ALBERT

Beschreibung: The most popular practice for analyzing nonstationarity of flood series is to use a fixed single-type probability distribution incorporated with the time-varying moments. However, the type of probability distribution could be both complex because of distinct flood populations and time-varying under changing environments. To allow the investigation of this complex nature, the time-varying two-component mixture distributions (TTMD) method is proposed in this study by considering the time variations of not only the moments of its component distributions but also the weighting coefficients. Having identified the existence of mixed flood populations based on circular statistics, the proposed TTMD was applied to model the annual maximum flood series (AMFS) of two stations in the Weihe River basin (WRB), with the model parameters calibrated by the meta-heuristic maximum likelihood (MHML) method. The performance of TTMD was evaluated by different diagnostic plots and indexes and compared with stationary single-type distributions, stationary mixture distributions and time-varying single-type distributions. The results highlighted the advantages of TTMD with physically-based covariates for both stations. Besides, the optimal TTMD models were considered to be capable of settling the issue of nonstationarity and capturing the mixed flood populations satisfactorily. This article is protected by copyright. All rights reserved.

Beschreibung: While the effects of land use change in urban areas have been widely examined, the combined effects of climate and land use change on the quality of urban and urbanizing streams have received much less attention. We describe a modeling framework that is applicable to the evaluation of potential changes in urban water quality and associated hydrologic changes in response to ongoing climate and landscape alteration. The grid-based spatially distributed model, DHSVM-WQ, is an outgrowth of the Distributed Hydrology-Soil-Vegetation Model (DHSVM) that incorporates modules for assessing hydrology and water quality in urbanized watersheds at a high spatial and temporal resolution. DHSVM-WQ simulates surface runoff quality and in-stream processes that control the transport of nonpoint-source (NPS) pollutants into urban streams. We configure DHSVM-WQ for three partially urbanized catchments in the Puget Sound region to evaluate the water quality responses to current conditions and projected changes in climate and/or land use over the next century. Here we focus on total suspended solids (TSS) and total phosphorus (TP) from nonpoint sources (runoff), as well as stream temperature. The projection of future land use is characterized by a combination of densification in existing urban or partially urban areas, and expansion of the urban footprint. The climate change scenarios consist of individual and concurrent changes in temperature and precipitation. Future precipitation is projected to increase in winter and decrease in summer, while future temperature is projected to increase throughout the year. Our results show that urbanization has a much greater effect than climate change on both the magnitude and seasonal variability of streamflow, TSS and TP loads largely due to substantially increased streamflow, and particularly winter flow peaks. Water temperature is more sensitive to climate warming scenarios than to urbanization and precipitation changes. Future urbanization and climate change together are predicted to significantly increase annual mean streamflow (up to 55%), water temperature (up to 1.9 °C), TSS load (up to 182%), and TP load (up to 74%). This article is protected by copyright. All rights reserved.

Beschreibung: The resolution of a digital elevation model (DEM) is a crucial factor in watershed hydrologic and environmental modeling. DEM resolution can cause significant variability in the representation of surface topography, which further affects quantification of hydrologic connectivity and simulation of hydrologic processes. The objective of this study is to examine the effects of DEM resolution on (1) surface microtopographic characteristics, (2) hydrologic connectivity, and (3) the spatial and temporal variations of hydrologic processes. A puddle-to-puddle (P2P) modeling system was utilized for surface delineation and modeling of the P2P overland flow dynamics, surface runoff, infiltration, and unsaturated flow for nine DEM resolution scenarios of a field plot surface. Comparisons of the nine modeling scenarios demonstrated that coarser DEM resolutions tended to eliminate topographic features, reduce surface depression storage, and strengthen hydrologic connectivity and surface runoff. We found that reduction in maximum depression storage and maximum ponding area was as high as 97.56% and 76.36%, respectively, as the DEM grid size increased from 2 cm to 80 cm. The paired t-test and fractal analysis demonstrated the existence of a threshold DEM resolution (10 cm for the field plot), within which the DEM-based hydrologic modeling was effective and acceptable. The effects of DEM resolution were further evaluated for a larger surface in the Prairie Pothole Region (PPR) subjected to observed rainfall events. It was found that simulations based on coarser resolution DEMs (〉10 m) tended to overestimate ponded areas and underestimate runoff discharge peaks. The simulated peak discharge from the PPR surface reduced by approximately 50% as the DEM resolution changed from 2 m to 90 m. Fractal analysis results elucidated scale dependency of hydrologic and topographic processes. In particular, scale analysis highlighted a unique constant-threshold-power relationship between DEM scale and topographic and hydrologic parameters/variables. Not only does this finding allow one to identify threshold DEM, but also further develop functional relationships for scaling to achieve valid topographic characterization as well as effective and efficient hydrologic modeling. This article is protected by copyright. All rights reserved.

