ALBERT

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: Model-based study of the role of rainfall and land use–land cover in the changes in the occurrence and intensity of Niger red floods in Niamey between 1953 and 2012 Claire Casse, Marielle Gosset, Théo Vischel, Guillaume Quantin, and Bachir Alkali Tanimoun Hydrol. Earth Syst. Sci., 20, 2841-2859, doi:10.5194/hess-20-2841-2016, 2016 Since 1950, the Niger River basin has overcome drastic changes. In Niamey city, the highest river levels and the longest flooded period ever recorded occurred in 2003, 2010, 2012 and 2013, with heavy casualties and property damage. The reasons for these changes, and the relative role of climate versus Land Use Land Cover changes are still debated and are investigated in this paper using observations and modelling. We conclude on the successive role of cover and then rainfall variability.

Beschreibung: Upscaling instantaneous to daily evapotranspiration using modelled daily shortwave radiation for remote sensing applications: an Artificial Neural Network approach Loise Wandera, Kaninska Mallick, Gerard Kiely, Olivier Roupsard, Matthias Peichl, and Vicenzo Magliulo Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016-344,2016 Manuscript under review for HESS (discussion: open, 0 comments) Upscaling instantaneous to daily evapotranspiration (ET i to ET d ) is one of the central challenge in regional vegetation water use mapping using polar orbiting satellites. Here we developed a robust ET i upscaling for global studies using the ratio between daily and instantaneous global radiation (R Sd /R Si ). Using data from 126 FLUXNET tower sites this study demonstrated R Sd /R si ratio to be the most robust factor explaining ET d /ET i variability across variable sky conditions and multiple biomes.

Beschreibung: On the propagation of diel signals in river networks using analytic solutions of flow equations Morgan Fonley, Ricardo Mantilla, Scott J. Small, and Rodica Curtu Hydrol. Earth Syst. Sci., 20, 2899-2912, doi:10.5194/hess-20-2899-2016, 2016 We design and implement a theoretical experiment to show that, under low-flow conditions, observed streamflow discrepancies between early and late summer can be attributed to different flow velocities in the river network. By developing an analytic solution to represent flow along a given river network, we emphasize the dependence of streamflow amplitude and time delay on the geomorphology of the network. We also simulate using a realistic river network to highlight the effects of scale.

Beschreibung: Simultaneous calibration of hydrological models in geographical space András Bárdossy, Yingchun Huang, and Thorsten Wagener Hydrol. Earth Syst. Sci., 20, 2913-2928, doi:10.5194/hess-20-2913-2016, 2016 This paper explores the simultaneous calibration method to transfer model parameters from gauged to ungauged catchments. It is hypothesized that the model parameters can be separated into two categories: one reflecting the dynamic behavior and the other representing the long-term water balance. The results of three numerical experiments indicate that a good parameter transfer to ungauged catchments can be achieved through simultaneous calibration of models for a number of catchments.

Beschreibung: Improving together: better science writing through peer learning Mathew A. Stiller-Reeve, Céline Heuzé, William T. Ball, Rachel H. White, Gabriele Messori, Karin van der Wiel, Iselin Medhaug, Annemarie H. Eckes, Amee O'Callaghan, Mike J. Newland, Sian R. Williams, Matthew Kasoar, Hella Elisa Wittmeier, and Valerie Kumer Hydrol. Earth Syst. Sci., 20, 2965-2973, doi:10.5194/hess-20-2965-2016, 2016 Scientific writing must improve and the key to long-term improvement of scientific writing lies with the early-career scientist (ECS). We introduce the ClimateSnack project, which aims to motivate ECSs to start writing groups around the world to improve their skills together. Writing groups offer many benefits but can be a challenge to keep going. Several ClimateSnack writing groups formed, and this paper examines why some of the groups flourished and others dissolved.

Beschreibung: A practical approach to lake water density from electrical conductivity and temperature Santiago Moreira, Martin Schultze, Karsten Rahn, and Bertram Boehrer Hydrol. Earth Syst. Sci., 20, 2975-2986, doi:10.5194/hess-20-2975-2016, 2016 Density calculations in lakes are essential in the study of stability, stratification and numerical modelling. However, density equations which use fixed constant coefficients are still common in use, despite their limitations. We present a practical approach which can specifically include the effect of dissolved substances by fitting two coefficients. The method has been tested against density equations and density measurements in lakes.

Beschreibung: Validation of Terrestrial Water Storage Variations as Simulated by Different Global Numerical Models with GRACE Satellite Observations Liangjing Zhang, Henryk Dobslaw, Tobias Stacke, Andreas Güntner, Robert Dill, and Maik Thomas Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016-330,2016 Manuscript under review for HESS (discussion: open, 0 comments) Global numerical models perform differently as has been found in some model intercomparison studies, which mainly focused on components as evapotranspiration, soil moisture or runoff. We have applied terrestrial water storage that is estimated from state-of-art post-processing method to validate four global numerical models and try to identify the advantages and deficiencies of a certain model. GRACE based TWS demonstrates its additional benefits to improve models in future.

Beschreibung: Model Study on Potential Contributions of the Proposed Huangpu Gate to Flood Control in Taihu Lake Basin Hanghui Zhang, Shuguang Liu, Jianchun Ye, and Pat Yeh Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016-310,2016 Manuscript under review for HESS (discussion: open, 0 comments) The Taihu Lake basin is located in the hinterland of the Yangtze River Delta. Constructing a gate at the mouth of the Huangpu River is considered one of the effective solutions to the basin's flooding problems. The pattern of river flow changes from bi-directional to unidirectional flow when the estuary gate is operated. It is concluded that the Huangpu River with a proposed gate is more effective than a natural channel, and its beneficiaries include the lake and the related surrounding areas.

