Earth System Sciences Discussions, 12, 9205-9246, 2015 Author(s): I. Cartwright and H. Hofmann Understanding the location and magnitude of groundwater inflows to rivers is important for the protection of riverine ecosystems and the management of connected groundwater and surface water systems. Downstream trends in 222 Rn activities and Cl concentrations in the Avon River, southeast Australia, implies that it contains alternating gaining and losing reaches. 222 Rn activities of up to 3690 Bq m −3 imply that inflows are locally substantial (up to 3.1 m 3 m −1 day −1 ). However, if it assumed that these inflows are solely from groundwater, the net groundwater inflows during low-flow periods exceed the measured increase in streamflow along the Avon River by up to 490 %. Uncertainties in the 222 Rn activities of groundwater, the gas transfer coefficient, and the degree of hyporheic exchange cannot explain this discrepancy. It is proposed that a significant volume of the total calculated inflows into the Avon River represents water that exfiltrates from the river, flows through parafluvial sediments, and subsequently re-enters the river in the gaining reaches. This returning parafluvial flow has high 222 Rn activities due to 222 Rn emanations from the alluvial sediments. The riffle sections of the Avon River commonly have steep longitudinal gradients and may transition from losing at their upstream end to gaining at the downstream end and parafluvial flow through the sediment banks on meanders and point bars may also occur. Parafluvial flow is likely to be important in rivers with coarse-grained alluvial sediments on their floodplains and failure to quantify the input of 222 Rn from parafluvial flow will result in overestimating groundwater inflows to rivers.

Description: Subsurface flow mixing in coarse, braided river deposits Hydrology and Earth System Sciences Discussions, 12, 9295-9316, 2015 Author(s): E. Huber and P. Huggenberger Coarse, braided river deposits show a large hydraulic heterogeneity at the metre scale. One of the main depositional elements found in such deposits is a trough structure filled with open-framework–bimodal gravel couplet cross-beds. Several studies investigated the impact of the highly permeable open-framework gravel texture mainly in terms of concentration breakthrough curves. However, although the trough fills are expected to be significant mixing agents for the subsurface flow, their impact on the three-dimensional flow field has not draw much attention. This study aims to evaluate the subsurface flow mixing caused by overlapping trough fills embedded in a poorly-sorted gravel matrix. Below the river bed of the Tagliamento River (northeast Italy), trough fills were identified with ground-penetrating radar (GPR) probing. Based on field observations of coarse, braided river deposits, a simple three-dimensional geometrical model with associated hydraulic properties was fitted to the interpreted GPR reflectors. Then, steady-state subsurface flow and advective transport simulations were performed on the small-scale, high-resolution model (size: 45 m × 50 m × 10.26 m). The impact of trough fills on the flow field is visualised by the injection of a conservative tracer at three different depths.

Description: Sustainability of water uses in managed hydrosystems: human- and climate-induced changes for the mid-21st century Hydrology and Earth System Sciences Discussions, 12, 9247-9293, 2015 Author(s): J. Fabre, D. Ruelland, A. Dezetter, and B. Grouillet This paper assesses the sustainability of planned water uses in mesoscale river basins under multiple climate change scenarios, and contributes to determining the possible causes of unsustainability. We propose an assessment grounded in real-world water management issues, with water management scenarios built in collaboration with local water agencies. Furthermore we present an analysis through indicators that relate to management goals and present the implications of climate uncertainty for our results, furthering the significance of our study for water management. A modeling framework integrating hydro-climatic and human dynamics and accounting for interactions between resource and demand was developed and applied in two basins of different scales and with contrasting water uses: the Herault (2500 km 2 , France) and the Ebro (85 000 km 2 , Spain) basins. Natural streamflow was evaluated using a conceptual hydrological model. A demand-driven reservoir management model was designed to account for streamflow regulations from the main dams. Human water demand was estimated from time series of demographic, socio-economic and climatic data. Environmental flows were accounted for by defining streamflow thresholds under which withdrawals were strictly limited. Finally indicators comparing water availability to demand at strategic resource and demand nodes were computed. This framework was applied under different combinations of climatic and water use scenarios for the mid-21st century to differentiate the impacts of climate- and human-induced changes on streamflow and water balance. Results showed that objective monthly environmental flows would be guaranteed in current climate conditions in both basins, yet in several areas this could imply limiting human water uses more than once every five years. The impact of the tested climate projections on both water availability and demand could question the water allocations and environmental requirements currently planned for the coming decades. Water shortages for human use could become more frequent and intense, and the pressure on water resources and aquatic ecosystems could intensify. The causes of unsustainability vary across sub-basins and scenarios, and in most areas results are highly dependent on the climate change scenario.

Description: Investigation of hydrological time series using copulas for detecting catchment characteristics and anthropogenic impacts Hydrology and Earth System Sciences Discussions, 12, 9157-9203, 2015 Author(s): T. Sugimoto, A. Bárdossy, G. S. S. Pegram, and J. Cullmann Global climate change can have impacts on characteristics of rainfall-runoff events and subsequently on the hydrological regime. Meanwhile, the catchment itself changes due to anthropogenic influences. In this context, it can be meaningful to detect the temporal changes of catchments independent from climate change by investigating existing long term discharge records. For this purpose, a new stochastic system based on copulas for time series analysis is introduced. While widely used time series models are based on linear combinations of correlations assuming a Gaussian behavior of variables, a statistical tool like copula has the advantage to scrutinize the dependence structure of the data in the uniform domain independent of the marginal. Two measures in the copula domain are introduced herein: 1. Copula asymmetry is defined for copulas and calculated for discharges; this measure describes the non symmetric property of the dependence structure and differs from one catchment to another due to the intrinsic nature of both runoff and catchment. 2. Copula distance is defined as Cramér-von Mises type distance calculated between two copula densities of different time scales. This measure describes the variability and interdependency of dependence structures similar to variance and covariance, which can assist in identifying the catchment changes. These measures are calculated for 100 years of daily discharges for the Rhine rivers. Comparing the results of copula asymmetry and copula distance between an API and simulated discharge time series by a hydrological model we can show the interesting signals of systematic modifications along the Rhine rivers in the last 30 years.

Description: Soil–aquifer phenomena affecting groundwater under vertisols: a review Hydrology and Earth System Sciences Discussions, 12, 9571-9598, 2015 Author(s): D. Kurtzman, S. Baram, and O. Dahan Vertisols are cracking clayey soils that: (i) usually form in alluvial lowlands where normally, groundwater pools into aquifers, (ii) have different types of voids (due to cracking) which make flow and transport of water, solutes and gas complex, and (iii) are regarded as fertile soils in many areas. The combination of these characteristics results in the unique soil–aquifer phenomena that are highlighted and summarized in this review. The review is divided into the following four sections: (1) soil cracks as preferential pathways for water and contaminants; in this section lysimeter- to basin-scale observations that show the significance of cracks as preferential flow paths in vertisols which bypass matrix blocks in the unsaturated zone are summarized. Relatively fresh-water recharge and groundwater contamination from these fluxes and their modeling are reviewed, (2) soil cracks as deep evaporators and unsaturated-zone salinity; deep sediment samples under uncultivated vertisols in semiarid regions reveal a dry (immobile), saline matrix, partly due to enhanced evaporation through soil cracks. Observations of this phenomenon are compiled in this section and the mechanism of evapoconcentration due to air flow in the cracks is discussed, (3) impact of cultivation on flushing of the unsaturated zone and aquifer salinization; the third section examines studies reporting that land-use change of vertisols from native land to cropland promotes greater fluxes through the saline unsaturated-zone matrix, eventually flushing salts to the aquifer. Different degrees of salt flushing are assessed as well as aquifer salinization on different scales, and a comparison is made with aquifers under other soils, (4) relatively little nitrate contamination in aquifers under vertisols; In this section we turn the light on observations showing that aquifers under cultivated vertisols are somewhat resistant to groundwater contamination by nitrate (the major agriculturally related groundwater problem). Denitrification is probably the main mechanism supporting this resistance, whereas a certain degree of anion-exchange capacity may have a retarding effect as well.

Description: A framework for testing the use of electric and electromagnetic data to reduce the prediction error of groundwater models Hydrology and Earth System Sciences Discussions, 12, 9599-9653, 2015 Author(s): N. K. Christensen, S. Christensen, and T. P. A. Ferre Despite geophysics is being used increasingly, it is still unclear how and when the integration of geophysical data improves the construction and predictive capability of groundwater models. Therefore, this paper presents a newly developed HY drogeophysical TE st- B ench (HYTEB) which is a collection of geological, groundwater and geophysical modeling and inversion software wrapped to make a platform for generation and consideration of multi-modal data for objective hydrologic analysis. It is intentionally flexible to allow for simple or sophisticated treatments of geophysical responses, hydrologic processes, parameterization, and inversion approaches. It can also be used to discover potential errors that can be introduced through petrophysical models and approaches to correlating geophysical and hydrologic parameters. With HYTEB we study alternative uses of electromagnetic (EM) data for groundwater modeling in a hydrogeological environment consisting of various types of glacial deposits with typical hydraulic conductivities and electrical resistivities covering impermeable bedrock with low resistivity. It is investigated to what extent groundwater model calibration and, often more importantly, model predictions can be improved by including in the calibration process electrical resistivity estimates obtained from TEM data. In all calibration cases, the hydraulic conductivity field is highly parameterized and the estimation is stabilized by regularization. For purely hydrologic inversion (HI, only using hydrologic data) we used Tikhonov regularization combined with singular value decomposition. For joint hydrogeophysical inversion (JHI) and sequential hydrogeophysical inversion (SHI) the resistivity estimates from TEM are used together with a petrophysical relationship to formulate the regularization term. In all cases, the regularization stabilizes the inversion, but neither the HI nor the JHI objective function could be minimized uniquely. SHI or JHI with regularization based on the use of TEM data produced estimated hydraulic conductivity fields that bear more resemblance to the reference fields than when using HI with Tikhonov regularization. However, for the studied system the resistivities estimated by SHI or JHI must be used with caution as estimators of hydraulic conductivity or as regularization means for subsequent hydrological inversion. Much of the lack of value of the geophysical data arises from a mistaken faith in the power of the petrophysical model in combination with geophysical data of low sensitivity, thereby propagating geophysical estimation errors into the hydrologic model parameters. With respect to reducing model prediction error, it depends on the type of prediction whether it has value to include geophysical data in the model calibration. It is found that all calibrated models are good predictors of hydraulic head. When the stress situation is changed from that of the hydrologic calibration data, then all models make biased predictions of head change. All calibrated models turn out to be a very poor predictor of the pumping well's recharge area and groundwater age. The reason for this is that distributed recharge is parameterized as depending on estimated hydraulic conductivity of the upper model layer which tends to be underestimated. Another important insight from the HYTEB analysis is thus that either recharge should be parameterized and estimated in a different way, or other types of data should be added to better constrain the recharge estimates.

Description: Co-evolution of volcanic catchments in Japan Hydrology and Earth System Sciences Discussions, 12, 9655-9700, 2015 Author(s): T. Yoshida and P. A. Troch Present day landscapes have evolved over time through interactions between the prevailing climates and geological settings. Understanding the linkage between spatial patterns of landforms, soils, and vegetation in landscapes and their hydrological response is critical to make quantitative predictions in ungaged basins. Catchment co-evolution is a theoretical framework that seeks to formulate hypotheses about the mechanisms and conditions that determine the historical development of catchments and how such evolution affects their hydrological response. In this study, we selected 14 volcanic catchments of different ages (from 0.225 to 82.2 Ma) in Japan. We derived indices of landscape properties (drainage density) as well as hydrological response (annual water balance, baseflow index, and flow duration curves) and examined their relation with catchment age and climate (through the aridity index). We found significant correlation between drainage density and baseflow index with age, but not with climate. The age of the catchments was also significantly related to intra-annual flow variability. Younger catchments tend to have lower peak flows and higher low flows, while older catchments exhibit more flashy runoff. The decrease of baseflow with catchment age confirms previous studies that hypothesized that in volcanic landscapes the major flow pathways have changed over time, from deep groundwater flow to shallow subsurface flow. The drainage density of our catchments decreased with age, contrary to previous findings in similar volcanic catchments but of significant younger age than the ones explored here. In these younger catchments, an increase in drainage density with age was observed, and it was hypothesized that this was because of more landscape incision due to increasing near-surface lateral flow paths in more mature catchments. Our results suggests two hypotheses on the evolution of drainage density in matured catchments. One is that as catchments further evolve, hydrologically active channels retreat as less recharge leads to lower average aquifer levels and less baseflow; the other is that it does not significantly change after catchments reached maturity in terms of surface dissection.

Description: Identifying hydrological responses of micro-catchments under contrasting land use in the Brazilian Cerrado Hydrology and Earth System Sciences Discussions, 12, 9915-9975, 2015 Author(s): R. L. B. Nobrega, A. C. Guzha, G. N. Torres, K. Kovacs, G. Lamparter, R. S. S. Amorim, E. Couto, and G. Gerold In recent decades, the Brazilian Cerrado biome has been affected by intense land-use change, particularly the conversion of natural forest to agricultural land. Understanding the environmental impacts of this land-use change on landscape hydrological dynamics is one of the main challenges in the Amazon agricultural frontier, where part of the Brazilian Cerrado biome is located and where most of the deforestation has occurred. This study uses empirical data from field measurements to characterize controls on hydrological processes from three first-order micro-catchments 〈 1 km 2 in the Cerrado biome. These micro-catchments were selected on the basis of predominant land use including native cerrado vegetation, pasture grass with cattle ranching, and cash crop land. We continuously monitored precipitation, streamflow, soil moisture, and meteorological variables from October 2012 to September 2014. Additionally, we determined the physical and hydraulic properties of the soils, and conducted topographic surveys. We used these data to quantify the water balance components of the study catchments and to relate these water fluxes to land use, catchment physiographic parameters, and soil hydrophysical properties. The results of this study show that runoff coefficients were 0.27, 0.40, and 0.16 for the cerrado, pasture, and cropland catchments, respectively. Baseflow is shown to play a significant role in streamflow generation in the three study catchments, with baseflow index values of more than 0.95. The results also show that evapotranspiration was highest in the cerrado (986 mm yr −1 ) compared to the cropland (828 mm yr −1 ) and the pasture (532 mm yr −1 ). However, discharges in the cropland catchment were unexpectedly lower than that of the cerrado catchment. The normalized discharge was 55 % higher and 57 % lower in the pasture and cropland catchments, respectively, compared with the cerrado catchment. We attribute this finding to the differences in soil type and topographic characteristics, and low-till farming techniques in the cropland catchment, additionally to the buffering effect of the gallery forests in these catchments. Although the results of this study provide a useful assessment of catchment rainfall–runoff controls in the Brazilian Cerrado landscape, further research is required to include quantification of the influence of the gallery forests on both hydrological and hydrochemical fluxes, which are important for watershed management and ecosystem services provisioning.