Beschreibung: This paper presents a methodology for hydrograph separation in mountain watersheds, which aims at identifying flow sources among ungauged headwater sub-catchments through a combination of observed streamflow and data on natural tracers including isotope and dissolved solids. Daily summer and bi-daily spring season water samples obtained at the outlet of the Juncal River Basin in the Andes of Central Chile were analyzed for all major ions as well as stable water isotopes, δ 18 O and δD. Additionally, various samples from rain, snow, surface streams and exfiltrating subsurface water (springs), were sampled throughout the catchment. A principal component analysis (PCA) was performed in order to address cross-correlation in the tracer dataset, reduce the dimensionality of the problem and to uncover patterns of variability. Potential sources were identified in a two-component U-space that explains 94% of the observed tracer variability at the catchment outlet. Hydrograph separation was performed through an Informative - Bayesian model. Our results indicate that the Juncal Norte Glacier headwater sub-catchment contributed at least 50% of summer flows at the Juncal River Basin outlet during the 2011–12 water year (a hydrologically dry period in the Region), even though it accounts for only 27% of the basin area. Our study confirms the value of combining solute and isotope information for estimating source contributions in complex hydrologic systems, and provides insights regarding experimental design in high-elevation semi-arid catchments. The findings of this study can be useful for evaluating modeling studies of the hydrological consequences of the rapid decrease in glacier cover observed in this region, by providing insights into the origin of river water in basins with little hydrometeorological information.

Beschreibung: The average flow of Silver Springs, one of the largest magnitude springs in central Florida, declined 32% from 2000 to 2012. The average groundwater head in the springshed declined 0.14 m, and the spring pool altitude increased 0.24 m during the same period. This paper presents a novel explanation of the spring flow recession curve for Silver Springs using Torricelli model, which uses the groundwater head at a sentinel well, the spring pool altitude, and the net recharge to groundwater. The effective springshed area and net recharge (defined as recharge minus groundwater pumping and evapotranspiration) were estimated based on the observed recession slopes for spring flow, groundwater head, and spring pool altitude. The results show that the effective springshed area continuously declined since 1989, and the net recharge declined since the 1970s with a significant drop in 2002. Subsequent to 2002, the net recharge increased modestly but not to the levels prior to the 1990s. The reduction in net recharge was caused by changes in hydroclimatic conditions including precipitation and temperature, along with groundwater withdrawals, which contributed to the declined spring flow.

Beschreibung: Discharge simulation from snow-dominated catchments seems to be an easy task. Any spatially-explicit precipitation-runoff model coupled to a temperature-index snow model generally yields simulations that mimic well the observed daily discharges. The robustness of such models is, however, questionable: in presence of strong annual discharge cycles, small model residuals do not guarantee high explanatory power of the underlying model. This paper proposes a methodology for snow hydrological model identification within a limits-of-acceptability framework, where acceptable model simulations are the ones that reproduce a set of signatures within an a priori specified range. The signatures proposed here namely include the relationship between the air temperature regime and the discharge regime, a new snow hydrology signature that can be readily transferred to other Alpine settings. The discriminatory power of all analyzed signatures is assessed with a new measure of their discriminatory power in the model prediction domain. The value of the proposed snow hydrology signatures and of the limits-of-acceptability approach is demonstrated for the Dischma river in Switzerland, whose discharge shows a strong temporal variability of hydrologic forcing conditions over the last 30 years. The signature-based model identification for this case study leads to the surprising conclusion that the observed discharge data contains a multi-year period that cannot be reproduced with the model at hand. This model-data mismatch might well result from a yet to be identified problem with the discharge observations, which would have been difficult to detect in a classical residual-based model identification approach. Overall, the detailed results for this case study underline the robustness of the limits-of-acceptability approach in the presence of error-prone observations if it is applied in combination with relatively robust signatures. Future work will show whether snow hydrology signatures and their limits-of-acceptability can be regionalized to ungauged catchments, which would make this model selection approach particularly powerful for Alpine environments. This article is protected by copyright. All rights reserved.