Beschreibung: Monitoring surface water quality using social media in the context of citizen science Hang Zheng, Hong Yang, Di Long, and Hua Jing Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016-359,2016 Manuscript under review for HESS (discussion: open, 0 comments) Do you feel angry if the river in your living place is polluted by industries? Do you want to do something to save your environment? Just log in http://www.thuhjjc.com and use the Tsinghua Environment Monitoring Platform (TEMP) to photograph the water pollution actives and make your report. This study established a social media platform to monitor and report surface water quality. The effectiveness of the platform was demonstrated by the 265 water quality reports across 29 provinces in China.

Beschreibung: Consequences and mitigation of saltwater intrusion induced by short-circuiting during aquifer storage and recovery (ASR) in a coastal subsurface Koen Gerardus Zuurbier and Pieter Jan Stuyfzand Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016-343,2016 Manuscript under review for HESS (discussion: open, 0 comments) The subsurface is increasingly perforated for exploitation of water and energy. This has increased the risk of leakage between originally separated aquifers. It is shown how this leakage can have a very negative impact on the recovery of freshwater during aquifer storage and recovery (ASR) in brackish-saline aquifers. Deep interception of intruding brackish-saline water can mitigate the negative effects and buoyancy of freshwater to some extent, but not completely.

Beschreibung: Response of water temperatures and stratification to changing climate in three lakes with different morphometry Madeline R. Magee and Chin H. Wu Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016-262,2016 Manuscript under review for HESS (discussion: open, 0 comments) Water temperatures and stratification in three morphometrically different lakes over the century are examined. It is found that surface temperatures increased, bottom temperatures decreased, and duration of the stratified increased. Abrupt changes in latent heat flux appear to be independent of morphometry, indicating that the timing of change is driven by climate. Increasing air temperature warmed bottom waters while decreasing wind speed cooled bottom waters, suggesting that the change of bott

Beschreibung: The European 2015 drought from a hydrological perspective Gregor Laaha, Tobias Gauster, Lena M. Tallaksen, Jean-Philippe Vidal, Kerstin Stahl, Christel Prudhomme, Benedikt Heudorfer, Radek Vlnas, Monica Ionita, Henny A. J. Van Lanen, Mary-Jeanne Adler, Laurie Caillouet, Claire Delus, Miriam Fendekova, Sebastien Gailliez, Jamie Hannaford, Daniel Kingston, Anne F. Van Loon, Luis Mediero, Marzena Osuch, Renata Romanowicz, Eric Sauquet, James H. Stagge, and Wai K. Wong Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016-366,2016 Manuscript under review for HESS (discussion: open, 0 comments) In 2015 large parts of Europe were affected by a drought. In terms of low flow magnitude, a region around the Czech Republic was most affected with annual low flows that exhibited return intervals of 100 years and more. In terms of deficit volumes, the geographical centre of the event was in the area of Southern Germany where the drought lasted particularly long. For an assessment of drought impacts on water resources hydrological data is required in addition to the hydro-meteorological indices.

Beschreibung: Insights into the water mean transit time in a high-elevation tropical ecosystem Giovanny M. Mosquera, Catalina Segura, Kellie B. Vaché, David Windhorst, Lutz Breuer, and Patricio Crespo Hydrol. Earth Syst. Sci., 20, 2987-3004, doi:10.5194/hess-20-2987-2016, 2016 This study focuses on the investigation of baseflow mean transit times (MTTs) in a high-elevation tropical ecosystem (páramo) using stable water isotopes. Results showed short MTTs (

Beschreibung: Identifying water deficit and vegetation response during the 2009/10 drought over North China: Implications for the South-to-North Water Diversion project Bowen Zhu, Xianhong Xie, and Kang Zhang Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016-313,2016 Manuscript under review for HESS (discussion: open, 0 comments) This study attempts to identify water deficit using GRACE data and vegetation response during the 2009/10 drought over North China. There was a decling trend in total water storage for the past decade based on GRACE data, and the regional deficit of water storage was approximately 25 km 3 in 2009/10. This drought event has led to suppression of vegetation growth in North China. The SNWD project may ease the water storage deficit in North China for this level of drought intensity.

Beschreibung: Hydrological threats for riparian wetlands of international importance – a global quantitative and qualitative analysis Christof Schneider, Martina Flörke, Lucia De Stefano, and Jacob D. Petersen-Perlman Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016-350,2016 Manuscript under review for HESS (discussion: open, 0 comments) Riparian wetlands are disappearing worldwide due to altered river hydrology. The WaterGAP3 model is used to compare modified to natural flow regimes at 93 Ramsar sites. Results show that water resource management seriously impairs inundation patterns at 29 % of the sites. New dam initiatives are likely to affect especially wetlands located in South America, Africa, Asia and the Balkan Peninsula. Hotspots for climate change impacts could be Eastern Europe and South America.

Beschreibung: Numerical Solution and Application of Time-Space Fractional Governing Equations of One-Dimensional Unsteady Open Channel Flow Process Ali Ercan and M. Levent Kavvas Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016-364,2016 Manuscript under review for HESS (discussion: open, 0 comments) A finite difference numerical approach is proposed to solve the time-space fractional governing equations of one-dimensional unsteady/non-uniform open channel flow process. Numerical simulations showed that flow discharge and water depth can exhibit heavier tails in downstream locations as space and time fractional derivative powers decrease from 1. The fractional governing equations under consideration are generalizations of the well-known Saint Venant equations.