Description: Stochastic or statistic? Comparing flow duration curve models in ungauged basins and changing climates Hydrology and Earth System Sciences Discussions, 12, 9765-9811, 2015 Author(s): M. F. Müller and S. E. Thompson The prediction of flow duration curves (FDCs) in ungauged basins remains an important task for hydrologists given the practical relevance of FDCs for water management and infrastructure design. Predicting FDCs in ungauged basins typically requires spatial interpolation of statistical or model parameters. This task is complicated if climate becomes non-stationary, as the prediction challenge now also requires extrapolation through time. In this context, process-based models for FDCs that mechanistically link the streamflow distribution to climate and landscape factors may have an advantage over purely statistical methods to predict FDCs. This study compares a stochastic (process-based) and statistical method for FDC prediction in both stationary and non-stationary contexts, using Nepal as a case study. Under contemporary conditions, both models perform well in predicting FDCs, with Nash–Sutcliffe coefficients above 0.80 in 75 % of the tested catchments. The main drives of uncertainty differ between the models: parameter interpolation was the main source of error for the statistical model, while violations of the assumptions of the process-based model represented the main source of its error. The process-based approach performed better than the statistical approach in numerical simulations with non-stationary climate drivers. The predictions of the statistical method under non-stationary rainfall conditions were poor if (i) local runoff coefficients were not accurately determined from the gauge network, or (ii) streamflow variability was strongly affected by changes in rainfall. A Monte Carlo analysis shows that the streamflow regimes in catchments characterized by a strong wet-season runoff and a rapid, strongly non-linear hydrologic response are particularly sensitive to changes in rainfall statistics. In these cases, process-based prediction approaches are strongly favored over statistical models.

Description: Reviving the "Ganges Water Machine": where and how much? Hydrology and Earth System Sciences Discussions, 12, 9741-9763, 2015 Author(s): L. Muthuwatta, U. A. Amarasinghe, A. Sood, and S. Lagudu Surface runoff generated in the monsoon months in the upstream parts of the Ganges River Basin contributes substantially to downstream floods, while water shortages in the dry months affect agricultural production in the basin. This paper examines the parts (sub-basins) of the Ganges that have the potential for augmenting subsurface storage (SSS), increase the availability of water for agriculture and other uses, and mitigate the floods in the downstream areas. The Soil and Water Assessment Tool (SWAT) is used to estimate sub-basin-wise water availability. The water availability estimated is then compared with the sub-basin-wise un-met water demand for agriculture. Hydrological analyses revealed that five sub-basins produced more than 10 billion cubic meters (B m 3 ) of annual surface runoff consistently during the simulation period. In these sub-basins, less than 50 % of the annual surface runoff is sufficient to irrigate all irrigable land in both the \textit{Rabi} (November to March) and summer (April to May) seasons. Further, for most of the sub-basins, there is sufficient water to meet the un-met water demand, provided that it is possible to capture the surface runoff during the wet season. It is estimated that the average flow to Bihar State from the upstream of the Ganges, a downstream basin location, is 277 ± 121 B m 3 , which is more than double the rainfall in the state alone. Strong relationships between outflows from the upstream sub-basins and the inflows to Bihar State suggested that flood inundation in the state could be reduced by capturing a portion of the upstream flows during the peak runoff periods.

Description: Monitoring infiltration processes with high-resolution surface-based Ground-Penetrating Radar Hydrology and Earth System Sciences Discussions, 12, 12215-12246, 2015 Author(s): P. Klenk, S. Jaumann, and K. Roth In this study, we present a series of high resolution Ground-Penetrating Radar (GPR) measurements monitoring two artificially induced infiltration pulses into two different sands with dual-frequency ground-based GPR. After the application of the second infiltration pulse, the water table in the subsoil was raised by pumping in water from below. The longterm relaxation of the system was then monitored over the course of several weeks. We focused on the capillary fringe reflection and on observed variations in soil water content as derived from direct wave travel times. We discuss the advantages of this dual-frequency approach and show the attainable precision in longterm monitoring of such relaxation processes. Reaching a relative precision of better than 0.001 [–] in water content, we can clearly discern the relaxation of the two investigated sands.

Description: Macropore flow at the field scale: predictive performance of empirical models and X-ray CT analyzed macropore characteristics Hydrology and Earth System Sciences Discussions, 12, 12089-12120, 2015 Author(s): M. Naveed, P. Moldrup, M. Schaap, M. Tuller, R. Kulkarni, H.-J. Vögel, and L. Wollesen de Jonge Predictions of macropore flow is important for maintaining both soil and water quality as it governs key related soil processes e.g. soil erosion and subsurface transport of pollutants. However, macropore flow currently cannot be reliably predicted at the field scale because of inherently large spatial variability. The aim of this study was to perform field scale characterization of macropore flow and investigate the predictive performance of (1) current empirical models for both water and air flow, and (2) X-ray CT derived macropore network characteristics. For this purpose, 65 cylindrical soil columns (6 cm diameter and 3.5 cm height) were extracted from the topsoil (5 to 8.5 cm depth) in a 15 m × 15 m grid from an agricultural loamy field located in Silstrup, Denmark. All soil columns were scanned with an industrial CT scanner (129 μm resolution) and later used for measurements of saturated water permeability, air permeability and gas diffusivity at −30 and −100 cm matric potentials. Distribution maps for both water and air permeabilities and gas diffusivity reflected no spatial correlation irrespective of the soil texture and organic matter maps. Empirical predictive models for both water and air permeabilities showed poor performance as they were not able to realistically capture macropore flow because of poor correlations with soil texture and bulk density. The tested empirical model predicted well gas diffusivity at −100 cm matric potential, but relatively failed at −30 cm matric potential particularly for samples with biopore flow. Image segmentation output of the four employed methods was nearly the same, and matched well with measured air-filled porosity at −30 cm matric potential. Many of the CT derived macropore network characteristics were strongly interrelated. Most of the macropore network characteristics were also strongly correlated with saturated water permeability, air permeability, and gas diffusivity. The correlations between macropore network characteristics and macropore flow parameters were further improved on dividing soil samples into samples with biopore and matrix flow. Observed strong correlations between macropore network characteristics and macropore flow highlighted the need of further research on numerical simulations of macropore flow based on X-ray CT images. This could pave the way for the digital soil physics laboratory in the future.

Description: Identification of the main attribute of river flow temporal variations in the Nile Basin Hydrology and Earth System Sciences Discussions, 12, 12167-12214, 2015 Author(s): C. Onyutha and P. Willems Temporal variation of monthly flows was investigated at 18 Discharge Measurement Stations (DMS) within the Nile Basin in Africa. The DMS were grouped using a clustering procedure based on the similarity in the flow variation patterns. The co-variation of the rainfall and flow was assessed in each group. To investigate the possible change in catchment behavior, which may interfere with the flow–rainfall relationship, three rainfall–runoff models were applied to the major catchment in each group based on the data time period falling within 1940–2003. The co-occurrence of the changes in the observed and simulated overland flow was examined using the cumulative rank difference (CRD) technique and the quantile perturbation method (QPM). Two groups of the DMS were obtained. Group 1 comprises the DMS from the equatorial region and/or South Sudan, while those in Sudan, Ethiopia and Egypt form group 2. In the selected catchment of each group, the patterns of changes in terms of the CRD sub-trends and QPM anomalies for both the observed and simulated flows were in a close agreement. These results indicate that change in catchment behavior due to anthropogenic influence in the Nile basin over the selected time period was minimal. Thus, the overall rainfall–runoff generation processes of the catchments were not impacted upon in a significant way. The temporal flow variations could be attributed mainly to the rainfall variations.

Description: The socio-ecohydrology of rainwater harvesting in India: understanding water storage and release dynamics at tank and catchment scales Hydrology and Earth System Sciences Discussions, 12, 12121-12165, 2015 Author(s): K. J. Van Meter, N. B. Basu, D. L. McLaughlin, and M. Steiff Rainwater harvesting (RWH), the small-scale collection and storage of runoff for irrigated agriculture, is recognized as a sustainable strategy for ensuring food security, especially in monsoonal landscapes in the developing world. In south India, these strategies have been used for millennia to mitigate problems of water scarcity. However, in the past 100 years many traditional RWH systems have fallen into disrepair due to increasing dependence on groundwater. This dependence has contributed to an accelerated decline in groundwater resources, which has in turn led to increased efforts at the state and national levels to revive older RWH systems. Critical to the success of such efforts is an improved understanding of how these ancient systems function in contemporary landscapes with extensive groundwater pumping and shifted climatic regimes. Knowledge is especially lacking regarding the water-exchange dynamics of these RWH "tanks" at tank and catchment scales, and how these exchanges regulate tank performance and catchment water balances. Here, we use fine-scale water-level variation to quantify daily fluxes of groundwater, evapotranspiration (ET), and sluice outflows in four tanks over the 2013 northeast monsoon season in a tank cascade that covers a catchment area of 28 km 2 . At the tank scale, our results indicate that groundwater recharge and irrigation outflows comprise the largest fractions of the tank water budget, with ET accounting for only 13–22 % of the outflows. At the scale of the cascade, we observe a distinct spatial pattern in groundwater-exchange dynamics, with the frequency and magnitude of groundwater inflows increasing down the cascade of tanks. The significant magnitude of return flows along the tank cascade leads to the most downgradient tank in the cascade having an outflow-to capacity ratio greater than 2. The presence of tanks in the landscape dramatically alters the catchment water balance, with runoff decreasing by nearly 75 %, and recharge increasing by more than 40 %. Finally, while water from the tanks directly satisfies ~40 % of the crop water requirement across the northeast monsoon season via surface water irrigation, a large fraction of the tank water is "wasted," and more efficient management of sluice outflows could lead to tanks meeting a higher fraction of crop water requirements.

Description: Analytical approach for determining the mean water level profile in an estuary with substantial fresh water discharge Hydrology and Earth System Sciences Discussions, 12, 8381-8417, 2015 Author(s): H. Cai, H. H. G. Savenije, C. Jiang, L. Zhao, and Q. Yang Although modestly, the mean water level in estuaries rises in landward direction induced by a combination of the salinity gradient, the tidal asymmetry, and the backwater effect. The water level slope is increased by the fresh water discharge. However, the interactions between tide and river flow and their individual contributions to the rise of the mean water level along the estuary are not yet completely understood. In this study, we adopt an analytical approach to describe the tidal wave propagation under the influence of fresh water discharge, in which the friction term is approximated by a Chebyshev polynomials approach. The analytical model is used to quantify the contributions made by tide, river, and tide–river interaction to the water level slope along the estuary. Subsequently, the method is applied to the Yangtze estuary under a wide range of river discharge conditions and the influence of tidal amplitude and fresh water discharge on the longitudinal variation of mean water level is explored. The proposed method is particularly useful for accurately predicting water levels and the frequency of extreme high water, relevant for water management and flood control.

Description: Trends in West African floods: a comparative analysis with rainfall and vegetation indices Hydrology and Earth System Sciences Discussions, 12, 5083-5121, 2015 Author(s): B. N. Nka, L. Oudin, H. Karambiri, J. E. Paturel, and P. Ribstein After the drought of the 1970s in West Africa, the variability of rainfall and land use changes affected mostly flow, and recently flooding has been said to be an increasingly common occurrence throughout the whole of West Africa. These changes raised many questions about the impact of climate change on the flood regimes in West African countries. This paper investigates whether floods are becoming more frequent or more severe, and to what extent climate patterns have been responsible for these changes. We analyzed the trends in the floods occurring in 14 catchments within West Africa's main climate zone. The methodology includes two methods for sampling flood events, namely the AM (annual maximum) method and the POT (peak over threshold), and two perspectives of analysis are presented: long-term analysis based on two long flood time series, and a regional perspective involving 14 catchments with shorter series. The Mann–Kendall trend test and the Pettitt break test were used to assess time series stationarity. The trends detected in flood time series were compared to the rainfall index trends and vegetation indices using contingency tables, in order to identify the main driver of change in flood magnitude and flood frequency. The relation between the flood index and the physiographic index was evaluated through a success criterion and the Cramer criterion calculated from the contingency tables. The results point out the existence of trends in flood magnitude and flood frequency time series with two main patterns. Sahelian floods show increasing flood trends and some Sudanian catchments present decreasing flood trends. For the overall catchments studied, the maximum 5 day consecutive rainfall index (Rx5d) seems to follow the flood trend, while the NDVI indices do not show a significant link with the flood trends, meaning that this index has no impact in the behavior of floods in the region.

Description: Evaluation of a multi-satellite soil moisture product and the Community Land Model 4.5 simulation in China Hydrology and Earth System Sciences Discussions, 12, 5151-5186, 2015 Author(s): B. Jia, J. Liu, and Z. Xie Twenty years of in situ soil moisture data from more than 300 stations located in China are used to perform an evaluation of two surface soil moisture datasets: a microwave-based multi-satellite product (ECV-SM) and the land surface model simulation from the Community Land Model 4.5 (CLM4.5). Both soil moisture products generally show a good agreement with in situ observations. The ECV-SM product has a low bias, with a root mean square difference (RMSD) of 0.075 m 3 m -3 , but shows a weak correlation with in situ observations ( R = 0.41). In contrast, the CLM4.5 simulation, forced by an observation-based atmospheric forcing data, produces better temporal variation of surface soil moisture ( R = 0.52), but shows a clear overestimation (bias = 0.05 m 3 m -3 ) and larger RMSD (0.09 m 3 m -3 ), especially in eastern China, caused by inaccurate descriptions of soil characteristics. The ECV-SM product is more likely to be superior in semi-arid regions, mainly because of the accurate retrievals and high observation density, but inferior over areas covered by dense vegetation. Furthermore, it shows a stable to slightly increasing performance in China, except for a decrease during the 2007–2010 blending period. Results from this study can provide comprehensive insight into the performances of the two soil moisture datasets in China, which will be useful for their improvements in merging algorithms or model simulations and for applications in soil moisture data assimilation.

Description: Optimality and inference in hydrology from entropy production considerations: synthetic hillslope numerical experiments Hydrology and Earth System Sciences Discussions, 12, 5123-5149, 2015 Author(s): S. J. Kollet In this study, entropy production optimization and inference principles are applied to a synthetic semi-arid hillslope in high-resolution, physics-based simulations. The results suggest that entropy or power is indeed maximized, because of the strong nonlinearity of variably saturated flow and competing processes related to soil moisture fluxes, the depletion of gradients, and the movement of a free water table. Thus, it appears that the maximum entropy production (MEP) principle may indeed be applicable to hydrologic systems. In the application to hydrologic system, the free water table constitutes an important degree of freedom in the optimization of entropy production and may also relate the theory to actual observations. In an ensuing analysis, an attempt is made to transfer the complex, "microscopic" hillslope model into a macroscopic model of reduced complexity using the MEP principle as an interference tool to obtain effective conductance coefficients and forces/gradients. The results demonstrate a new approach for the application of MEP to hydrologic systems and may form the basis for fruitful discussions and research in future.

Description: Creating long term gridded fields of reference evapotranspiration in Alpine terrain based on a re-calibrated Hargreaves method Hydrology and Earth System Sciences Discussions, 12, 5055-5082, 2015 Author(s): K. Haslinger and A. Bartsch A new approach for the construction of high resolution gridded fields of reference evapotranspiration for the Austrian domain on a daily time step is presented. Forcing fields of gridded data of minimum and maximum temperatures are used to estimate reference evapotranspiration based on the formulation of Hargreaves. The calibration constant in the Hargreaves equation is recalibrated to the Penman–Monteith equation, which is recommended by the FAO, in a monthly and station-wise assessment. This ensures on one hand eliminated biases of the Hargreaves approach compared to the formulation of Penman–Monteith and on the other hand also reduced root mean square errors and relative errors on a daily time scale. The resulting new calibration parameters are interpolated in time to a daily temporal resolution for a standard year of 365 days. The overall novelty of the approach is the conduction of surface elevation as a predictor to estimate the re-calibrated Hargreaves parameter in space. A third order spline is fitted to the re-calibrated parameters against elevation at every station and yields the statistical model for assessing these new parameters in space by using the underlying digital elevation model of the temperature fields. Having newly calibrated parameters for every day of year and every grid point, the Hargreaves method is applied to the temperature fields, yielding reference evapotranspiration for the entire grid and time period from 1961–2013. With this approach it is possible to generate high resolution reference evapotranspiration fields starting when only temperature observations are available but re-calibrated to meet the requirements of the recommendations defined by the FAO.