Beschreibung: Changes in land use and land cover are major drivers of hydrological alteration in the tropical Andes. However, quantifying their impacts is fraught with difficulties because of the extreme diversity in meteorological boundary conditions, which contrasts strongly with the lack of knowledge about local hydrological processes. Although local studies have reduced data scarcity in certain regions, the complexity of the tropical Andes poses a big challenge to regional hydrological prediction. This study analyses data generated from a participatory monitoring network of 25 headwater catchments covering three of the major Andean biomes ( páramo , jalca , and puna ), and link their hydrological responses to main types of human interventions (cultivation, afforestation and grazing). A paired catchment setup was implemented to evaluate the impacts of change using a “trading space-for-time” approach. Catchments were selected based on regional representativeness and contrasting land use types. Precipitation and discharge have been monitored and analysed at high temporal resolution for a time period between 1 and 5 years. The observed catchment responses clearly reflect the extraordinarily wide spectrum of hydrological processes of the tropical Andes. They range from perennially humid páramos in Ecuador and northern Peru with extremely large specific discharge and baseflows, to highly seasonal, flashy catchments in the drier punas of southern Peru and Bolivia. The impacts of land use are similarly diverse and their magnitudes are a function of catchment properties, original and replacement vegetation, and management type. Cultivation and afforestation consistently affect the entire range of discharges, particularly low flows. The impacts of grazing are more variable, but have the largest effect on the catchment hydrological regulation. Overall, anthropogenic interventions result in increased streamflow variability and significant reductions in catchment regulation capacity and water yield, irrespective of the hydrological properties of the original biome. This article is protected by copyright. All rights reserved.

Beschreibung: Temporal streamflow variability in an inland hydrologic station and temporal trends and frequency changes at three weather stations in a semiarid river basin located in Loess Plateau, China were detected using linear regression, Mann-Kendall (MK) analysis, and wavelet transform methods. Double Cumulative Curve and Ordered Clustering were used to identify the hydrological periods of upper Sang-kan (USK) basin between 1957 and 2012. The results indicate that: (1) precipitation in the USK basin over the study period did not show any trend while the temperature showed a significant increase; (2) streamflow flowing out of the USK basin indicated a significant decrease; (3) two distinct hydrological periods; the “natural period” from 1957 to 1984 and the “human impact period” from 1985 to 2012 were present; and (4) the contribution of climate change and human activities to reduce the streamflow was 36.9% and 63.1%, respectively. The results indicate that human activities may be contributing to a decrease in streamflow in the USK basin. This article is protected by copyright. All rights reserved.

Beschreibung: Hydrochemistry methods were used to decipher the weathering and geochemical processes controlling solute acquisition of river waters in the dry season in the middle Loess Plateau (MLP), one of the most severely eroded areas and turbid riverine systems in the world. River waters were neutral to slightly alkaline with pH varying from 7.6 to 9.6. The total dissolved solids (TDS) decreased from northwest to southeast with a mean value of 804 mg/L, much higher than the global average and other large rivers in China. Ternary diagram showed that river waters were dominated by Na + , HCO 3 - , and Cl - with the main water-type of HCO 3 - -Cl - -Na + . Saturation index (SI) values, Mg 2+ , Ca 2+ , and HCO 3 - analyses indicated the preferential Ca 2+ removal by calcite precipitation. Gibbs plots and stoichiometry plots indicated that the dissolved solutes were mainly derived from rock weathering with minor anthropogenic and atmospheric inputs. Samples in the northwestern basin are also influenced by evaporation. A forward model of mass budget calculation showed that, owing to high soluble characteristics, evaporite dissolution was a major feature of river waters and contributed 41% to the total dissolved cations on average, while carbonate and silicate weathering contributed 28%, and 25% on average, respectively. Beside evaporite dissolution, cation exchange is also responsible for the high concentrations of Na + in river water. Spatial variations showed that evaporite dissolution and silicate weathering were higher in the northern basin, whereas carbonate weathering was higher in the southern basin. Different from most rivers in the world, the physical erosion rates (varying from 117.7 to 4116.6 t/km 2 y) are much higher than the chemical weathering rates (varying from 3.54 to 6.76 t/km 2 y) in the MLP due to the loose structure of loess and poor vegetation in the basin. In the future, studies on comparison of water geochemistry in different seasons and on influence of different types of land use and soil salinization on water geochemistry, denudation rates, and water quality should be strengthened in the MLP. These results shed some lights on processes responsible for modern loess weathering and also indicated the importance of time-series sampling strategy for river water chemistry.