Beschreibung: Vegetative impacts upon bedload transport capacity and channel stability for differing alluvial planforms in the Yellow River source zone Zhi Wei Li, Guo An Yu, Gary Brierley, and Zhao Yin Wang Hydrol. Earth Syst. Sci., 20, 3013-3025, doi:10.5194/hess-20-3013-2016, 2016 Influence of vegetation upon bedload transport and channel morphodynamics is examined along a channel stability gradient ranging from meandering to anabranching to anabranching–braided to fully braided planform conditions along trunk and tributary reaches of the Yellow River source zone in western China. This innovative work reveals complex interactions between channel planform, bedload transport capacity, sediment supply in the flood season, and the hydraulic role of vegetation.

Beschreibung: Using rainfall thresholds and ensemble precipitation forecasts to issue and improve urban inundation alerts Tsun-Hu Yang, Gong-Do Hwang, Chin-Cheng Tsai, and Jui-Yi Ho Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016-340,2016 Manuscript under review for HESS (discussion: open, 0 comments) Taiwan continues to suffer from floods. This study proposes the integration of rainfall thresholds and ensemble precipitation forecasts to provide probabilistic urban inundation forecasts. Utilization of ensemble precipitation forecasts can extend forecast lead times to 72%thinsp;h, preceding peak flows and allowing response agencies to take necessary preparatory measures. This study also develops a hybrid of real-time observation and rainfall forecasts to improve the first 24-h inundation forecasts.

Beschreibung: Contrasting transit times of water from peatlands and eucalypt forests in the Australian Alps determined by tritium: implications for vulnerability and the source of water in upland catchments I. Cartwright and U. Morgenstern Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016-361,2016 Manuscript under review for HESS (discussion: open, 0 comments) This research determines timescales over which water is stored in peatlands and adjacent eucalyptus forest catchments in upland river systems in southeast Australia using tritium. The mean transit times in the peatland catchments were in many cases less than one year which contrasts with the longer transit times (years to decades) in the eucalyptus catchments. The results indicate that the peat is susceptible to drying which renders it vulnerable to degradation and bushfire.

Beschreibung: Effect of restoration vegetation on the stochasticity of soil erosion in a semi-arid environment Ji Zhou, Bojie Fu, Guangyao Gao, Yihe Lü, and Shuai Wang Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016-386,2016 Manuscript under review for HESS (discussion: open, 0 comments) By monitoring soil erosion in the Loess Plateau, we constructed an integrated analysis system to quantify the stochasticity of environment, found that the morphological structures in vegetation types are the source of different stochasticities of soil erosion, and proved that Poisson model is fit for predicting erosion stochasticity at larger temporal scale. This study could be meaningful for the selection of restoration vegetation for conserving soil and water resources in the Loess Plateau.

Beschreibung: Modeling liquid water transport in snow under rain-on-snow conditions – considering preferential flow Sebastian Würzer, Nander Wever, Roman Juras, Michael Lehning, and Tobias Jonas Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016-351,2016 Manuscript under review for HESS (discussion: open, 0 comments) We discuss a dual-domain water transport model in a physics based snowpack model to account for preferential flow (PF) in addition to matrix flow. Yet so far no operationally used snow model was explicitly accounting for PF. The new approach is compared to existing water transport models and validated against in-situ data from sprinkling and natural rain-on-snow (ROS) events. Our work demonstrates the benefit of considering PF in modeling hourly snowpack runoff, especially during ROS conditions.

Beschreibung: Can integrative catchment management mitigate future water quality issues caused by climate change and socio-economic development? Mark Honti, Nele Schuwirth, Jörg Rieckermann, and Christian Stamm Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016-297,2016 Manuscript under review for HESS (discussion: open, 0 comments) Catchments are complex systems where water quantity, quality and the provided ecological services are determined by interacting physical, chemical, biological, economic, and social factors. The awareness of these interactions led to the prevailing catchment management paradigm of Integrated Water Resources Management. The design and evaluation of solutions for integrated water resources management requires to predict changes of local or regional water quality, which requires integrated approach for modeling too. On one hand, integrated models have to be comprehensive enough to cover the aspects relevant for management decisions, allow for mapping of global change processes – as climate change, population growth, migration, and socio-economic development – to the regional and local contexts. On the other hand, models have to be sufficiently simple and fast enough to apply proper methods of uncertainty analysis, which can consider model structure deficits and propagate errors through the chain of submodels. Here, we present an integrated catchment model satisfying both objectives. The conceptual "iWaQa" model was developed to support the integrated management of small streams. It can predict both traditional water quality parameters like nutrients and a wide set of organic micropollutants originating from plant and material protection products. Due to the model's simplicity, it allows for a full, propagative analysis of predictive uncertainty, including certain structural and input errors. The usefulness of the model is demonstrated by predicting future water quality in a small catchment with mixed land use in the Swiss Plateau. The focus of our study is the change of water quality over the next decades driven by climate change, population growth or decline, socio-economic development and the implementation of management strategies for improving water quality. Our results indicate that input and model structure uncertainties are the most influential factors on certain water quality parameters and in these cases the uncertainty of modeling is already very high for the present conditions. Nevertheless, a proper quantification of today's uncertainty can make the management fairly robust for the foreseen range of possible evolution into the next decades. With a time-horizon of 2050, it seems that human land use and management decisions have a larger influence on water quality than climate change. However, the analysis of single climate model chains indicates that the importance of climate grows when a certain climate prediction is considered instead of the ensemble forecast.