Description: Assessing the quality of Digital Elevation Models obtained from mini-Unmanned Aerial Vehicles for overland flow modelling in urban areas Hydrology and Earth System Sciences Discussions, 12, 5629-5670, 2015 Author(s): J. P. Leitão, M. Moy de Vitry, A. Scheidegger, and J. Rieckermann Precise and detailed Digital Elevation Models (DEMs) are essential to accurately predict overland flow in urban areas. Unfortunately, traditional sources of DEM remain a bottleneck for detailed and reliable overland flow models, because the resulting DEMs are too coarse to provide DEMs of sufficient detail to inform urban overland flows. Interestingly, technological developments of Unmanned Aerial Vehicles (UAVs) suggest that they have matured enough to be a competitive alternative to satellites or airplanes. However, this has not been tested so far. In this this study we therefore evaluated whether DEMs generated from UAV imagery are suitable for urban drainage overland flow modelling. Specifically, fourteen UAV flights were conducted to assess the influence of four different flight parameters on the quality of generated DEMs: (i) flight altitude, (ii) image overlapping, (iii) camera pitch and (iv) weather conditions. In addition, we compared the best quality UAV DEM to a conventional Light Detection and Ranging (LiDAR)-based DEM. To evaluate both the quality of the UAV DEMs and the comparison to LiDAR-based DEMs, we performed regression analysis on several qualitative and quantitative metrics, such as elevation accuracy, quality of object representation (e.g., buildings, walls and trees) in the DEM, which were specifically tailored to assess overland flow modelling performance, using the flight parameters as explanatory variables. Our results suggested that, first, as expected, flight altitude influenced the DEM quality most, where lower flights produce better DEMs; in a similar fashion, overcast weather conditions are preferable, but weather conditions and other factors influence DEM quality much less. Second, we found that for urban overland flow modelling, the UAV DEMs performed competitively in comparison to a traditional LiDAR-based DEM. An important advantage of using UAVs to generate DEMs in urban areas is their flexibility that enables more frequent, local and affordable elevation data updates, allowing, for example, to capture different tree foliage conditions.

Description: Joint inference of groundwater-recharge and hydraulic-conductivity fields from head data using the Ensemble-Kalman filter Hydrology and Earth System Sciences Discussions, 12, 5565-5599, 2015 Author(s): D. Erdal and O. A. Cirpka Regional groundwater flow strongly depends on groundwater recharge and hydraulic conductivity. Both are spatially variable fields, and their estimation is an ongoing topic in groundwater research and practice. In this study, we use the Ensemble Kalman filter as an inversion method to jointly estimate spatially variable recharge and conductivity fields from head observations. The success of the approach strongly depends on the assumed prior knowledge. If the structural assumptions underlying the initial ensemble of the parameter fields are correct, both estimated fields resemble the true ones. However, erroneous prior knowledge may not be corrected by the data. In the worst case, the estimated recharge field resembles the true conductivity field, resulting in a model that meets the observations but has very poor predictive power. The study exemplifies the importance of prior knowledge in the joint estimation of parameters from ambiguous measurements.

Description: A global approach to defining flood seasons Hydrology and Earth System Sciences Discussions, 12, 4595-4630, 2015 Author(s): D. Lee, P. Ward, and P. Block Globally, flood catastrophes lead all natural hazards in terms of impacts on society, causing billions of dollars of damages annually. While short-term flood warning systems are improving in number and sophistication, forecasting systems on the order of months to seasons are a rarity, yet may lead to further disaster preparedness. To lay the groundwork for prediction, dominant flood seasons must be adequately defined. A global approach is adopted here, using the PCR-GLOBWB model to define spatial and temporal characteristics of major flood seasons globally. The main flood season is identified using a volume-based threshold technique. In comparison with observations, 40% (50%) of locations at a station (sub-basin) scale have identical peak months and 81% (89%) are within 1 month, indicating strong agreement between model and observed flood seasons. Model defined flood seasons are additionally found to well represent actual flood records from the Dartmouth Flood Observatory, further substantiating the models ability to reproduce the appropriate flood season. Minor flood seasons are also defined for regions with bi-modal streamflow climatology. Properly defining flood seasons can lead to prediction through association of streamflow with local and large-scale hydroclimatic indicators, and eventual integration into early warning systems for informed advanced planning and management. This is especially attractive for regions with limited observations and/or little capacity to develop early warning flood systems.

Description: Variability in snow cover phenology in China from 1952 to 2010 Hydrology and Earth System Sciences Discussions, 12, 4471-4506, 2015 Author(s): C. Q. Ke, X. C. Li, H. Xie, X. Liu, and C. Kou Daily snow observation data from 672 stations, particularly the 352 stations with over ten annual mean snow cover days (SCD), during 1952–2010 in China, are used in this study. We first examine spatiotemporal variations and trends of SCD, snow cover onset date (SCOD), and snow cover end date (SCED). We then investigate SCD relationships with number of days with temperature below 0 °C (TBZD), mean air temperature (MAT), and Arctic Oscillation (AO) index, the latter two being constrained to the snow season of each snow year. The results indicate that the heavy-snow years for the entire country include 1955, 1957, 1964, and 2010, and light-snow years include 1953, 1965, 1999, 2002, and 2009. The reduced TBZD and increased MAT are the main reasons for the overall delay of SCOD and advance of SCED since 1952, although it is not necessary for one station to experience both significantly delayed SCOD and early SCED. This explains why only 15% of the stations show significant shortening of SCD, while 75% of the stations show no significant change in the SCD trends. This differs with the overall shortening of the snow period in the Northern Hemisphere previously reported. Our analyses indicate that the SCD distribution pattern and trends in China are very complex and are not controlled by any single climate variable examined (i.e. TBZD, MAT, or AO), but a combination of multiple variables. It is found that the AO index has the maximum impact on the SCD shortening trends in Shandong Peninsula, Changbai Mountains, and North Xinjiang, while the combined TBZD and MAT have the maximum impact on the SCD shortening trends in the Loess Plateau, Xiaoxingganling, and Sanjiang Plain.

Description: Quantifying energy and water fluxes in dry dune ecosystems of the Netherlands Hydrology and Earth System Sciences Discussions, 12, 4541-4594, 2015 Author(s): B. R. Voortman, R. P. Bartholomeus, S. E. A. T. M. van der Zee, M. F. P. Bierkens, and J. P. M. Witte Coastal and inland dunes provide various ecosystem services that are related to groundwater, such as drinking water production and biodiversity. To manage groundwater in a sustainable manner, knowledge of actual evapotranspiration ( ET a ) for the various land covers in dunes is essential. Aiming at improving the parameterization of dune vegetation in hydro-meteorological models, this study explores the magnitude of energy and water fluxes in an inland dune ecosystem in the Netherlands. Hydro-meteorological measurements were used to parameterize the Penman–Monteith evapotranspiration model for four different surfaces: bare sand, moss, grass and heather. We found that the net longwave radiation ( R nl ) was the largest energy flux for most surfaces during daytime. However, modelling this flux by a calibrated FAO-56 R nl model for each surface and for hourly time steps was unsuccessful. Our R nl model, with a novel sub-model using solar elevation angle and air temperature to describe the diurnal pattern in radiative surface temperature, improved R nl simulations considerably. Model simulations of evaporation from moss surfaces showed that the modulating effect of mosses on the water balance is species dependent. We demonstrate that dense moss carpets ( Campylopus introflexus ) evaporate more (5%, +14 mm) than bare sand (total of 258 mm in 2013), while more open structured mosses ( Hypnum cupressiforme ) evaporate less (−30%, −76 mm) than bare sand. Additionally, we found that a drought event in the summer of 2013 showed a pronounced delayed signal on lysimeter measurements of ET a for the grass and heather surfaces respectively. Due to the desiccation of leaves after the drought event, and their feedback on the parameters of the Penman–Monteith equation, the potential evapotranspiration in the year 2013 dropped with 9% (−37mm) and 10% (−61 mm) for the grass and heather surfaces respectively, which subsequently led to lowered ET a of 8% (−29 mm) and 7% (−29 mm). These feedbacks are of importance to water resources, especially during a changing climate with increasing number of drought days. Therefore, such feedbacks need to be integrated into a coupled plant physiological and hydro-meteorological model to accurately simulate ET a . In addition, our study showed that groundwater recharge in dunes can be increased considerably by promoting moss vegetation, especially of open structured moss species.

Description: Kalman filter approach for estimating water level time series over inland water using multi-mission satellite altimetry Hydrology and Earth System Sciences Discussions, 12, 4813-4855, 2015 Author(s): C. Schwatke, D. Dettmering, W. Bosch, and F. Seitz Satellite altimetry has been designed for sea level monitoring over open ocean areas. However, since some years, this technology is also used for observing inland water levels of lakes and rivers. In this paper, a new approach for the estimation of inland water level time series is described. It is used for the computation of time series available through the web service "Database for Hydrological Time Series over Inland Water" (DAHITI). The method is based on a Kalman filter approach incorporating multi-mission altimeter observations and their uncertainties. As input data, cross-calibrated altimeter data from Envisat, ERS-2, Jason-1, Jason-2, Topex/Poseidon, and SARAL/AltiKa are used. The paper presents water level time series for a variety of lakes and rivers in North and South America featuring different characteristics such as shape, lake extent, river width, and data coverage. A comprehensive validation is performed by comparison with in-situ gauge data and results from external inland altimeter databases. The new approach yields RMS differences with respect to in-situ data between 4 and 38 cm for lakes and 12 and 139 cm for rivers, respectively. For most study cases, more accurate height information than from available other altimeter data bases can be achieved.

Description: A review of applications of satellite SAR, optical, altimetry and DEM data for surface water modelling, mapping and parameter estimation Hydrology and Earth System Sciences Discussions, 12, 4857-4878, 2015 Author(s): Z. N. Musa, I. Popescu, and A. Mynett Hydrological data collection requires deployment of physical infrastructure like rain gauges, water level gauges, as well as use of expensive equipment like echo sounders. Many countries around the world have recorded a decrease in deployment of physical infrastructure for hydrological measurements; developing countries especially have less of this infrastructure and where they exist, they are poorly maintained. Satellite remote sensing can bridge this gap, and has been applied by hydrologists over the years, with the earliest applications in water body and flood mapping. With the availability of more optical satellites with relatively low temporal resolutions globally, satellite data is commonly used for: mapping of water bodies, testing of inundation models, precipitation monitoring, and mapping of flood extent. Use of satellite data to estimate hydrological parameters continues to increase due to use of better sensors, improvement in knowledge of/and utilization of satellite data, and expansion of research topics. A review of applications of satellite remote sensing in surface water modelling, mapping and estimation is presented, and its limitations for surface water applications are also discussed.

Description: Estimation of crop water requirements: extending the one-step approach to dual crop coefficients Hydrology and Earth System Sciences Discussions, 12, 4933-4963, 2015 Author(s): J. P. Lhomme, N. Boudhina, M. M. Masmoudi, and A. Chehbouni Crop water requirements are commonly estimated with the FAO-56 methodology based upon a "two-step" approach: first a reference evapotranspiration (ET 0 ) is calculated from weather variables with the Penman–Monteith equation; then ET 0 is multiplied by a tabulated crop-specific coefficient ( K c ) to determine the water requirement (ET c ) of a given crop under standard conditions. This method has been challenged to the benefit of a "one-step" approach, where crop evapotranspiration is directly calculated from a Penman–Monteith equation, its surface resistance replacing the crop coefficient. Whereas the transformation of the two-step approach into a one-step approach has been well documented when a single crop coefficient ( K c ) is used, the case of dual crop coefficients ( K cb for the crop and K e for the soil) has not been treated yet. The present paper examines this specific case. Using a full two-layer model as a reference, it is shown that the FAO-56 dual crop coefficient approach can be translated into a one-step approach based upon a modified combination equation. This equation has the basic form of the Penman–Monteith equation, but its surface resistance is calculated as the parallel sum of a foliage resistance (replacing K cb ) and a soil surface resistance (replacing K e ). We also show that the foliage resistance, which depends on leaf stomatal resistance and leaf area, can be inferred from the basal crop coefficient ( K cb ) in a way similar to the Matt–Shuttleworth method.

Description: A method for calculating the duration and intensity of salt intrusions: the Yangtze River estuary Hydrology and Earth System Sciences Discussions, 12, 4909-4932, 2015 Author(s): M. Webber, M. T. Li, J. Chen, B. Finlayson, D. Chen, Z. Y. Chen, M. Wang, and J. Barnett Studies of intrusions of salt water into estuaries are typically constrained by both the short duration of discharge records and the paucity of observations of discharge and salinity. Thus studies of intrusions of salt water into estuaries typically seek to identify the conditions under which intrusions occur, using detailed observations for periods of 20–60 days. This paper demonstrates a method by which to identify the conditions under which intense intrusions of long duration occur and applies that method to the Yangtze River estuary. The paper constructs a model of the relationship between salinity and discharge and then employs Monte Carlo simulation methods to reconstruct the probability of observing intrusions of differing intensities and durations in relation to discharge. The model predicts that the duration of intrusions with chlorinity ≥250 mg L −1 increases as the number of consecutive days with discharge ≤12 000 m 3 s −1 increases; consecutive days of discharges ≤8000 m 3 s −1 predict the duration of intrusions with chlorinity ≥400 or 500 mg L −1 . In 26 of the 64 years analysed, the probability of an intrusion of at least 60 days at ≥250 mg L −1 is greater than 1 in 1000; in 17 years is greater than 1 in 100; and in ten years is greater than 1 in 10.

Description: Spatial and temporal runoff processes in the degraded Ethiopian Highlands: the Anjeni Watershed Hydrology and Earth System Sciences Discussions, 12, 4387-4411, 2015 Author(s): H. K. Bayabil, T. Y. Tebebu, C. R. Stoof, and T. S. Steenhuis As runoff mechanisms in the Ethiopian highlands are not well understood, performance of many soil and water conservation measures is inadequate because of ineffective placement outside the major runoff source areas. To improve understanding of the runoff generating mechanisms in these highlands, we monitored runoff volumes from 24 runoff plots constructed in the 113 ha Anjeni watershed, where historic data of rainfall and stream discharge were available. In addition, we assessed the effectiveness of charcoal and crop rooting depth in reducing runoff, in which we compared the effect of lupine (a deep-rooted crop) to that of barley. Daily rainfall, surface runoff, and root zone moisture content were measured during the monsoon seasons of 2012 and 2013 (with all plots being tilled in 2012, but only barley plots in 2013). In addition, long-term surface runoff (from four plots) and outlet discharge data from the research site (1989–1993) was analyzed and compared with our observations. Results showed that the degree of soil degradation and soil disturbance (tillage) were significant factors affecting plot runoff responses. As expected runoff was greater from more degraded soils, while tilled plots had greater soil storage and thus less runoff. Overall, barley plots produced significantly less runoff than lupine plots. Specifically, considerable difference was observed for smaller rainfall events (ca.

Description: Quantitative historical hydrology in Europe Hydrology and Earth System Sciences Discussions, 12, 4413-4469, 2015 Author(s): G. Benito, R. Brázdil, J. Herget, and M. J. Machado In the last decades, the quantification of flood hydrological characteristics (peak discharge, hydrograph shape, and runoff volume) from documentary evidence has gained scientific recognition as a method to lengthen flood records of rare and extreme events. This paper describes the methodological evolution of the quantitative historical hydrology under the influence of developments in hydraulics and statistics. In the 19th century, discharge calculations based on flood marks was the only source of hydrological data for engineering design, but later was left aside on favour of systematic gauge records and conventional hydrological procedures. In the last two decades, there is growing scientific and public interest to understand long-term patterns of rare floods, maintain the flood heritage and memory of extremes, and to develop methods for deterministic and statistical application to different scientific and engineering problems. A compilation of 45 case studies across Europe with reconstructed discharges demonstrates that (1) in most cases present flood magnitudes are not unusual within the context of the last millennium, although recent floods may exceed past floods in some temperate European rivers (e.g. the Vltava and Po rivers), (2) frequency of extreme floods have decreased since the 1950s, although some rivers (e.g. the Gardon and Ouse rivers) show a reactivation of rare events over the last two decades. There is a great potential of gaining understanding of individual extreme events based on a combined multiproxy approach (palaeoflood and documentary records) providing high-resolution time flood series and their environmental and climatic changes; and to develop non-systematic and non-stationary statistical models based on relations of past floods with external and internal covariates under natural low-frequency climate variability.

Description: An index of floodplain surface complexity Hydrology and Earth System Sciences Discussions, 12, 4507-4540, 2015 Author(s): M. W. Scown, M. C. Thoms, and N. R. De Jager Floodplain surface topography is an important component of floodplain ecosystems. It is the primary physical template upon which ecosystem processes are acted out. There has been a limited appreciation of floodplain surface complexity because of the traditional focus on temporal variability in floodplains as well as limitations to quantifying spatial complexity. An index of floodplain surface complexity (FSC) is developed in this paper and applied to eight floodplains from different geographic settings. The index is based on the two key indicators of complexity; variability in surface geometry (VSG) and the spatial organization of surface conditions (SOC) and was determined at three sampling scales. Relationships between these measures of spatial complexity and environmental drivers, namely; flow variability (mean daily discharge [ Q ], the coefficient of variation of daily discharge [ Q CV ], the coefficient of variation of mean annual discharge [ Q CVAnn ], the coefficient of variation of maximum annual discharge [ Q CVMax ]), sediment yield (SY), valley slope (Vs), and floodplain width (Fpw) were examined. FSC, VSG, and SOC varied between the eight floodplains and this was dependent upon sampling scale. All complexity values declined with increasing Fpw in either a power, logarithmic, or exponential function. There was little change in surface complexity with floodplain widths greater than 10 km. VSG was significantly related to SY and no significant relationships were determined between any of the hydrological variables and floodplain surface complexity.

Description: Effects of mountain agriculture on nutrient cycling at upstream watersheds Hydrology and Earth System Sciences Discussions, 12, 4785-4811, 2015 Author(s): T.-C. Lin, P. L. Shaner, L.-J. Wang, Y.-T. Shih, C.-P. Wang, G.-H. Huang, and J.-C. Huang The expansion of agriculture to rugged mountains can exacerbate negative impacts of agriculture activities on ecosystem function. In this study, we monitored streamwater chemistry of four watersheds with varying proportions of agricultural lands (0.4, 3, 17, 22%) and rainfall chemistry of two of the four watersheds at Feitsui Reservoir Watershed in northern Taiwan to examine the effects of agriculture on watershed nutrient cycling. We found that the greater the proportions of agricultural lands, the higher the ion concentrations, which is evident for fertilizer-associated ions (NO 3 - , K + ) but not for ions that are rich in soils (SO 4 2- , Ca 2+ , Mg 2+ ), suggesting that agriculture enriched fertilizer-associated nutrients in streamwater. The watershed with the highest proportion of agricultural lands had higher concentrations of ions in rainfall and lower nutrient retention capacity (i.e. higher output–input ratio of ions) compared to the relatively pristine watershed, suggesting that agriculture can influence atmospheric deposition of nutrients and a system's ability to retain nutrients. Furthermore, we found that a forested watershed downstream of agricultural activities can dilute the concentrations of fertilizer-associated ions (NO 3 − , K + ) in streamwater by more than 70%, indicating that specific landscape configurations help mitigate nutrient enrichment to aquatic systems. We estimated that agricultural lands at our study site contributed approximately 400 kg ha −1 yr −1 of NO 3 -N and 260 kg ha −1 yr −1 of PO 4 -P output via streamwater, an order of magnitude greater than previously reported around the globe and can only be matched by areas under intense fertilizer use. Furthermore, we re-constructed watershed nutrient fluxes to show that excessive leaching of N and P, and additional loss of N to the atmosphere via volatilization and denitrification, can occur under intense fertilizer use. In summary, this study demonstrated the pervasive impacts of agriculture activities, especially excessive fertilization, on ecosystem nutrient cycling at mountain watersheds.

Description: Reconciling high altitude precipitation in the upper Indus Basin with glacier mass balances and runoff Hydrology and Earth System Sciences Discussions, 12, 4755-4784, 2015 Author(s): W. W. Immerzeel, N. Wanders, A. F. Lutz, J. M. Shea, and M. F. P. Bierkens Mountain ranges in Asia are important water suppliers, especially if downstream climates are arid, water demands are high and glaciers are abundant. In such basins, the hydrological cycle depends heavily on high altitude precipitation. Yet direct observations of high altitude precipitation are lacking and satellite derived products are of insufficient resolution and quality to capture spatial variation and magnitude of mountain precipitation. Here we use glacier mass balances to inversely infer the high altitude precipitation in the upper Indus Basin and show that the amount of precipitation required to sustain the observed mass balances of the large glacier systems is far beyond what is observed at valley stations or estimated by gridded precipitation products. An independent validation with observed river flow confirms that the water balance can indeed only be closed when the high altitude precipitation is up to a factor ten higher than previously thought. We conclude that these findings alter the present understanding of high altitude hydrology and will have an important bearing on climate change impact studies, planning and design of hydropower plants and irrigation reservoirs and the regional geopolitical situation in general.

Description: Groundwater-dependent ecosystems: recent insights, new techniques and an ecosystem-scale threshold response Hydrology and Earth System Sciences Discussions, 12, 4677-4754, 2015 Author(s): D. Eamus, S. Zolfaghar, R. Villalobos-Vega, J. Cleverly, and A. Huete Groundwater-dependent ecosystems (GDEs) are at risk globally due to unsustainable levels of groundwater extraction, especially in arid and semi-arid regions. In this review, we examine recent developments in the ecohydrology of GDEs with a focus on three knowledge gaps: (1) how do we locate GDEs, (2) how much water is transpired from shallow aquifers by GDEs; and (3) what are the responses of GDEs to excessive groundwater extraction? The answers to these questions will determine water allocations that are required to sustain functioning of GDEs and to guide regulations on groundwater extraction to avoid negative impacts on GDEs. We discuss three methods for identifying GDEs: (1) fluctuations in depth-to-groundwater that are associated with diurnal variations in transpiration, (2) stable isotope analysis of water sources in the transpiration stream; and (3) remote sensing methods. We then discuss several methods for estimating rates of GW use, including direct measurement using sapflux or eddy covariance technologies, estimation of a climate wetness index within a Budyko framework, spatial distribution of ET using remote sensing, groundwater modelling and stable isotopes. Remote sensing methods often rely on direct measurements to calibrate the relationship between vegetation indices and ET. ET from GDEs is also determined using hydrologic models of varying complexity, from the "White method" to fully coupled, variable saturation models. Combinations of methods are typically employed to obtain clearer insight into the components of groundwater discharge in GDEs, such as the proportional importance of transpiration vs. evaporation (e.g., using stable isotopes) or from groundwater vs. rainwater sources. Groundwater extraction can have severe consequences on structure and function of GDEs. In the most extreme cases, phreatophytes experience crown dieback and death following groundwater drawdown. We provide a brief review of two case studies of the impacts of GW extraction and discuss the use of C isotope ratios in xylem to reveal past influences of GW extraction. We conclude with a discussion of a depth-to-groundwater threshold in mesic and semi-arid GDEs. Across this threshold, significant changes occur in ecosystem structure and function.

Description: High-resolution modelling of interactions between soil moisture and convection development in mountain enclosed Tibetan basin Hydrology and Earth System Sciences Discussions, 12, 4631-4675, 2015 Author(s): T. Gerken, W. Babel, M. Herzog, K. Fuchs, F. Sun, Y. Ma, T. Foken, and H.-F. Graf The Tibetan Plateau plays a significant role in the atmospheric circulation and the Asian monsoon system. Turbulent surface fluxes and the evolution of boundary layer clouds to deep and moist convection provide a feedback system that modifies the Plateau's surface energy balance on scales that are currently unresolved in mesoscale models. This work analyses the land surface's role and specifically the influence of soil moisture on the triggering of convection at a cross-section of the Nam Co Lake basin, 150 km north of Lhasa using a cloud resolving atmospheric model with a fully coupled surface. The modelled turbulent fluxes and development of convection compare reasonably well with the observed weather. The simulations span Bowen-ratios of 0.5 to 2.5. It is found that convection development is strongest at intermediate soil moistures. Dry cases with soils close to the permanent wilting point are moisture limited in the convection development, while convection in wet soil moisture cases is limited by cloud cover reducing incoming solar radiation and sensible heat fluxes. This has a strong impact on the surface energy balance. This study also shows that local development of convection is an important mechanism for the upward transport of water vapour that originates from the lake basin that can then be transported to dryer regions of the plateau. Both processes demonstrate the importance of soil moisture and surface–atmosphere interactions on the energy and hydrological cycles of the Tibetan Plateau.

Description: Annual canopy interception at artificial lowland tropical forest Hydrology and Earth System Sciences Discussions, 12, 4879-4907, 2015 Author(s): A. B. Azinoor-Azida and L. Minjiao The objective of this paper is to present the application of interception model developed in artificial lowland tropical forest. This model estimates annual canopy interception loss with temporal resolution effects. A 12-month data from 2 plots in study area were collected and the measured interception loss was compared with results calculated using original Gash, modified Gash and the interception model developed. The results show that the model can be applied to estimate annual interception loss.

Description: Understanding runoff processes in a semi-arid environment through isotope and hydrochemical hydrograph separations Hydrology and Earth System Sciences Discussions, 12, 975-1015, 2015 Author(s): V. V. Camacho, A. M. L Saraiva Okello, J. W. Wenninger, and S. Uhlenbrook The understanding of runoff generation mechanisms is crucial for the sustainable management of river basins such as the allocation of water resources or the prediction of floods and droughts. However, identifying the mechanisms of runoff generation has been a challenging task, even more so in arid and semi-arid areas where high rainfall and streamflow variability, high evaporation rates, and deep groundwater reservoirs increase the complexity of hydrological process dynamics. Isotope and hydrochemical tracers have proven to be useful in identifying runoff components and their characteristics. Moreover, although widely used in humid-temperate regions, isotope hydrograph separations have not been studied in detail in arid and semi-arid areas. Thus the purpose of this study is to determine if isotope hydrograph separations are suitable for the quantification and characterization of runoff components in a semi-arid catchment considering the hydrological complexities of these regions. Through a hydrochemical characterization of the surface water and groundwater sources of the catchment and two and three component hydrograph separations, runoff components of the Kaap Catchment in South Africa were quantified using both, isotope and hydrochemical tracers. No major disadvantages while using isotope tracers over hydrochemical tracers were found. Hydrograph separation results showed that runoff in the Kaap catchment is mainly generated by groundwater sources. Two-component hydrograph separations revealed groundwater contributions between 64 and 98% of total runoff. By means of three-component hydrograph separations, runoff components were further separated into direct runoff, shallow and deep groundwater components. Direct runoff, defined as the direct precipitation on the stream channel and overland flow, contributed up to 41% of total runoff during wet catchment conditions. Shallow groundwater defined as the soil water and near-surface water component, contributed up to 45% of total runoff, and deep groundwater contributed up to 84% of total runoff. A strong correlation for the four studied events was found between the antecedent precipitation conditions and direct runoff. These findings suggest that direct runoff is enhanced by wetter conditions in the catchment which trigger saturation excess overland flow as observed in the hydrograph separations.

Description: Climate response to Amazon forest replacement by heterogeneous crop cover Hydrology and Earth System Sciences Discussions, 12, 879-910, 2015 Author(s): A. M. Badger and P. A. Dirmeyer Previous modeling studies with atmospheric general circulation models and basic land surface schemes to balance energy and water budgets have shown that by removing the natural vegetation over the Amazon, the region's climate becomes warmer and drier. In this study we use a fully coupled Earth System Model and replace tropical forests by a distribution of six common tropical crops with variable planting dates, physiological parameters and irrigation. There is still general agreement with previous studies as areal averages show a warmer (+1.4 K) and drier (−0.35 mm day −1 ) climate. Using an interactive crop model with a realistic crop distribution shows that regions of vegetation change experience different responses dependent upon the initial tree coverage and whether the replacement vegetation is irrigated, with seasonal changes synchronized to the cropping season. Areas with initial tree coverage greater than 80% show an increase in coupling with atmosphere after deforestation, suggesting land use change could heighten sensitivity to climate anomalies, while irrigation acts to dampen coupling with atmosphere.

Description: Uncertainty in hydrological signatures Hydrology and Earth System Sciences Discussions, 12, 4233-4270, 2015 Author(s): I. K. Westerberg and H. K. McMillan Information about rainfall–runoff processes is essential for hydrological analyses, modelling and water-management applications. A hydrological, or diagnostic, signature quantifies such information from observed data as an index value. Signatures are widely used, including for catchment classification, model calibration and change detection. Uncertainties in the observed data – including measurement inaccuracy and representativeness as well as errors relating to data management – propagate to the signature values and reduce their information content. Subjective choices in the calculation method are a further source of uncertainty. We review the uncertainties relevant to different signatures based on rainfall and flow data. We propose a generally applicable method to calculate these uncertainties based on Monte Carlo sampling and demonstrate it in two catchments for common signatures including rainfall–runoff thresholds, recession analysis and basic descriptive signatures of flow distribution and dynamics. Our intention is to contribute to awareness and knowledge of signature uncertainty, including typical sources, magnitude and methods for its assessment. We found that the uncertainties were often large (i.e. typical intervals of ±10–40% relative uncertainty) and highly variable between signatures. There was greater uncertainty in signatures that use high-frequency responses, small data subsets, or subsets prone to measurement errors. There was lower uncertainty in signatures that use spatial or temporal averages. Some signatures were sensitive to particular uncertainty types such as rating-curve form. We found that signatures can be designed to be robust to some uncertainty sources. Signature uncertainties of the magnitudes we found have the potential to change the conclusions of hydrological and ecohydrological analyses, such as cross-catchment comparisons or inferences about dominant processes.

Description: Updating hydraulic properties and layer thicknesses in hydrogeological models using groundwater model calibration results Hydrology and Earth System Sciences Discussions, 12, 4191-4231, 2015 Author(s): A. Lourens, M. F. P. Bierkens, and F. C. van Geer Usually, subsoil data for groundwater models are generated from borehole data, using upscaling techniques. Since the assumed hydraulic properties for litho-classes in boreholes are uncertain, and upscaling may add inaccuracies, the groundwater model has to be calibrated. In this paper, a method is presented that uses a calibrated groundwater model to improve the quality of a hydrogeological model (layer thickness and hydraulic properties) as obtained from borehole data. To achieve this, all borehole data are defined by random variables and related to aquifer and aquitard properties at the same support as the groundwater model, using complete probability density functions. Subsequently, the calibrated parameter values of the groundwater model are assumed to be the truth and are used to find the most likely combination of layer thicknesses and hydraulic conductivities for the lithological layers making up the aquifer or aquitard. The presented example is an application of the proposed method to aquitards. Nevertheless, the method can be applied to aquifers as well. The analysis of the results gives rise to the discussion about the correctness of the hydrogeological interpretation of the borehole data as well as the correctness of the calibration results of the groundwater flow model. In order to make the problem tractable, computationally feasible, and avoid assumptions about the distribution form, piecewise linear probability density functions are used, instead of parametrized functions.