Beschreibung: CFSv2-based sub-seasonal precipitation and temperature forecast skill over the contiguous United States Di Tian, Eric F. Wood, and Xing Yuan Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016-376,2016 Manuscript under review for HESS (discussion: open, 0 comments) Forecasts from global seasonal climate forecast models can be potentially exploited for sub-seasonal forecasts of precipitation and 2-m temperature. The probabilistic sub-seasonal forecast skill of ten precipitation and temperature indices is investigated using the 28-years’ hindcasts of the Climate Forecast System version 2 (CFSv2) over the contiguous United States (CONUS). The forecast skill is highly dependent on the forecast indices, regions, seasons, leads, and methods. Indices characterizing mean precipitation and temperature as well as measuring frequency or duration of precipitation and temperature extremes for 7-, 14-, and 30-day forecasts were skillful depending on seasons, regions, and forecast leads. Forecasts for 7- and 14-day temperature indices showed skill even at weeks 3 and 4, and generally more skillful than precipitation indices. Overall, temperature indices showed higher skill than precipitation indices over the entire CONUS region. While the forecast skill related to mean precipitation indices were low in summer over the CONUS, the number of rainy days, number of consecutive rainy days, and the number of consecutive dry days showed considerable high skill for the west coast region. The 30-day forecasts of precipitation and temperature indices calculated from the downscaled monthly CFSv2 forecasts are less skillful than those calculated from the daily CFSv2 forecasts, suggesting the potential usefulness of the CFSv2 daily forecasts for hydrological applications relative to the temporally disaggregated CFSv2 monthly forecasts. While the presence of active Madden-Julian Oscillation (MJO) events improves CFSv2 weekly mean precipitation forecast skill over major areas of CONUS, MJO or El Niño Southern Oscillation did not have same strong effects on weekly mean temperature forecasts.

Beschreibung: Mekong River flow and hydrological extremes under climate change Long Phi Hoang, Hannu Lauri, Matti Kummu, Jorma Koponen, Michelle T. H. van Vliet, Iwan Supit, Rik Leemans, Pavel Kabat, and Fulco Ludwig Hydrol. Earth Syst. Sci., 20, 3027-3041, doi:10.5194/hess-20-3027-2016, 2016 We modelled hydrological changes under climate change in the Mekong River, focusing on extreme events. The scenario ensemble shows an intensification of the hydrological cycle under climate change. Annual river flow increases between 5 and 16 % depending on locations. Extreme high flows increase substantially in both magnitude and frequency, posing threats to flood safety in the basin. Extreme low-flow events are projected to reduce as a result of increased river flow during the dry season.

Beschreibung: Technical note: Inference in hydrology from entropy balance considerations Stefan J. Kollet Hydrol. Earth Syst. Sci., 20, 2801-2809, doi:10.5194/hess-20-2801-2016, 2016 In this study, the method of inference of macroscale thermodynamic potentials, forces, and exchange coefficients for variably saturated groundwater flow is outlined based on the entropy balance. The theoretical basis of the method of inference is the explicit calculation of the internal entropy production from microscale, thermodynamic flux–force relationships using, e.g., hyper-resolution variably saturated groundwater flow models. Emphasis is placed on the two-scale nature of the entropy balance equation that allows simultaneously incorporating movement equations at the micro- and macroscale. The method is illustrated with simple hydrologic cross sections at steady state and periodic sources/sinks at dynamic equilibrium, and provides a thermodynamic point of view of upscaling in variably saturated groundwater flow. The current limitations in the connection with observable variables and predictive capabilities are discussed, and some perspectives for future research are provided.

Beschreibung: Evaluating uncertainty in estimates of soil moisture memory with a reverse ensemble approach Dave MacLeod, Hannah Cloke, Florian Pappenberger, and Antje Weisheimer Hydrol. Earth Syst. Sci., 20, 2737-2743, doi:10.5194/hess-20-2737-2016, 2016 Soil moisture memory is a key aspect of seasonal climate predictions, through feedback between the land surface and the atmosphere. Estimates have been made of the length of soil moisture memory; however, we show here how estimates of memory show large variation with uncertain model parameters. Explicit representation of model uncertainty may then improve the realism of simulations and seasonal climate forecasts.

Beschreibung: Estimating drought risk across Europe from reported drought impacts, drought indices, and vulnerability factors Veit Blauhut, Kerstin Stahl, James Howard Stagge, Lena M. Tallaksen, Lucia De Stefano, and Jürgen Vogt Hydrol. Earth Syst. Sci., 20, 2779-2800, doi:10.5194/hess-20-2779-2016, 2016 Drought is one of the most costly natural hazards in Europe. Due to its complexity, drought risk, meant as the combination of the natural hazard and societal vulnerability, is difficult to define and challenging to detect and predict, as the impacts of drought are very diverse, covering the breadth of socioeconomic and environmental systems. Pan-European maps of drought risk could inform the elaboration of guidelines and policies to address its documented severity and impact across borders. This work tests the capability of commonly applied drought indices and vulnerability factors to predict annual drought impact occurrence for different sectors and macro regions in Europe and combines information on past drought impacts, drought indices, and vulnerability factors into estimates of drought risk at the pan-European scale. This hybrid approach bridges the gap between traditional vulnerability assessment and probabilistic impact prediction in a statistical modelling framework. Multivariable logistic regression was applied to predict the likelihood of impact occurrence on an annual basis for particular impact categories and European macro regions. The results indicate sector- and macro-region-specific sensitivities of drought indices, with the Standardized Precipitation Evapotranspiration Index (SPEI) for a 12-month accumulation period as the overall best hazard predictor. Vulnerability factors have only limited ability to predict drought impacts as single predictors, with information about land use and water resources being the best vulnerability-based predictors. The application of the hybrid approach revealed strong regional and sector-specific differences in drought risk across Europe. The majority of the best predictor combinations rely on a combination of SPEI for shorter and longer accumulation periods, and a combination of information on land use and water resources. The added value of integrating regional vulnerability information with drought risk prediction could be proven. Thus, the study contributes to the overall understanding of drivers of drought impacts, appropriateness of drought indices selection for specific applications, and drought risk assessment.

Beschreibung: Redox controls on methane formation, migration and fate in shallow aquifers Pauline Humez, Bernhard Mayer, Michael Nightingale, Veith Becker, Andrew Kingston, Stephen Taylor, Guy Bayegnak, Romain Millot, and Wolfram Kloppmann Hydrol. Earth Syst. Sci., 20, 2759-2777, doi:10.5194/hess-20-2759-2016, 2016 Development of unconventional energy resources if often associated with public concerns regarding potential contamination of shallow groundwater due to methane leakage. We combined chemical and isotopic analyses of gas and water samples obtained from shallow aquifers in Alberta (Canada) to assess baseline methane sources and found that 〉 67 % of the samples contained biogenic methane formed in situ in the aquifers. There was no evidence of deep thermogenic methane migration into shallow aquifers.

Beschreibung: Ordinary kriging as a tool to estimate historical daily streamflow records William H. Farmer Hydrol. Earth Syst. Sci., 20, 2721-2735, doi:10.5194/hess-20-2721-2016, 2016 The potential of geostatistical tools, leveraging the spatial structure and dependency of correlated time series, for the prediction of daily streamflow time series at unmonitored locations is explored. Simple geostatistical tools improve on traditional estimates of daily streamflow. The temporal evolution of spatial structure, including seasonal fluctuations, is also explored. The proposed method is contrasted with more advanced geostatistical methods and shown to be comparable.

Beschreibung: Spatial and Temporal Variability in Baseflow in the Mattole River Headwaters, California, USA Nathan Queener and Andrew P. Stubblefield Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016-300,2016 Manuscript under review for HESS (discussion: open, 0 comments) Understanding controls on stream low flow is critical for managing aquatic resources. Baseflows in small river basins in northern California, USA were found to be quite divergent. The difference in water yield was much greater than the difference in precipitation. Steep, high elevation areas, dissected by many channels, had the highest baseflow. The positive correlation between basin steepness and flow is thought to result from greater basin storage capacity in thick weathered bedrock layers.

Beschreibung: Looking beyond general metrics for model comparison – lessons from an international model intercomparison study Tanja de Boer-Euser, Laurène Bouaziz, Jan De Niel, Claudia Brauer, Benjamin Dewals, Gilles Drogue, Fabrizio Fenicia, Benjamin Grelier, Jiri Nossent, Fernando Pereira, Hubert Savenije, Guillaume Thirel, and Patrick Willems Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016-339,2016 Manuscript under review for HESS (discussion: open, 0 comments) In this study the rainfall–runoff models of eight international research groups were compared for a set of subcatchments of the Meuse basin to investigate the influence of certain model components on the modelled discharge. Although the models showed similar performances based on general metrics, clear differences could be observed for specific events. The differences during drier conditions could indeed be linked back to differences in model structures.

Beschreibung: Morphological dynamics of an englacial channel Geir Vatne and Tristram D. L. Irvine-Fynn Hydrol. Earth Syst. Sci., 20, 2947-2964, doi:10.5194/hess-20-2947-2016, 2016 Ten years of direct observations of an englacial conduit in a cold based glacier in Svalbard document for the first time how a vertical meltwater waterfall (moulin) is formed from gradual incision of a meltwater channel. This evolution appears to be dominated by knickpoints that incise upstream at rates several times faster than the vertical incision in adjacent near horizontal channel sections.

Beschreibung: Can controlled drainage control agricultural nutrient emissions? Evidence from a BACI experiment combined with a dual isotope approach Mette V. Carstensen, Jane R. Poulsen, Niels B. Ovesen, Christen D. Børgesen, Søren K. Hvid, and Brian Kronvang Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016-303,2016 Manuscript under review for HESS (discussion: open, 0 comments) This study investigates whether the groundwater management technique called controlled drainage can be used as a mitigation measure to reduce the N loss from fields. Controlled drainage did lead to a significant reduction of the drain water flow (38–52 %) as well as nitrate-N losses (36–48 %) compared to traditional drainage systems without affecting crop growth. Enhanced denitrification was only documented for a shorter period, thus the reduction of N loss was primarily due to reduced drain flow.

Beschreibung: The Budyko functions under non-steady state conditions: new approach and comparison with previous formulations Roger Moussa and Jean-Paul Lhomme Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016-379,2016 Manuscript under review for HESS (discussion: open, 0 comments) This paper presents a new approach to transform any Budyko function under steady-state conditions into a function valid for non-steady state conditions. It involves an additional parameter which accounts for change in soil water storage. Compared to previous formulations it gives similar results as the analytical solution of Greve et al. (2016). However, the domain of variation of the new formulation differs from those of Chen et al. (2013) and Du et al. (2016).