Description: Non–stationarity in annual maxima rainfall across Australia – implications for Intensity–Frequency–Duration (IFD) relationships Hydrology and Earth System Sciences Discussions, 12, 3449-3475, 2015 Author(s): D. C. Verdon-Kidd and A. S. Kiem Rainfall Intensity–Frequency–Duration (IFD) relationships are commonly required for the design and planning of water supply and management systems around the world. Currently IFD information is based on the "stationary climate assumption" – that weather at any point in time will vary randomly and that the underlying climate statistics (including both averages and extremes) will remain constant irrespective of the period of record. However, the validity of this assumption has been questioned over the last 15 years, particularly in Australia, following an improved understanding of the significant impact of climate variability and change occurring on interannual to multidecadal timescales. This paper provides evidence of non-stationarity in annual maxima rainfall timeseries using 96 daily rainfall stations and 66 sub-daily rainfall stations across Australia. Further, the effect of non-stationarity on the resulting IFD estimates are explored for three long-term sub-daily rainfall records (Brisbane, Sydney and Melbourne) utilising insights into multidecadal climate variability. It is demonstrated that IFD relationships may under- or over-estimate the design rainfall depending on the length and time period spanned by the rainfall data used to develop the IFD information. It is recommended that non-stationarity in annual maxima rainfall be explicitly considered and appropriately treated in the ongoing revisions of Engineers Australia's guide to estimating and utilising IFD information, "Australian Rainfall and Runoff", and that clear guidance needs to be provided on how to deal with the issue of non-stationarity of extreme events (irrespective of whether that non-stationarity is due to natural or anthropogenic climate change). The findings of our study also have important implications for other regions of the world that exhibit considerable hydroclimatic variability and where IFD information is based on relatively short data sets.

Description: Predicting land use and soil controls on erosion and sediment redistribution in agricultural loess areas: model development and cross scale verification Hydrology and Earth System Sciences Discussions, 12, 3527-3592, 2015 Author(s): U. Scherer and E. Zehe This study quantifies soil and land use controls on sediment mobilisation and redistribution in cultivated loess soil landscapes, as these landscapes are frequently used for intensive cultivation and are highly susceptible to erosion. To this end we developed and verified a process based model named CATFLOW-SED at the plot, hillslope and catchment scales. The model relies on an explicit representation of hillslopes and their dominant physiographical characteristics which control overland flow formation, particle detachment and sediment redistribution (transport and sedimentation). Erosion processes are represented by means of the steady state approximation of the sediment continuity equation, their interaction is conceptualized based on the sediment transport capacity of overland flow. Particle detachment is represented by means of a threshold approach accounting for the attacking forces of rainfall and overland flow which need to exceed a threshold in soil erosion resistance to mobilize soil particles (Scherer et al., 2012). Transport capacity of overland flow is represented as proposed by Engelund and Hansen (1967). Top soil particles and aggregates are detached and transported according to their share in the particle size distribution. Size selective deposition of soil particles is determined based on the sink velocity of the various particle size classes. CATFLOW-SED was verified on the plot, hillslope and catchment scale, where either particle detachment or lateral redistribution or sedimentation is the limiting factor, to check whether the respective parameterizations are transferable for simulations at the next higher scale. For verification we used the Weiherbach data set providing plot scale rainfall simulation experiments, long term monitoring of sediment yields on a selected hillslope as well as observed sediment fluxes at the catchment outlet. Our findings corroborate that CATFLOW-SED predicted the sediment loads at all scales within the error margin of the measurements. An accurate prediction of overland flow turned out as being necessary and sufficient to guarantee spatial transferability of erosion parameters optimized at smaller scales to the next higher scale without need for further calibration. Based on the verified model setup, we investigate the efficiency of land use management to mitigate measures in erosion scenarios for cultivated loess landscapes.

Description: Phosphorus dynamics in lowland streams as a response to climatic, hydrological and agricultural land use gradients Hydrology and Earth System Sciences Discussions, 12, 3349-3390, 2015 Author(s): G. Goyenola, M. Meerhoff, F. Teixeira-de Mello, I. González-Bergonzoni, D. Graeber, C. Fosalba, N. Vidal, N. Mazzeo, N. B. Ovesen, E. Jeppesen, and B. Kronvang Climate and hydrology are relevant control factors for determining the timing and amount of nutrient losses from agricultural fields to freshwaters. In this study, we evaluated the effect of agricultural intensification on the concentrations, dynamics and export of phosphorus (P) in streams in two contrasting climate and hydrological regimes (temperate Denmark and subtropical Uruguay). We applied two alternative nutrient sampling programmes (high frequency composite sampling and low frequency instantaneous-grab sampling) and three alternative methods to estimate exported P from the catchments. A source apportionment model was applied to evaluate the contribution derived from point and diffuse sources in all four catchments studied. Climatic and hydrological characteristics of catchments expressed as flow responsiveness (flashiness), exerted control on catchment and stream TP dynamics, having consequences that were more significant than the outcome of different TP monitoring and export estimation strategies. The impact of intensification of agriculture differed between the two contrasting climate zones. Intensification had a significant impact on subtropical climate with much higher total (as high as 4436 μg P L −1 ), particulate, dissolved and reactive soluble P concentrations and higher P export (as high as 5.20 kg P ha −1 year −1 ). However, we did not find an increased contribution of particulate P to total P as consequence of higher stream flashiness and intensification of agriculture. The high P concentrations at low flow and predominance of dissolved P in subtropical streams actually exacerbate the environmental and sanitary risks associated with eutrophication. In the other hand, temperate intensively farmed stream had lower TP than extensively farmed stream. Our results suggest that the lack of environmental regulations of agricultural production has more severe consequences on water quality, than climatic and hydrological differences between the analysed catchments.

Description: Temporal parameter sensitivity guided verification of process dynamics Hydrology and Earth System Sciences Discussions, 12, 1729-1764, 2015 Author(s): M. Pfannerstill, B. Guse, D. Reusser, and N. Fohrer To ensure reliable results of a hydrological model, it is essential that the model reproduces the hydrological processes adequately. Information about process dynamics is provided by looking at the temporal sensitivities of the corresponding model parameters. For this, the temporal dynamics of parameter sensitivity are used to describe the dominance of parameters for each time step. The parameter dominance is then related to the corresponding hydrological process, since the temporal parameter sensitivity represents the modelled hydrological process. For a reliable model application it has to be verified that the modelled hydrological processes match the expectations of real-world hydrological processes. We present a framework, which distinguishes between a verification of single model components and of the overall model behaviour. We analyse the temporal dynamics of parameter sensitivity of a modified groundwater component of a hydrological model. The results of the single analysis for the modified component show that the behaviour of the parameters of the modified groundwater component is consistent with the idea of the structural modifications. Additionally, the appropriate simulation of all relevant hydrological processes is verified as the temporal dynamics of parameter sensitivity represent these processes according to the expectations. Thus, we conclude that temporal dynamics of parameter sensitivity are helpful for verifying modifications of hydrological models.

Description: What are the key drivers of regional differences in the water balance on the Tibetan Plateau? Hydrology and Earth System Sciences Discussions, 12, 4271-4314, 2015 Author(s): S. Biskop, F. Maussion, P. Krause, and M. Fink Lake-level fluctuations in closed basins on the Tibetan Plateau (TP) indicate climate-induced changes in the regional water balance. However, little is known about the region's key hydrological parameters, hampering the interpretation of these changes. The purpose of this study is to contribute to a more quantitative understanding of these controls. Four lakes in the south-central part of the TP were selected to analyze the spatiotemporal variations of water-balance components: Nam Co and Tangra Yumco (indicating increasing water levels), and Mapam Yumco and Paiku Co (indicating stable or slightly decreasing water levels). We present the results of an integrated approach combining hydrological modeling, atmospheric-model output and remote-sensing data. The hydrological model J2000g was adapted and extended according to the specific characteristics of closed lake basins on the TP and driven with "High Asia Refined analysis (HAR)" data at 10 km resolution for the period 2001–2010. Our results reveal that because of the small portion of glacier areas (1 to 7% of the total basin area) the contribution of glacier melt water accounts for only 14–30% of total runoff during the study period. Precipitation is found to be the principal factor controlling the water-balance in the four studied basins. The positive water balance in the Nam Co and Tangra Yumco basins was primarily related to larger precipitation amounts and thus higher runoff rates in comparison with the Paiku Co and Mapam Yumco basins. This study highlights the benefits of combining atmospheric and hydrological modeling. The presented approach can be readily transferred to other ungauged lake basins on the TP, opening new directions of research. Future work should go towards increasing the atmospheric model's spatial resolution and a better assessment of the model-chain uncertainties, especially in this region where observational data is missing.

Description: Modelling water, sediment and nutrient fluxes from a mixed land-use catchment in New Zealand: effects of hydrologic conditions on SWAT model performance Hydrology and Earth System Sciences Discussions, 12, 4315-4352, 2015 Author(s): W. Me, J. M. Abell, and D. P. Hamilton The Soil Water Assessment Tool (SWAT) was configured for the Puarenga Stream catchment (77 km 2 ), Rotorua, New Zealand. The catchment land use is mostly plantation forest, some of which is spray-irrigated with treated wastewater. A Sequential Uncertainty Fitting (SUFI-2) procedure was used to auto-calibrate unknown parameter values in the SWAT model which was applied to the Puarenga catchment. Discharge, sediment, and nutrient variables were then partitioned into two components (base flow and quick flow) based on hydrograph separation. A manual procedure (one-at a-time sensitivity analysis) was then used to quantify parameter sensitivity for the two hydrologically-separated regimes. Comparison of simulated daily mean discharge, sediment and nutrient concentrations with high-frequency, event-based measurements allowed the error in model predictions to be quantified. This comparison highlighted the potential for model error associated with quick-flow fluxes in flashy lower-order streams to be underestimated compared with low-frequency (e.g. monthly) measurements derived predominantly from base flow measurements. To overcome this problem we advocate the use of high-frequency, event-based monitoring data during calibration and dynamic parameter values with some dependence on discharge regime. This study has important implications for quantifying uncertainty in hydrological models, particularly for studies where model simulations are used to simulate responses of stream discharge and composition to changes in irrigation and land management.

Description: Technical Note: Groundwater flow modeling in coastal aquifers – the influence of submarine groundwater discharge on the position of the saltwater–freshwater interface Hydrology and Earth System Sciences Discussions, 12, 3753-3785, 2015 Author(s): H. A. Shishaye An investigation of the impact of submarine groundwater discharge on the position of saltwater–freshwater interface is presented in this manuscript. Two conceptualizations were considered and analyzed using both analytic and numerical techniques, for comparison purposes. The first conceptualization assumes that the tip of the saltwater–freshwater interface occurs at the shoreline, and the second conceptualization allows for the tip to extend off-shore. Analytic solutions exist for both conceptualizations, i.e., Strack (1976) for conceptualization 1 and Bakker (2006) for conceptualization 2. Results from both analytic and numeric analysis for the two conceptualizations are presented. Results from the first conceptualization were found to overestimate the inland distance to the interface toe, compared to the second conceptualization, for it ignores the influence of submarine groundwater discharge on the interface location. Moreover, results from the analytic solutions as a whole were found to overestimate the interface location compared to the numerical modeling results, for analytic solutions are based on the sharp interface approximations. Therefore, an empirically derived dispersion factor should be used to correct the analytic solution results so as to compare them with the numerically simulated values. Furthermore, offshore model extents should be incorporated when modeling coastal aquifer systems to include the influence of submarine groundwater discharge on the saltwater–freshwater interface position.

Description: Effects of changes in moisture source and the upstream rainout on stable isotopes in summer precipitation – a case study in Nanjing, East China Hydrology and Earth System Sciences Discussions, 12, 3919-3944, 2015 Author(s): Y. Tang, H. Pang, W. Zhang, Y. Li, S. Wu, and S. Hou In the Asian monsoon region, variations in the stable isotopic composition of speleothems have often been attributed to the "amount effect". However, an increasing number of studies suggest that the "amount effect" in local precipitation is insignificant or even non-existent. To explore this issue further, we examined the variability of daily stable isotopic composition (δ 18 O) in summer precipitation of 2012–2014 in Nanjing, East China. We found that δ 18 O was not significantly correlated with local rainfall amount, but could be linked to changes in the location and rainout processes of precipitation source regions. Our findings suggest that the stable isotopes in precipitation could signal the location shift of precipitation source regions in the intertropical convergence zone (ITCZ) over the course of the monsoon season. As a result, changes in moisture source location and upstream rainout effect should be taken into account when interpreting the stable isotopic composition of speleothems in the Asian monsoon region.

Description: A three-pillar approach to assessing climate impacts on low flows Hydrology and Earth System Sciences Discussions, 12, 13069-13122, 2015 Author(s): G. Laaha, J. Parajka, A. Viglione, D. Koffler, K. Haslinger, W. Schöner, J. Zehetgruber, and G. Blöschl The objective of this paper is to present a new strategy for assessing climate impacts on low flows and droughts. The strategy is termed a three-pillar approach as it combines different sources of information. The first pillar, trend extrapolation, exploits the temporal patterns of observed low flows and extends them into the future. The second pillar, rainfall–runoff projections uses precipitation and temperature scenarios from climate models as an input to rainfall–runoff models to project future low flows. The third pillar, stochastic projections, exploits the temporal patterns of observed precipitation and air temperature and extends them into the future to drive rainfall–runoff projections. These pieces of information are combined by expert judgement based on a synoptic view of data and model outputs, taking the respective uncertainties of the methods into account. The viability of the approach is demonstrated for four example catchments from Austria that represent typical climate conditions in Central Europe. The projections differ in terms of their signs and magnitudes. The degree to which the methods agree depends on the regional climate and the dominant low flow seasonality. In the Alpine region where winter low flows dominate, trend projections and climate scenarios yield consistent projections of increasing low flows, although of different magnitudes. In the region north of the Alps, consistently small changes are projected by all methods. In the regions in the South and Southeast, more pronounced and mostly decreasing trends are projected but there is disagreement in the magnitudes of the projected changes. These results suggest that conclusions drawn from only one pillar of information would be highly uncertain. The three-pillar approach offers a systematic framework of combining different sources of information aiming at more robust projections than obtained from each pillar alone.

Description: Dominant climatic factor driving annual runoff change at catchments scale over China Hydrology and Earth System Sciences Discussions, 12, 12911-12945, 2015 Author(s): Z. Huang and H. Yang With global climate changes intensifying, the hydrological response to climate changes has attracted more attentions. It is beneficial not only for hydrology and ecology but also for water resources planning and management to reveal the impacts of climate change on runoff. It is of great significance of climate elasticity of runoff to estimate the impacts of climatic factors on runoff. In addition, there are large spatial variations in climate type and geography characteristics over China. To get a better understanding the spatial variation of runoff response to climate variables change and detect the dominant climatic factor driving annual runoff change, we chose the climate elasticity method proposed by Yang and Yang (2011), where the impact of the catchment characteristics on runoff was represented by a parameter n . The results show that the dominant climatic factor driving annual runoff is precipitation in the most part of China, net radiation in the lower reach of Yangtze River Basin, the Pearl River Basin, the Huai River Basin and the southeast area, and wind speed in part of the northeast China.

Description: The impact of road and railway embankments on runoff and soil erosion in eastern Spain Hydrology and Earth System Sciences Discussions, 12, 12947-12985, 2015 Author(s): P. Pereira, A. Gimeìnez-Morera, A. Novara, S. Keesstra, A. Jordán, R. E. Masto, E. Brevik, C. Azorin-Molina, and A. Cerdà Road and railway infrastructure increased in the Mediterranean region during the last three decades. This included the building of embankments, which are assumed to be a~large source of sediments and runoff. However, little is known about soil erosion rates, the factors that control them, and the processes that contribute to detachment, transport and deposition of sediments from road and railway embankments. The objective of this study was therefore to assess the impacts of road and railway embankments as a source of sediment and water, and compare them to other land use types (citrus plantations and shrublands) representative of the Cànyoles watershed to evaluate the importance of road embankments as a~source of water and sediment under high magnitude low frequency rainfall events. Sixty rainfall experiments (1 m 2 plots; 60 min duration; 78 mm h −1 rainfall intensity) were carried out on these land use types: 20 on two railway embankments (10 + 10), 20 on two road embankments (10 + 10), and 10 on citrus and 10 on shrubland. Road and railway embankments were characterized by bare soils with low organic matter and high bulk density. Erosion processes were more active in road, railway and citrus plots, and null in the shrublands. The non-sustainable soil erosion rates of 3 Mg ha −1 y −1 measured on the road embankments were due to the efficient runoff connectivity plus low infiltration rates within the plot as the runoff took less than one minute to reach the runoff outlet. Road and railway embankments are both an active source of sediments and runoff, and soil erosion control strategies must be applied. The citrus plantations also act as a~source of water and sediments (1.5 Mg ha −1 y −1 ), while shrublands are sediment sinks, as no overland flow was observed due to the high infiltration rates.

Description: Controls on hydrologic drought duration in near-natural streamflow in Europe and the USA Hydrology and Earth System Sciences Discussions, 12, 12877-12910, 2015 Author(s): E. Tijdeman, S. Bachmair, and K. Stahl Climate classification systems, such as Köppen–Geiger and the aridity index, are often used in large-scale drought modeling studies and in drought monitoring and early warning systems to stratify regions with similar hydro-climatic drought properties. What is currently lacking is a large-scale evaluation of the relation between climate and hydrologic drought characteristics. In this study we explored how suitable common climate classifications are for differentiating river basins according to their characteristic hydrologic drought duration and whether drought durations within the same climate classes are comparable between different regions. This study uses a dataset of 808 near-natural streamflow records from Europe and the USA to answer these questions. First, we grouped drought duration distributions of each record over different classes of climate classification systems and individual climate and catchment controls. Then, we compared these drought duration distributions of all classes within each climate classification system or classification based on individual controls. Results showed that climate classification systems that include absolute precipitation in their classification scheme (e.g., the aridity index) are most suitable to differentiate basins according to drought duration within both the USA and Europe. However, differences in duration distributions were found for the same climate classes in Europe and the USA. These differences are likely caused by differences in precipitation, in catchment controls as expressed by the base flow index and in differences in climate beyond the total water balance (e.g., seasonality in precipitation), which have shown to exert a control on drought duration as well. Climate classification systems that include an absolute precipitation control can be tailored into drought monitoring and early warning systems for Europe and the USA to define regions with different sensitivities to hydrologic droughts, which, for example, have been found to be higher in basins with a low aridity index. However, stratification of basins according to these climate classification systems is likely to be complemented with information of other climate classification systems (Köppen–Geiger) and individual controls (precipitation and the base flow index), especially in a comparative study between Europe and the USA.

Description: Correction of real-time satellite precipitation with satellite soil moisture observations Hydrology and Earth System Sciences Discussions, 12, 5749-5787, 2015 Author(s): W. Zhan, M. Pan, N. Wanders, and E. F. Wood Rainfall and soil moisture are two key elements in modeling the interactions between the land surface and the atmosphere. Accurate and high-resolution real-time precipitation is crucial for monitoring and predicting the on-set of floods, and allows for alert and warning before the impact becomes a disaster. Assimilation of remote sensing data into a flood-forecasting model has the potential to improve monitoring accuracy. Space-borne microwave observations are especially interesting because of their sensitivity to surface soil moisture and its change. In this study, we assimilate satellite soil moisture retrievals using the Variable Infiltration Capacity (VIC) land surface model, and a dynamic assimilation technique, a particle filter, to adjust the Tropical Rainfall Measuring Mission Multi-satellite Precipitation Analysis (TMPA) real-time precipitation estimates. We compare updated precipitation with real-time precipitation before and after adjustment and with NLDAS gauge-radar observations. Results show that satellite soil moisture retrievals provide additional information by correcting errors in rainfall bias. High accuracy soil moisture retrievals, when merged with precipitation, generally increase both rainfall frequency and intensity, and are most effective in the correction of rainfall under dry to normal surface condition while limited/negative improvement is seen over wet/saturated surfaces. Errors from soil moisture, mixed among the real signal, may generate a false rainfall signal approximately 2 mm day −1 and thus lower the precipitation accuracy after adjustment.

Description: The effect of empirical-statistical correction of intensity-dependent model errors on the climate change signal Hydrology and Earth System Sciences Discussions, 12, 5671-5701, 2015 Author(s): A. Gobiet, M. Suklitsch, and G. Heinrich This study discusses the effect of empirical-statistical bias correction methods like quantile mapping (QM) on the change signals of climate simulations. We show that QM regionally alters the mean temperature climate change signal (CCS) derived from the ENSEMBLES multi-model dataset by up to 15%. Such modification is currently strongly discussed and is often regarded as deficiency of bias correction methods. However, an analytical analysis reveals that this modification corresponds to the effect of intensity-dependent model errors on the CCS. Such errors cause, if uncorrected, biases in the CCS. QM removes these intensity-dependent errors and can therefore potentially lead to an improved CCS. A similar analysis as for the multi-model mean CCS has been conducted for the variance of CCSs in the multi-model ensemble. It shows that this indicator for model uncertainty is artificially inflated by intensity-dependent model errors. Therefore, QM has also the potential to serve as an empirical constraint on model uncertainty in climate projections. However, any improvement of simulated CCSs by empirical-statistical bias correction methods can only be realized, if the model error characteristics are sufficiently time-invariant.

Description: Effective damage zone volume of fault zones and initial salinity distribution determine intensity of shallow aquifer salinization in geological underground utilization Hydrology and Earth System Sciences Discussions, 12, 5703-5748, 2015 Author(s): M. Langer, E. Tillner, T. Kempka, and M. Kühn Injection of fluids into deep saline aquifers causes a pore pressure increase in the storage formation, and thus displacement of resident brines. Via hydraulically conductive faults, brine may migrate upwards into shallower aquifers, and lead to unwanted salinization of potable groundwater resources. In the present study, we investigated different scenarios for a prospective storage site close to the city of Beeskow in the Northeast German Basin by using a 3-D regional scale model (100 km × 100 km × 1.34 km) that includes four ambient fault zones. The focus was on assessing the impact of fault length and the effect of an overlying secondary reservoir as well as model boundary conditions on the potential salinization of shallow groundwater resources. We employed numerical simulations of brine injection as a representative fluid using the simulator TOUGH2-MP. Our simulation results demonstrate that pressure build-up within the reservoir determines the intensity and duration of fluid flow through the faults, and hence salinization of shallower aquifers. Application of different boundary conditions proved that these have a crucial impact on reservoir fluid displacement. If reservoir boundaries are closed, the fluid migrated upwards into the shallow aquifer, corresponds to the overall injected fluid mass. In that case, a short hydraulically conductive fault length and the presence of an overlying secondary reservoir leads only to retardation in brine displacement up to a factor of five and three, respectively. If the reservoir boundaries are open, salinization is considerably reduced: in the presence of a secondary reservoir, 33% of equivalent brine mass migrates into the shallow aquifer, if all four faults are hydraulically open over their entire length, whereas the displaced equivalent brine mass is only 12% for a single fault of two kilometres length. Taking into account the considered geological boundary conditions, the brine originates in maximum from the upper 4 to 298 m of the investigated faults. Hence, the initial salt–freshwater interface present in the fault is of high relevance for the resulting shallow aquifer salinization. The present study demonstrates that the existence of hydraulically conductive faults is not necessarily an exclusion criterion for potential injection sites, because salinization of shallower aquifers strongly depends on initial salinity distribution, location of hydraulically conductive faults and their length as well as geological boundary conditions. These constraints are location specific, and need to be explored thoroughly in advance of any field activity. They provide the basis for scenario analyses and a reliable risk assessment.

Description: Estimation of flood warning runoff thresholds in ungauged basins with asymmetric error functions Hydrology and Earth System Sciences Discussions, 12, 6011-6041, 2015 Author(s): E. Toth In many real-world flood forecasting systems, the runoff thresholds for activating warnings or mitigation measures correspond to the flow peaks with a given return period (often the 2-year one, that may be associated with the bankfull discharge). At locations where the historical streamflow records are absent or very limited, the threshold can be estimated with regionally-derived empirical relationships between catchment descriptors and the desired flood quantile. Whatever is the function form, such models are generally parameterised by minimising the mean square error, that assigns equal importance to overprediction or underprediction errors. Considering that the consequences of an overestimated warning threshold (leading to the risk of missing alarms) generally have a much lower level of acceptance than those of an underestimated threshold (leading to the issuance of false alarms), the present work proposes to parameterise the regression model through an asymmetric error function, that penalises more the overpredictions. The estimates by models (feedforward neural networks) with increasing degree of asymmetry are compared with those of a traditional, symmetrically-trained network, in a rigorous cross-validation experiment referred to a database of catchments covering the Italian country. The analysis shows that the use of the asymmetric error function can substantially reduce the number and extent of overestimation errors, if compared to the use of the traditional square errors. Of course such reduction is at the expense of increasing underestimation errors, but the overall accurateness is still acceptable and the results illustrate the potential value of choosing an asymmetric error function when the consequences of missed alarms are more severe than those of false alarms.

Description: The cost of ending groundwater overdraft on the North China Plain Hydrology and Earth System Sciences Discussions, 12, 5931-5966, 2015 Author(s): C. Davidsen, S. Liu, X. Mo, D. Rosbjerg, and P. Bauer-Gottwein Over-exploitation of groundwater reserves is a major environmental problem around the world. In many river basins, groundwater and surface water are used conjunctively and joint optimization strategies are required. A hydroeconomic modelling approach is used to find cost-optimal sustainable surface water and groundwater allocation strategies for a river basin, given an arbitrary initial groundwater level in the aquifer. A simplified management problem with conjunctive use of scarce surface water and groundwater under inflow and recharge uncertainty is presented. Because of head-dependent groundwater pumping costs the optimization problem is non-linear and non-convex, and a genetic algorithm is used to solve the 1-step-ahead sub-problems with the objective of minimizing the sum of immediate and expected future costs. A real-world application in the Ziya River Basin in northern China is used to demonstrate the model capabilities. Persistent overdraft from the groundwater aquifers on the North China Plain has caused declining groundwater tables, salinization and infiltration of \hack{\break} wastewater. The model maps the opportunity cost of water in different scenarios, and the cost of ending groundwater overdraft in the basin is estimated to be 5.47 billion CNY yr −1 . The model can be used to guide decision makers to ensure long-term sustainability of groundwater and surface water resources management in the basin in an economically optimal way.

Description: Influence of environmental factors on spectral characteristic of chromophoric dissolved organic matter (CDOM) in Inner Mongolia Plateau, China Hydrology and Earth System Sciences Discussions, 12, 5895-5929, 2015 Author(s): Z. D. Wen, K. S. Song, Y. Zhao, J. Du, and J. H. Ma Spectral characteristics of chromophoric dissolved organic matter (CDOM) were examined in conjunction with environmental factors in the waters of 22 rivers and 26 terminal waters in Hulun Buir plateau, northeast China. Dissolved organic carbon (DOC), total nitrogen (TN), and total phosphorous (TP) were significantly higher in terminal waters than rivers waters ( p 〈 0.01). Principal component analysis (PCA) indicated that non-water light absorption and anthropogenic nutrient disturbances might be the causes of the diversity of water quality parameters in Hulun Buir plateau. CDOM absorption in river waters was significantly lower than terminal waters ( p 〈 0.01). Analysis of ratio of absorption at 250–365 nm ( E 250 : 365 ), specific UV absorbance (SUVA 254 ), and spectral slope ratio ( S r ) indicated that CDOM in river waters had higher aromaticity, molecular weight, and vascular plant contribution than in terminal waters. Furthermore, results showed that DOC concentration, CDOM light absorption, and the proportion of autochthonous sources of CDOM in plateau waters were all higher than in other freshwater rivers reported in the literature. The strong evapoconcentration, intense ultraviolet irradiance and landscape features of Hulun Buir plateau may be responsible for the above phenomenon. Redundancy analysis (RDA) indicated that the environmental variables TSM, TN, and EC had a strong correlation with light absorption characteristics, followed by TDS and chlorophyll a . In most sampling locations, CDOM was the dominant non-water light-absorbing substance. Light absorption by non-algal particles often exceeded that by phytoplankton in the plateau waters. Study of these optical-physicochemical correlations is helpful in the evaluation of the potential influence of water quality factors on non-water light absorption in cold plateau water environments. And the study on organic carbon in plateau lakes had a vital contribution to global carbon balance estimation.

Description: Evaluating the utility of satellite soil moisture retrievals over irrigated areas and the ability of land data assimilation methods to correct for unmodeled processes Hydrology and Earth System Sciences Discussions, 12, 5967-6009, 2015 Author(s): S. V. Kumar, C. D. Peters-Lidard, J. A. Santanello, R. H. Reichle, C. S. Draper, R. D. Koster, G. Nearing, and M. F. Jasinski The Earth's land surface is characterized by tremendous natural heterogeneity and human engineered modifications, both of which are challenging to represent in land surface models. Satellite remote sensing is often the most practical and effective method to observe the land surface over large geographical areas. Agricultural irrigation is an important human induced modifications to natural land surface processes, as it is pervasive across the world and because of its significant influence on the regional and global water budgets. In this article, irrigation is used as an example of a human engineered, unmodeled land surface process, and the utility of satellite soil moisture retrievals over irrigated areas in the continental US is examined. Such retrievals are based on passive or active microwave observations from the Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR-E), the Advanced Microwave Scanning Radiometer 2 (AMSR2), the Soil Moisture Ocean Salinity (SMOS) mission, WindSat and the Advanced Scatterometer (ASCAT). The analysis suggests that the skill of these retrievals for representing irrigation artifacts is mixed, with ASCAT based products somewhat more skillful than SMOS and AMSR2 products. The article then examines the suitability of typical bias correction strategies in current land data assimilation systems when unmodeled processes dominate the bias between the model and the observations. Using a suite of synthetic experiments that includes bias correction strategies such as quantile mapping and trained forward modeling, it is demonstrated that the bias correction practices lead to the exclusion of the signals from unmodeled processes, if these processes are the major source of the biases. It is further shown that new methods are needed to preserve the observational information about unmodeled processes during data assimilation.