Beschreibung: Application of tritium in precipitation and baseflow in Japan: a case study of groundwater transit times and storage in Hokkaido watersheds Maksym A. Gusyev, Uwe Morgenstern, Michael K. Stewart, Yusuke Yamazaki, Kazuhisa Kashiwaya, Terumasa Nishihara, Daisuke Kuribayashi, Hisaya Sawano, and Yoichi Iwami Hydrol. Earth Syst. Sci., 20, 3043-3058, doi:10.5194/hess-20-3043-2016, 2016 Tritium-estimated groundwater mean transit times (MTTs) and storage volumes provide useful information for water resources management especially during droughts. In Hokkaido, we find that (1) one tritium measurement at baseflow is already sufficient to estimate MTT for some catchments, (2) the hydrogeological settings control tritium transit times of subsurface groundwater storage at baseflow, and (3) in future, one tritium measurement will be sufficient to estimate MTT in most Japanese catchments.

Beschreibung: Acclimatizing Fast Orthogonal Search (FOS) Model for River Stream-flow Forecasting Abdalla Osman, Mohammed Falah Allawi, Haitham Abdulmohsin Afan, Aboelmagd Noureldin, and Ahmed El-shafie Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016-347,2016 Manuscript under review for HESS (discussion: open, 0 comments) The current research is significant not only for researchers but also for decision-makers for water resources system management more specifically for dam and reservoir operations. Fast Orthogonal Search (FOS) model might result in a system that could be used in several river basins. Supported by efficient FOS-based pattern recognition model, accurate information about future river stream flow could be utilized by the decision-maker to formulate better water resources system management.

Beschreibung: Variation of soil hydraulic properties with alpine grassland degradation in the Eastern Tibetan Plateau Tao Pan, Shuai Hou, Shaohong Wu, Yanhua Liu, Xintong Zou, Anna Herzberger, and Jianguo Liu Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016-333,2016 Manuscript under review for HESS (discussion: open, 0 comments) The study show that soil hydraulic properties, especially those of the top soil, varied greatly with alpine swamp meadow degradation. Soil porosity is the dominant influencing factor of soil hydraulic properties. The results suggest that alpine swamp meadow degradation would inevitably lead to negative hydrological effects. Hydrological modelling in Tibetan Plateau and similar regions are recommended to understand the effects of degradation on soil hydraulic properties.

Beschreibung: Assessing land–ocean connectivity via submarine groundwater discharge (SGD) in the Ria Formosa Lagoon (Portugal): combining radon measurements and stable isotope hydrology Carlos Rocha, Cristina Veiga-Pires, Jan Scholten, Kay Knoeller, Darren R. Gröcke, Liliana Carvalho, Jaime Anibal, and Jean Wilson Hydrol. Earth Syst. Sci., 20, 3077-3098, doi:10.5194/hess-20-3077-2016, 2016 We combine radon and stable isotopes in water to determine total submarine groundwater discharge (SGD) in the Ria Formosa and discriminate its component modes. We show that tidal action filters the entire water volume in the lagoon through local beaches 3.5 times a year, driving an estimated 350Ton nitrogen/year into the system. Conversely, fresh groundwater is discharged into the lagoon only occasionally, adding ~ 61 Ton nitrogen/year, but is capable of driving new production in the system.

Beschreibung: Sustainability of water uses in managed hydrosystems: human- and climate-induced changes for the mid-21st century Julie Fabre, Denis Ruelland, Alain Dezetter, and Benjamin Grouillet Hydrol. Earth Syst. Sci., 20, 3129-3147, doi:10.5194/hess-20-3129-2016, 2016 We assess the sustainability of planned water uses in complex mesoscale river basins by modeling water demand and availability under climatic and anthropogenic changes. We present an analysis through indicators that relate to water management goals. The impacts of climate projections on both water availability and demand question the water allocations and environmental constraints currently planned for the coming decades. This work brings an essential long-term perspective to water sharing plans.

Beschreibung: Real-time monitoring of nitrate transport in the deep vadose zone under a crop field – implications for groundwater protection Tuvia Turkeltaub, Daniel Kurtzman, and Ofer Dahan Hydrol. Earth Syst. Sci., 20, 3099-3108, doi:10.5194/hess-20-3099-2016, 2016 Efficient groundwater protection from pollution originating in agriculture requires effective monitoring means capable of tacking pollution processes in the vadose zone, long before groundwater pollution turns into an unavoidable fact. In this study, a vadose zone monitoring system that was installed under a crop field fertilized by dairy slurry enabled real-time tracking of nitrate plum migration down the vadose zone from the land surface to the water table at 18m depth.

Beschreibung: Willingness-to-pay for a probabilistic flood forecast: a risk-based decision-making game Louise Arnal, Maria-Helena Ramos, Erin Coughlan de Perez, Hannah Louise Cloke, Elisabeth Stephens, Fredrik Wetterhall, Schalk Jan van Andel, and Florian Pappenberger Hydrol. Earth Syst. Sci., 20, 3109-3128, doi:10.5194/hess-20-3109-2016, 2016 Forecasts are produced as probabilities of occurrence of specific events, which is both an added value and a challenge for users. This paper presents a game on flood protection, "How much are you prepared to pay for a forecast?", which investigated how users perceive the value of forecasts and are willing to pay for them when making decisions. It shows that users are mainly influenced by the perceived quality of the forecasts, their need for the information and their degree of risk tolerance.

Beschreibung: Fresh groundwater resources in a large sand replenishment Sebastian Huizer, Gualbert H. P. Oude Essink, and Marc F. P. Bierkens Hydrol. Earth Syst. Sci., 20, 3149-3166, doi:10.5194/hess-20-3149-2016, 2016 The anticipation of sea-level rise has led to an innovative project called the Sand Engine, where a large volume of sand was placed on the Dutch coast. The intention is that the sand is redistributed by wind, current and tide, reinforcing coastal defence structures. Model simulations show that this large sand replenishment can result in a substantial growth of fresh groundwater resources. Thus, sand replenishments could combine coastal protection with an increase of fresh groundwater resources.