Description: Initial assessment of a multi-model approach to spring flood forecasting in Sweden Hydrology and Earth System Sciences Discussions, 12, 6077-6113, 2015 Author(s): J. Olsson, C. B. Uvo, K. Foster, and W. Yang Hydropower is a major energy source in Sweden and proper reservoir management prior to the spring flood onset is crucial for optimal production. This requires useful forecasts of the accumulated discharge in the spring flood period (i.e. the spring-flood volume, SFV). Today's SFV forecasts are generated using a model-based climatological ensemble approach, where time series of precipitation and temperature from historical years are used to force a calibrated and initialised set-up of the HBV model. In this study, a number of new approaches to spring flood forecasting, that reflect the latest developments with respect to analysis and modelling on seasonal time scales, are presented and evaluated. Three main approaches, represented by specific methods, are evaluated in SFV hindcasts for three main Swedish rivers over a 10-year period with lead times between 0 and 4 months. In the first approach, historically analogue years with respect to the climate in the period preceding the spring flood are identified and used to compose a reduced ensemble. In the second, seasonal meteorological ensemble forecasts are used to drive the HBV model over the spring flood period. In the third approach, statistical relationships between SFV and the large-sale atmospheric circulation are used to build forecast models. None of the new approaches consistently outperform the climatological ensemble approach, but for specific locations and lead times improvements of 20–30 % are found. When combining all forecasts in a weighted multi-model approach, a mean improvement over all locations and lead times of nearly 10 % was indicated. This demonstrates the potential of the approach and further development and optimisation into an operational system is ongoing.

Description: Satellite signal shows storage-unloading subsidence in North China Hydrology and Earth System Sciences Discussions, 12, 6043-6075, 2015 Author(s): J. P. Moiwo and F. Tao Worsening water storage depletion (WSD) contributes to environmental degradation, land subsidence and earthquake and could disrupt food production/security and social stability. There is need for efficient water use strategies in North China, a pivotal agrarian, industrial and political base in China with a widespread WSD. This study integrates satellite, model and field data products to investigate WSD and land subsidence in North China. In the first step, GRACE (Gravity Recovery and Climate Experiment) mass rates are used to show WSD in the region. Next, GRACE total water storage (TWS) is corrected for soil water storage (SWS) to derive groundwater storage (GWS) using GLDAS (Global Land Data Assimilation System) data products. The derived GWS is compared with GWS obtained from field-measured groundwater level to show land subsidence in the study area. Then GPS (Global Positioning System) data of relative land surface change (LSC) are used to confirm the subsidence due to WSD. A total of ~ 96 near-consecutive months (January 2002 through December 2009) of datasets are used in the study. Based on GRACE mass rates, TWS depletion is 23.76 ± 1.74 mm yr −1 or 13.73 ± 1.01 km 3 yr −1 in the 578 000 km 2 study area. This is ~ 31 % of the slated 45 km 3 yr −1 water delivery in 2050 via the South–North Water Diversion Project. Analysis of relative LSC shows subsidence of 7.29 ± 0.35 mm yr −1 in Beijing and 2.74 ± 0.16 mm yr −1 in North China. About 11.53 % (2.74 ± 0.18 mm or 1.58 ± 0.12 km 3 ) of the TWS and 8.37 % (1.52 ± 0.70 mm or 0.88 ± 0.03 km 3 ) of the GWS are attributed to storage reductions accompanying subsidence in the region. Although interpretations of the findings require caution due to the short temporal and large spatial coverage, the concurrence of WSD and land subsidence could have adverse implications for the study area. It is critical that the relevant stakeholders embark on resource-efficient measures to ensure water availability, food security, ecological sustainability and social stability in this pivotal region.

Description: In-situ unsaturated zone stable water isotope ( 2 H and 18 O) measurements in semi-arid environments using tunable off-axis integrated cavity output spectroscopy Hydrology and Earth System Sciences Discussions, 12, 6115-6149, 2015 Author(s): M. Gaj, M. Beyer, P. Koeniger, H. Wanke, J. Hamutoko, and T. Himmelsbach Stable isotopes (deuterium, 2 H, and oxygen-18, 18 O) of soil pore water were measured directly in the field using tunable off-axis integrated cavity output spectroscopy (OA-ICOS) and commercially available soil gas probes in a semi-arid region of the Cuvelai-Etosha-Basin, Namibia. High spatial and temporal resolution was achieved in the study area with reasonable accuracy and measurements were in agreement with laboratory-based cryogenic vacuum extraction and subsequent cavity ring down laser spectroscopic isotope analysis (CRDS). After drift correction of the isotope data, mean precision for over 140 measurements of two consecutive field campaigns in June and November 2014 were 1.8 and 0.46 ‰ for δ 2 H and 18 O, respectively. Mean Accuracy using quality check standards was 5 and 0.3 ‰ for δ 2 H and δ 18 O, respectively. Results support the applicability of an in-situ measurement system for the determination of stable isotopes in soil pore water. Spatio-temporal variability could be deduced with the observed data in an extremely dry evaporation dominated environment which was sporadically affected by intermittent rainfall.

Description: Diagnosing hydrological limitations of a Land Surface Model: application of JULES to a deep-groundwater chalk basin Hydrology and Earth System Sciences Discussions, 12, 7541-7582, 2015 Author(s): N. Le Vine, A. Butler, N. McIntyre, and C. Jackson Land Surface Models (LSMs) are prospective starting points to develop a global hyper-resolution model of the terrestrial water, energy and biogeochemical cycles. However, there are some fundamental limitations of LSMs related to how meaningfully hydrological fluxes and stores are represented. A diagnostic approach to model evaluation is taken here that exploits hydrological expert knowledge to detect LSM inadequacies through consideration of the major behavioural functions of a hydrological system: overall water balance, vertical water redistribution in the unsaturated zone, temporal water redistribution and spatial water redistribution over the catchment's groundwater and surface water systems. Three types of information are utilised to improve the model's hydrology: (a) observations, (b) information about expected response from regionalised data, and (c) information from an independent physics-based model. The study considers the JULES (Joint UK Land Environmental Simulator) LSM applied to a deep-groundwater chalk catchment in the UK. The diagnosed hydrological limitations and the proposed ways to address them are indicative of the challenges faced while transitioning to a global high resolution model of the water cycle.

Description: High-frequency monitoring reveals nutrient sources and transport processes in an agriculture-dominated lowland water system Hydrology and Earth System Sciences Discussions, 12, 8337-8380, 2015 Author(s): B. van der Grift, H. P. Broers, W. L. Berendrecht, J. C. Rozemeijer, L. A. Osté, and J. Griffioen Many agriculture-dominated lowland water systems worldwide suffer from eutrophication caused by high nutrient loads. Insight in the hydrochemical functioning of embanked polder catchments is highly relevant for improving the water quality in such areas. This paper introduces new insights in nutrient sources and transport processes in a low elevated polder in the Netherlands using high-frequency monitoring technology at the outlet, where the water is pumped into a higher situated lake, combined with a low-frequency water quality monitoring program at six locations within the drainage area. Seasonal trends and short scale temporal dynamics in concentrations indicated that the NO 3 concentration at the pumping station originated from N-loss from agricultural lands. The NO 3 loads appear as losses with drain water discharge after intensive rainfall events during the winter months due to preferential flow through the cracked clay soil. Transfer function-noise modelling of hourly NO 3 concentrations reveals that a large part of the dynamics in NO 3 concentrations during the winter months can be related to rainfall. The total phosphorus (TP) concentration almost doubled during operation of the pumping station which points to resuspension of particulate P from channel bed sediments induced by changes in water flow due to pumping. Rainfall events that caused peaks in NO 3 concentrations did not results in TP concentration peaks. The by rainfall induced and NO 3 enriched quick interflow, may also be enriched in TP but this is then buffered in the water system due to sedimentation of particulate P. Increased TP concentrations associated with run-off events is only observed during a rainfall event at the end of a freeze–thaw cycle. All these observations suggest that the P retention potential of polder water systems is highly due to the artificial pumping regime that buffers high flows. As the TP concentration is affected by operation of the pumping station, timing of sampling relative to the operating hours of the pumping station should be accounted for when calculating P export loads, determining trends in water quality or when judging water quality status of polder water systems.

Description: The Normalized Difference Infrared Index (NDII) as a proxy for soil moisture storage in hydrological modelling Hydrology and Earth System Sciences Discussions, 12, 8419-8457, 2015 Author(s): N. Sriwongsitanon, H. Gao, H. H. G. Savenije, E. Maekan, S. Saengsawang, and S. Thianpopirug With remote sensing we can readily observe the Earth's surface, but looking under the surface into the root zone of vegetation is still a major challenge. Yet knowledge on the dynamics of soil moisture in the root zone is essential for agriculture, land–atmosphere interaction and hydrological modelling, alike. In this paper we develop a novel approach to monitor the soil moisture storage deficit in the root zone of vegetation, by using the remotely sensed Normalised Difference Infrared Index (NDII) in the Upper Ping River Basin (UPRB) in northern Thailand. Satellite data from the Moderate Resolution Imaging Spectro-radiometer (MODIS) was used to evaluate the NDII over an 8 day period, covering the study area from 2001 to 2013. The results show that NDII values decrease sharply at the end of the wet season in October and reach lowest values near the end of the dry season in March. The values then increase abruptly after rains have started, but vary in an insignificant manner from the middle to the late rainy season. The NDII proves to be a very strong proxy for moisture storage deficit in the root zone, which is a crucial component of hydrological models. In addition, the NDII appears to be a reliable indicator for the temporal and spatial distribution of drought conditions in the UPRB. The 8 day average NDII values were found to correlate very well with the 8 day average soil moisture content ( S U ) simulated by FLEX L (rainfall–runoff model) at 8 runoff stations during the dry season – giving an average R 2 value 0.87 on an exponential relationship, while for the wet season it reduced to be around 0.61. Apparently, the NDII is an effective index for the moisture storage in the root zone during the time of moisture deficit, and a powerful indicator to assess droughts. In the dry season, when plants are exposed to water stress, the leaf-water deficit increases steadily. Once leaf-water is close to saturation – mostly at the end of the wet season – leaf characteristics and NDII values do not vary significantly, causing lower correlation between NDII and S u in the wet season. However, the correlations between NDII and S u still remain high for both seasons and therefore the product can be used to define drought situations throughout the year and be of use to water management.

Description: Investigating effects of different evapotranspiration (ET) schemes on soil water dynamics and ET partitioning: a large lysimeter case of summer maize in a semi-arid environment northwest of China Hydrology and Earth System Sciences Discussions, 12, 9977-10022, 2015 Author(s): L. Yu, Y. Zeng, Z. Su, H. Cai, and Z. Zheng Different evapotranspiration (ET) schemes can affect significantly the performance of land surface models in capturing the soil water dynamics and ET partitioning over various land cover and climates, the accurate understanding of which is crucial to determine the effective irrigation. In this study, a land model considering the coupled transfer of water, vapor and heat in the soil, with two alternative ET schemes, was used to investigate how the coupled mechanism can affect the soil water dynamics in a crop field and how the ET partitioning was influenced. There are two different evapotranspiration (ET) schemes, one is based on reference crop evapotranspiration (ET 0 ) and use LAI to partition into soil evaporation and transpiration, denoted as the ET ind scheme; the other is one-step calculation of actual soil evaporation and potential transpiration by incorporating canopy minimum resistance and actual soil resistance into Penman–Monteith model, denoted as the ET dir scheme. Results indicated that the coupled model with the two different ET schemes differed in simulating soil water content and crop evapotranspiration components while agreed well for the simulation of soil temperature. Considering the aerodynamic and surface resistance terms made the ET dir scheme better in simulating soil evaporation especially after irrigations. Furthermore, the results of different crop growth scenarios indicated that the uncertainty in LAI played an important role in estimating the relative transpiration and evaporation fraction. The soil drying seemed to intensify the disturbance of maximum rooting depth and root growth rate in calculating ET components. The former was more important at the late growing season while the latter dominated at the early growing season.

Description: Quantifying the nutrient flux within a lowland karstic catchment Hydrology and Earth System Sciences Discussions, 12, 10221-10260, 2015 Author(s): T. McCormack, O. Naughton, P. M. Johnston, and L. W. Gill Nutrient contamination of surface and groundwaters is an issue of growing importance as the risks associated with agricultural runoff escalate due to increasing demands on global food production. In this study, the nutrient flux occurring within the surface and groundwaters of a lowland karst catchment in western Ireland was investigated with the aid of alkalinity sampling and a hydrological model. Water samples were tested from a variety of rivers, lakes (or turloughs), boreholes and springs at monthly intervals over three years. Alkalinity sampling was used to elucidate the contrasting hydrological functioning between different turloughs. Such disparate hydrological functioning was further investigated with the aid of a hydrological model which allowed for an estimate of allogenic and autogenic derived nutrient loading into the karst system. The model also allowed for an investigation of mixing within the turloughs, comparing observed behaviours with the hypothetical conservative behaviour allowed for by the model. Within the turloughs, nutrient concentrations were found to reduce over the flooded period, even though the turloughs hydrological functioning (and the hydrological model) suggested this should not occur. As such, it was determined that nutrient loss processes were occurring within the system. Denitrification during stable flooded periods (typically 3–4 months per year) was deemed to be the main process reducing nitrogen concentrations within the turloughs whereas phosphorus loss is thought to occur mostly via sedimentation and subsequent soil deposition. The results from this study suggest that, in stable conditions, ephemeral lakes can impart considerable nutrient losses on a karst groundwater system.

Description: Evaluation of five hydrological models across Europe and their suitability for making projections under climate change Hydrology and Earth System Sciences Discussions, 12, 10289-10330, 2015 Author(s): W. Greuell, J. C. M. Andersson, C. Donnelly, L. Feyen, D. Gerten, F. Ludwig, G. Pisacane, P. Roudier, and S. Schaphoff The main aims of this paper are the evaluation of five large-scale hydrological models across Europe and the assessment of the suitability of the models for making projections under climate change. For the evaluation, 22 years of discharge measurements from 46 large catchments were exploited. In the reference simulations forcing was taken from the E-OBS dataset for precipitation and temperature, and from the WFDEI dataset for other variables. On average across all catchments, biases were small for four of the models, ranging between −29 and +23 mm yr −1 (−9 and +8 %), while one model produced a large negative bias (−117 mm yr −1 ; −38 %). Despite large differences in e.g. the evapotranspiration schemes, the skill to simulate interannual variability did not differ much between the models, which can be ascribed to the dominant effect of interannual variation in precipitation on interannual variation in discharge. Assuming that the skill of a model to simulate interannual variability provides a measure for the model's ability to make projections under climate change, the skill of future discharge projections will not differ much between models. The quality of the simulation of the mean annual cycles, and low and high discharge was found to be related to the degree of calibration of the models, with the more calibrated models outperforming the crudely and non-calibrated models. The sensitivity to forcing was investigated by carrying out alternative simulations with all forcing variables from WFDEI, which increased biases by between +66 and +85 mm yr −1 (21–28 %), significantly changed the inter-model ranking of the skill to simulate the mean and increased the magnitude of interannual variability by 28 %, on average.

Description: Comment on "Using groundwater age and hydrochemistry to understand sources and dynamics of nutrient contamination through the catchment into Lake Rotorua, New Zealand" by Morgenstern et al. (2015) Hydrology and Earth System Sciences Discussions, 12, 10379-10388, 2015 Author(s): J. M. Abell, D. P. Hamilton, and C. G. McBride This Comment addresses a key conclusion in the paper entitled "Using groundwater age and hydrochemistry to understand sources and dynamics of nutrient contamination through the catchment into Lake Rotorua, New Zealand" by Morgenstern et al. (2015). The authors analyse hydrochemistry data and conclude that "the only effective way to limit algae blooms and improve lake water quality in such environments is by limiting the nitrate load". We undertook the crucial task of examining this conclusion because it contradicts the current strategy of limiting both phosphorus and nitrogen loads to the lake, supported by a multi-million dollar programme of action. Following careful consideration, we believe that the conclusion is invalid and outline four reasons to support our assessment. Our comments do not relate to the methodology or results that are presented by Morgenstern et al. (2015), and we recognise that their paper makes an otherwise highly valuable contribution to understanding hydro-chemical processes in the catchment.

Description: Assessment of small-scale variability of rainfall and multisatellite precipitation estimates using a meso-rain gauge network measurements from southern peninsular India Hydrology and Earth System Sciences Discussions, 12, 10389-10429, 2015 Author(s): K. Sunilkumar, T. Narayana Rao, and S. Satheeshkumar This paper describes the establishment of a dense rain gauge network and small-scale variability in rain storms (both in space and time) over a complex hilly terrain in southeast peninsular India. Three years of high-resolution gauge measurements are used to evaluate 3 hourly rainfall and sub-daily variations of four widely used multisatellite precipitation estimates (MPEs). The network consists of 36 rain gauges arranged in a near-square grid area of 50 km × 50 km with an intergauge distance of ~ 10 km. Morphological features of rainfall in two principal monsoon seasons (southwest monsoon: SWM and northeast monsoon: NEM) show marked seasonal differences. The NEM rainfall exhibits significant spatial variability and most of the rainfall is associated with large-scale systems (in wet spells), whereas the contribution from small-scale systems is considerable in SWM. Rain storms with longer duration and copious rainfall are seen mostly in the western quadrants in SWM and northern quadrants in NEM, indicating complex spatial variability within the study region. The diurnal cycle also exhibits marked spatiotemporal variability with strong diurnal cycle at all the stations (except for 1) during the SWM and insignificant diurnal cycle at many stations during the NEM. On average, the diurnal amplitudes are a factor 2 larger in SWM than in NEM. The 24 h harmonic explains about 70 % of total variance in SWM and only ~ 30 % in NEM. The late night-mid night peak (20:00–02:00 LT) observed during the SWM is attributed to the propagating systems from the west coast during active monsoon spells. Correlograms with different temporal integrations of rainfall data (1, 3, 12, 24 h) show an increase in the spatial correlation with temporal integration, but the correlation remains nearly the same after 12 h of integration in both the monsoons. The 1 h resolution data shows the steepest reduction in correlation with intergauge distance and the correlation becomes insignificant after ~30 km in both monsoons. Evaluation of high-resolution rainfall estimates from various MPEs against the gauge rainfall indicates that all MPEs underestimate the weak and heavy rain. The MPEs exhibit good detection skills of rain at both 3 and 24 h resolutions, however, considerable improvement is observed at 24 h resolution. Among different MPEs, Climate Prediction Centre morphing technique (CMORPH) performs better at 3 hourly resolution in both monsoons. The performance of TRMM multisatellite precipitation analysis (TMPA) is much better at daily resolution than at 3 hourly, as evidenced by better statistical metrics than the other MPEs. All MPEs captured the basic shape of diurnal cycle and the amplitude quite well, but failed to reproduce the weak/insignificant diurnal cycle in NEM.

Description: Recent changes in climate, hydrology and sediment load in the Wadi Abd, Algeria (1970–2010) Hydrology and Earth System Sciences Discussions, 12, 10457-10513, 2015 Author(s): M. Achite and S. Ouillon Here we investigate the changes of temperature, precipitation, river runoff and sediment transport in the Wadi Abd in NW Algeria over a time series of 40 hydrological years (1970–2010). Temperature increased and precipitation decreased with the reduction in rainfall being relatively higher during the rainy season. A shift towards an earlier onset of first rains during summer was also found with cascading effects on hydrology (hydrological regimes, vegetation etc) and thus on erosion and sediment yield. During the 1980s, the flow regime shifted from perennial to intermittent with an amplification of the variations of discharge and a modification of the sediment regime with higher and more irregular suspended particulate flux. Sediment flux was shown to almost double every decade from 1970s to 2000s. The sediment regime shifted from two equivalent seasons of sediment delivery (spring and autumn) to a single major season regime. In 2000s, autumn produced over 4 times more sediment than spring. The enhanced scatter denotes an increase of hysteresis phenomena in the Wadi Abd that is probably related to the change in the hydrologic regime. The increased erosion of the watershed is accompanied by a decrease in the coefficient b of its rating curves and a decrease in the erosive power of the river. At the end of the period, due to the irregularity of the discharge, the ability of a rating curve to derive suspended sediment concentration from river discharge was poor.

Description: Improved large-scale hydrological modelling through the assimilation of streamflow and downscaled satellite soil moisture observations Hydrology and Earth System Sciences Discussions, 12, 10559-10601, 2015 Author(s): P. Lopez Lopez, N. Wanders, J. Schellekens, L. J. Renzullo, E. H. Sutanudjaja, and M. F. P. Bierkens The coarse spatial resolution of global hydrological models (typically 〉 0.25°) limits their ability to resolve key water balance processes for many river basins and thus compromises their suitability for water resources management, especially when compared to locally-tuned river models. A possible solution to the problem may be to drive the coarse resolution models with locally available high spatial resolution meteorological data as well as to assimilate ground-based and remotely-sensed observations of key water cycle variables. While this would improve the resolution of the global model, the impact of prediction accuracy remains largely an open question. In this study we investigate the impact of assimilating streamflow and satellite soil moisture observations on the accuracy of global hydrological model estimations, when driven by either coarse- or high-resolution meteorological observations in the Murrumbidgee river basin in Australia. To this end, a 0.08° resolution version of the PCR-GLOBWB global hydrological model is forced with downscaled global meteorological data (from 0.5° downscaled to 0.08° resolution) obtained from the WATCH Forcing Data methodology applied to ERA-Interim (WFDEI) and a local high resolution gauging station based gridded dataset (0.05°). Downscaled satellite derived soil moisture (from approx. 0.5° downscaled to 0.08° resolution) from AMSR-E and streamflow observations collected from 23 gauging stations are assimilated using an ensemble Kalman filter. Several scenarios are analysed to explore the added value of data assimilation considering both local and global meteorological data. Results show that the assimilation of soil moisture observations results in the largest improvement of the model estimates of streamflow. The joint assimilation of both streamflow and downscaled soil moisture observations leads to further improvement in streamflow simulations (20 % reduction in RMSE). Furthermore, results show that the added contribution of data assimilation, for both soil moisture and streamflow, is more pronounced when the global meteorological data are used to force the models. This is caused by the higher uncertainty and coarser resolution of the global forcing. We conclude that it is possible to improve PCR-GLOBWB simulations forced by coarse resolution meteorological data with assimilation of downscaled spaceborne soil moisture and streamflow observations. These improved model results are close to the ones from a local model forced with local meteorological data. These findings are important in light of the efforts that are currently done to go to global hyper-resolution modelling and can help to advance this research.

Description: Improving flood forecasting capability of physically based distributed hydrological model by parameter optimization Hydrology and Earth System Sciences Discussions, 12, 10603-10649, 2015 Author(s): Y. Chen, J. Li, and H. Xu Physically based distributed hydrological models discrete the terrain of the whole catchment into a number of grid cells at fine resolution, and assimilate different terrain data and precipitation to different cells, and are regarded to have the potential to improve the catchment hydrological processes simulation and prediction capability. In the early stage, physically based distributed hydrological models are assumed to derive model parameters from the terrain properties directly, so there is no need to calibrate model parameters, but unfortunately, the uncertanties associated with this model parameter deriving is very high, which impacted their application in flood forecasting, so parameter optimization may also be necessary. There are two main purposes for this study, the first is to propose a parameter optimization method for physically based distributed hydrological models in catchment flood forecasting by using PSO algorithm and to test its competence and to improve its performances, the second is to explore the possibility of improving physically based distributed hydrological models capability in cathcment flood forecasting by parameter optimization. In this paper, based on the scalar concept, a general framework for parameter optimization of the PBDHMs for catchment flood forecasting is first proposed that could be used for all PBDHMs. Then, with Liuxihe model as the study model, which is a physically based distributed hydrological model proposed for catchment flood forecasting, the improverd Particle Swarm Optimization (PSO) algorithm is developed for the parameter optimization of Liuxihe model in catchment flood forecasting, the improvements include to adopt the linear decreasing inertia weight strategy to change the inertia weight, and the arccosine function strategy to adjust the acceleration coefficients. This method has been tested in two catchments in southern China with different sizes, and the results show that the improved PSO algorithm could be used for Liuxihe model parameter optimization effectively, and could improve the model capability largely in catchment flood forecasting, thus proven that parameter optimization is necessary to improve the flood forecasting capability of physically based distributed hydrological model. It also has been found that the appropriate particle number and the maximum evolution number of PSO algorithm used for Liuxihe model catchment flood forcasting is 20 and 30, respectively.

Description: The WACMOS-ET project – Part 1: Tower-scale evaluation of four remote sensing-based evapotranspiration algorithms Hydrology and Earth System Sciences Discussions, 12, 10739-10787, 2015 Author(s): D. Michel, C. Jiménez, D. G. Miralles, M. Jung, M. Hirschi, A. Ershadi, B. Martens, M. F. McCabe, J. B. Fisher, Q. Mu, S. I. Seneviratne, E. F. Wood, and D. Fernández-Prieto The WACMOS-ET project has compiled a forcing data set covering the period 2005–2007 that aims to maximize the exploitation of European Earth Observations data sets for evapotranspiration (ET) estimation. The data set was used to run 4 established ET algorithms: the Priestley–Taylor Jet Propulsion Laboratory model (PT-JPL), the Penman–Monteith algorithm from the MODIS evaporation product (PM-MOD), the Surface Energy Balance System (SEBS) and the Global Land Evaporation Amsterdam Model (GLEAM). In addition, in-situ meteorological data from 24 FLUXNET towers was used to force the models, with results from both forcing sets compared to tower-based flux observations. Model performance was assessed across several time scales using both sub-daily and daily forcings. The PT-JPL model and GLEAM provide the best performance for both satellite- and tower-based forcing as well as for the considered temporal resolutions. Simulations using the PM-MOD were mostly underestimated, while the SEBS performance was characterized by a systematic overestimation. In general, all four algorithms produce the best results in wet and moderately wet climate regimes. In dry regimes, the correlation and the absolute agreement to the reference tower ET observations were consistently lower. While ET derived with in situ forcing data agrees best with the tower measurements ( R 2 = 0.67), the agreement of the satellite-based ET estimates is only marginally lower ( R 2 = 0.58). Results also show similar model performance at daily and sub-daily (3-hourly) resolutions. Overall, our validation experiments against in situ measurements indicate that there is no single best-performing algorithm across all biome and forcing types. An extension of the evaluation to a larger selection of 85 towers (model inputs re-sampled to a common grid to facilitate global estimates) confirmed the original findings.

Description: Technical Note: The impact of spatial scale in bias correction of climate model output for hydrologic impact studies Hydrology and Earth System Sciences Discussions, 12, 10893-10920, 2015 Author(s): E. P. Maurer, D. L. Ficklin, and W. Wang Statistical downscaling is a commonly used technique for translating large-scale climate model output to a scale appropriate for assessing impacts. To ensure downscaled meteorology can be used in climate impact studies, downscaling must correct biases in the large-scale signal. A simple and generally effective method for accommodating systematic biases in large-scale model output is quantile mapping, which has been applied to many variables and shown to reduce biases on average, even in the presence of non-stationarity. Quantile mapping bias correction has been applied at spatial scales ranging from areas of hundreds of kilometers to individual points, such as weather station locations. Since water resources and other models used to simulate climate impacts are sensitive to biases in input meteorology, there is a motivation to apply bias correction at a scale fine enough that the downscaled data closely resembles historically observed data, though past work has identified undesirable consequences to applying quantile mapping at too fine a scale. This study explores the role of the spatial scale at which the quantile-mapping bias correction is applied, in the context of estimating high and low daily streamflows across the Western United States. We vary the spatial scale at which quantile mapping bias correction is performed from 2° (∼ 200 km) to 1/8° (∼ 12 km) within a statistical downscaling procedure, and use the downscaled daily precipitation and temperature to drive a hydrology model. We find that little additional benefit is obtained, and some skill is degraded, when using quantile mapping at scales finer than approximately 0.5° (∼ 50 km). This can provide guidance to those applying the quantile mapping bias correction method for hydrologic impacts analysis.

Description: Investigating the impact of land-use land-cover change on Indian summer monsoon daily rainfall and temperature during 1951–2005 using a regional climate model Hydrology and Earth System Sciences Discussions, 12, 6575-6633, 2015 Author(s): S. Halder, S. K. Saha, P. A. Dirmeyer, T. N. Chase, and B. N. Goswami Daily moderate rainfall events, that constitute a major portion of seasonal summer monsoon rainfall over central India, have decreased significantly during the period 1951 till 2005. Mean and extreme near surface daily temperature during the monsoon season have also increased by a maximum of 1–1.5 °C. Using simulations made with a high-resolution regional climate model (RegCM4) with prescribed vegetation cover of 1950 and 2005, it is demonstrated that part of the above observed changes in moderate rainfall events and temperature have been caused by land-use land-cover change (LULCC) which is mostly anthropogenic. Model simulations show that the increase in seasonal mean and extreme temperature over central India coincides with the region of decreased (increased) forest (crop) cover. The results also show that land-use land-cover alone causes warming in the extremes of daily mean and maximum temperatures by maximum of 1–1.2 °C, that is comparable with the observed increasing trend in the extremes. Decrease (increase) in forest (crop) cover reduces the evapotranspiration over land and large-scale convective instability, apart from decreasing the moisture convergence. These factors act together not only in reducing the moderate rainfall events over central India but also the amount of rainfall in that category, significantly. This is the most interesting result of this study. Additionally, the model simulations are repeated by removing the warming trend in sea surface temperatures. As a result, there is enhanced warming at the surface and decrease in moderate rainfall events over central India. Results from the additional experiments corroborate our initial findings and confirm the contribution of land-use land-cover change on increase in daily mean and extreme temperature and decrease in moderate rainfall events. This study not only demonstrates the important implications of LULCC over India, but also shows the necessity for inclusion of projected anthropogenic changes in land-use land-cover in future climate change scenarios for developing better adaptation and mitigation strategies.

Description: Distributed model of hydrological and sediment transport processes in large river basins in Southeast Asia Hydrology and Earth System Sciences Discussions, 12, 6755-6797, 2015 Author(s): S. Zuliziana, K. Tanuma, C. Yoshimura, and O. C. Saavedra Soil erosion and sediment transport have been modeled at several spatial and temporal scales, yet few models have been reported for large river basins (e.g., drainage areas 〉 100 000 km 2 ). In this study, we propose a process-based distributed model for assessment of sediment transport at a large basin scale. A distributed hydrological model was coupled with a process-based distributed sediment transport model describing soil erosion and sedimentary processes at hillslope units and channels. The model was tested on two large river basins: the Chao Phraya River Basin (drainage area: 160 000 km 2 ) and the Mekong River Basin (795 000 km 2 ). The simulation over 10 years showed good agreement with the observed suspended sediment load in both basins. The average Nash–Sutcliffe efficiency (NSE) and average correlation coefficient ( r ) between the simulated and observed suspended sediment loads were 0.62 and 0.61, respectively, in the Chao Phraya River Basin except the lowland section. In the Mekong River Basin, the overall average NSE and r were 0.60 and 0.78, respectively. Sensitivity analysis indicated that suspended sediment load is sensitive to detachability by raindrop ( k ) in the Chao Phraya River Basin and to soil detachability over land ( K f ) in the Mekong River Basin. Overall, the results suggest that the present model can be used to understand and simulate erosion and sediment transport in large river basins.