Beschreibung: Gauge-Adjusted Rainfall Estimates from Commercial Microwave Links Martin Fencl, Michal Dohnal, Jörg Rieckermann, and Vojtěch Bareš Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016-397,2016 Manuscript under review for HESS (discussion: open, 0 comments) Commercial microwave links (CMLs) can provide rainfall observations with high space-time resolution. Unfortunately, CML rainfall estimates are usually biased because we lack detailed information on the processes which attenuate the transmitted microwaves. We suggest removing the bias by dynamically adjusting CMLs to cumulative data from rain gauges (RGs), which can be remote from the CMLs. Our approach practically eliminates the bias, which we demonstrate on unique data from several CMLs a RGs.

Beschreibung: The socioecohydrology of rainwater harvesting in India: understanding water storage and release dynamics across spatial scales Kimberly J. Van Meter, Michael Steiff, Daniel L. McLaughlin, and Nandita B. Basu Hydrol. Earth Syst. Sci., 20, 2629-2647, doi:10.5194/hess-20-2629-2016, 2016 Although village-scale rainwater harvesting (RWH) structures have been used for millennia in India, many of these structures have fallen into disrepair due to increased dependence on groundwater. This dependence has contributed to declines in groundwater resources, and in turn to efforts to revive older RWH systems. In the present study, we use field data to quantify water fluxes in a cascade of irrigation tanks to better our understanding of the impact of RWH systems on the water balance in con

Beschreibung: Comparison of uncertainty in multi-parameter and multi-model ensemble hydrologic analysis of climate change Younggu Her, Seung-Hwan Yoo, Chounghyun Seong, Jaehak Jeong, Jaepil Cho, and Syewoon Hwang Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016-160,2016 Manuscript under review for HESS (discussion: open, 0 comments) This study demonstrated that the significance of GCM and hydrological parameter selection varied depending on the hydrologic components (e.g. direct runoff, soil moisture, and baseflow) of interest and the thresholds used to identify the behavioral parameter sets in a hydrologic analysis of climate change. A newly proposed analysis strategy enabled to investigate the contributions of each GCM to the overall uncertainty in a multi-GCM ensemble for hydrologic analysis.

Beschreibung: Towards a tracer-based conceptualization of meltwater dynamics and streamflow response in a glacierized catchment Daniele Penna, Michael Engel, Giacomo Bertoldi, and Francesco Comiti Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016-334,2016 Manuscript under review for HESS (discussion: open, 0 comments) In this research we used environmental tracers in the Saldur River catchment, Italian Alps to obtain new insight into the hydrology of glacierized catchments. We analysed the spatio-temporal variability of the tracer signature within the catchment, distinguished the contribution of groundwater, glacier melt and snowmelt to stream discharge, identified the sources of uncertainty in the estimation of streamflow components and presented a paradigm of hydrological function of glacierized catchments.

Beschreibung: A retrospective streamflow ensemble forecast for an extreme hydrologic event: a case study of Hurricane Irene and on the Hudson River basin Firas Saleh, Venkatsundar Ramaswamy, Nickitas Georgas, Alan F. Blumberg, and Julie Pullen Hydrol. Earth Syst. Sci., 20, 2649-2667, doi:10.5194/hess-20-2649-2016, 2016 An operational framework was implemented to generate retrospective ensemble streamflow forecasts for an extreme hydrological event, Hurricane Irene. The implications of this work benefit streamflow forecast efforts and can be used for numerous applications, such as forecasting the water resources variability, predicting fate of water quality and climate change scenarios. Socio-economic analysis may be used to weigh on how improved forecasts prevent life loss and minimize property damage.

Beschreibung: A comparison of the modern Lie scaling method to classical scaling techniques James Polsinelli and M. Levent Kavvas Hydrol. Earth Syst. Sci., 20, 2669-2678, doi:10.5194/hess-20-2669-2016, 2016 This article summarizes the theory and demonstrates the technique of a new scaling method known as the Lie scaling. In the course of applying the method to two example problems, classical notions of dynamical and kinematic scaling are incorporated. The two example problems are a 2-D unconfined groundwater problem in a heterogeneous soil and a 1-D contaminant transport problem. The article concludes with comments on the relative strengths and weaknesses of Lie scaling and classical scaling.

Beschreibung: 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) Jonathan M. Abell, David P. Hamilton, and Christopher G. McBride Hydrol. Earth Syst. Sci., 20, 2395-2401, doi:10.5194/hess-20-2395-2016, 2016 We 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). They propose that "the only effective way to limit algae blooms and improve lake water quality in such environments is by limiting the nitrate load". We outline four reasons why it is important to instead limit both phosphorus and nitrogen loads to this iconic lake, consistent with current policy.

Beschreibung: Improvement of hydrological model calibration by selecting multiple parameter ranges Qiaofeng Wu, Shuguang Liu, Yi Cai, Xinjian Li, and Yangming Jiang Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016-275,2016 Manuscript under review for HESS (discussion: open, 0 comments) We proposed a method to calibrate the hydrological model by selecting parameter range. The results show the probability distribution can be used to determine the optimal range of single parameter. Analysis of parameter sensitivity and correlation is helpful to obtain the optimal combination of multi-parameter ranges which contributes to a higher and more concentrated value of the objective function. The findings can provide references for enhancing the precision of hydrological process modeling.

Beschreibung: Estimation of flood warning runoff thresholds in ungauged basins with asymmetric error functions Elena Toth Hydrol. Earth Syst. Sci., 20, 2383-2394, doi:10.5194/hess-20-2383-2016, 2016 Runoff thresholds for activating flood warnings might be estimated with regionally derived relationships between catchment descriptors and assigned flood quantiles. Since the consequences of overestimated thresholds (leading to missing alarms) are generally more severe than those of an underestimation (leading to false alarms), the work proposes to parameterise the regression model with an asymmetric error function, instead of using a traditional, symmetric square errors sum.

Beschreibung: The cumulative effects of forest disturbance and climate variability on baseflow in a large watershed in British Columbia, Canada Qiang Li, Xiaohua Wei, Mingfang Zhang, Wenfei Liu, Krysta Giles-Hansen, Yi Wang, and Liangliang Duan Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016-291,2016 Manuscript under review for HESS (discussion: open, 0 comments) Assessing how forest disturbance and climate change affect baseflow or groundwater discharge is critical for understanding water resource supply and protecting aquatic functions. Previous studies have mainly evaluated the effects of forest disturbance on streamflow, with rare attention on baseflow, particularly in large watersheds. However, studying this topic is challenging as it requires explicit inclusion of climate into assessment due to their interactions at any large watersheds. In this study, we used Upper Similkameen River watershed (USR) (1810 km 2 ), located in the southern interior of British Columbia, Canada to examine how forest disturbance and climate variability affect baseflow. The conductivity mass balance method was first used for baseflow separation, and the modified double mass curves were then employed to quantitatively separate the relative contributions of forest disturbance and climate variability to annual baseflow. Our results showed that average annual baseflow and baseflow index (baseflow/streamflow) were about 85.2 ± 21.5 mm year -1 and 0.22 ± 0.05 for the study period of 1954–2013, respectively. The forest disturbance increased the annual baseflow of 18.4 mm, while climate variability decreased 19.4 mm. In addition, forest disturbance also shifted the baseflow regime with increasing of the spring baseflow and decreasing of the summer baseflow. We conclude that forest disturbance significantly altered the baseflow magnitudes and patterns, and its role in annual baseflow was equal to that caused by climate variability in the study watershed despite their opposite changing directions. The implications of our results are discussed in the context of future forest disturbance (or land cover changes) and climate changes.

Beschreibung: Geostatistical upscaling of rain gauge data to support uncertainty analysis of lumped urban hydrological models Manoranjan Muthusamy, Alma Schellart, Simon Tait, and Gerard B. M. Heuvelink Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016-279,2016 Manuscript under review for HESS (discussion: open, 0 comments) In this study we develop a method to estimate the spatially averaged rainfall intensity together with associated level of uncertainty using geostatistical upscaling. Rainfall data collected from a cluster of eight paired rain gauges in a small urban catchment are used in this study. Results show that the prediction uncertainty comes mainly from two sources: spatial variability of rainfall and measurement error. Results from this study can be used for uncertainty analyses of hydrologic modelling.

Beschreibung: Can local climate variability be explained by weather patterns? A multi-station evaluation for the Rhine basin Aline Murawski, Gerd Bürger, Sergiy Vorogushyn, and Bruno Merz Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016-286,2016 Manuscript under review for HESS (discussion: open, 0 comments) For understanding past flood changes in the Rhine catchment and the role of anthropogenic climate change for extreme flows, an attribution study relying on a proper GCM (General Circulation Model) downscaling is needed. A downscaling based on conditioning a stochastic weather generator on weather patterns is a promising approach. Here the link between patterns and local climate is tested, and the skill of GCMs to reproduce these patterns is evaluated.

Beschreibung: Technical Note: Monitoring of unsteady open channel flows using continuous slope-area method Kyutae Lee, Ali R. Firoozfar, and Marian Muste Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016-289,2016 Manuscript under review for HESS (discussion: open, 0 comments) Accurate estimation of stream/river flows is important for the public safety during floods as well as for the efficient use of limited water resources for hydropower generation and irrigation. In this paper, we have investigated a feasibility of the continuous slope area (CSA) method which utilizes the measurements of instantaneous water surface elevation changes, and the results showed promising capabilities of the suggested method for the accurate estimation of flows in natural streams/rivers.

Beschreibung: Estimating extreme river discharges in Europe through a Bayesian Network Dominik Paprotny and Oswaldo Morales Nápoles Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016-250,2016 Manuscript under review for HESS (discussion: open, 0 comments) Large-scale hydrological modelling of flood hazard requires adequate extreme discharge data. Models based on physics are applied alongside those utilizing only statistical analysis. The former requires enormous computation power, while the latter are most limited in accuracy and spatial coverage. In this paper we introduce an alternate, statistical approach based on Bayesian Networks (BN), a graphical model for dependent random variables. We use a non-parametric BN to describe the joint distribution of extreme discharges in European rivers and variables describing the geographical characteristics of their catchments. Data on annual maxima of daily discharges from more than 1800 river gauge stations were collected, together with information on terrain, land use and climate of catchments that drain to those locations. The (conditional) correlations between the variables are modelled through copulas, with the dependency structure defined in the network. The results show that using this method, mean annual maxima and return periods of discharges could be estimated with an accuracy similar to existing studies using physical models for Europe, and better than a comparable global statistical method. Performance of the model varies slightly between regions of Europe, but is consistent between different time periods, and is not affected by a split-sample validation. The BN was applied to a large domain covering all sizes of rivers in the continent, both for present and future climate, showing large variation in influence of climate change on river discharges, as well as large differences between emission scenarios. The method could be used to provide quick estimates of extreme discharges at any location for the purpose of obtaining input information for hydraulic modelling.