Beschreibung: The Tagus River Basin is an ultimately important water source for hydropower production, urban and agricultural water supply in Spain and Portugal. Growing electricity and water supply demands, over-regulation of the river and construction of new dams, as well as large inter-basin and intra-basin water transfers aggravated by strong natural variability of climate in the catchment have already imposed significant pressures on the river. The substantial reduction of discharge is observed already now, and projected climatic change is expected to alter the water budget of the catchment further. In this study we address the effects of projected climate change on the water resources availability in the Tagus River Basin, and influence of potential changes on hydropower generation of the three important reservoirs in the basin. The catchment scale, process-based eco-hydrological model Soil and Water Integrated Model (SWIM) was set up, calibrated and validated for the entire Tagus River Basin, taking into account fifteen large reservoirs in the catchment. The future climate projections were selected from those generated within the Inter-Sectoral Impact Model Intercomparison Project. They include five bias-corrected climatic datasets for the region, obtained from Global Circulation Models runs under two emissions scenario – moderate and extreme ones, and covered the whole century. The results show a strong agreement among model runs in projecting substantial decrease of discharge of the Tagus River discharge and, consequently, a strong decrease in hydropower production under both future climate scenarios.

Beschreibung: Simulations from hydrological models are affected by potentially large uncertainties stemming from various sources, including model parameters and observational uncertainty in the input/output data. Understanding the relative importance of such sources of uncertainty is essential to support model calibration, validation and diagnostic evaluation, and to prioritise efforts for uncertainty reduction. It can also support the identification of ‘disinformative data’ whose values are the consequence of measurement errors or inadequate observations. Sensitivity Analysis (SA) provides the theoretical framework and the numerical tools to quantify the relative contribution of different sources of uncertainty to the variability of the model outputs. In traditional applications of GSA, model outputs are aggregations of the full set of a simulated variable. For example, many GSA applications use a performance metric (e.g. the root mean squared error) as model output that aggregates the distances of a simulated time series to available observations. This aggregation of propagated uncertainties prior to GSA may lead to a significant loss of information and may cover up local behaviour that could be of great interest. Time-varying sensitivity analysis (TVSA), where the aggregation and SA are repeated at different time-steps, is a viable option to reduce this loss of information. In this work, we use TVSA to address two questions: [1] Can we distinguish between the relative importance of parameter uncertainty versus data uncertainty in time? [2] Do these influences change in catchments with different characteristics? To our knowledge, the results present one of the first quantitative investigation on the relative importance of parameter and data uncertainty across time. We find that the approach is capable of separating influential periods across data and parameter uncertainties, while also highlighting significant differences between the catchments analysed.

Beschreibung: In this paper we assess the performance of the catchment model SIMCAT, to predict nitrate and soluble reactive phosphorus (SRP) concentrations against four monitoring regimes with different spatial and temporal sampling frequencies. The GLUE uncertainty framework is used, along with a general sensitivity analysis to understand relative parameter sensitivity. Improvements to model calibration are explored by introducing more detailed process representation using the INCA water quality model, driven by the E-HYPE hydrological model. The results show how targeted sampling of headwater watercourses upstream of point discharges is essential for calibrating diffuse loads, and can exert a strong influence on the whole-catchment model performance. Further downstream, if the point discharges and loads are accurately represented, then the improvement in the catchment scale model performance is relatively small as more calibration points are added or frequency is increased. The higher order, dynamic model INCA-P, which incorporates sediment and biotic interaction, resulted in improved whole-catchment performance over SIMCAT, although there are still large epistemic uncertainties from land phase export coefficients and runoff. However, the very large sampling errors in routine monitoring make it difficult to invest confidence in the modelling, especially since we know phosphorous transport to be very episodic and driven by high flow conditions for which there are few samples. The environmental modelling community seems to have been stuck in this position for some time, and whilst it is useful to use an uncertainty framework to highlight these issues, it has not widely been adopted, perhaps because there is no clear mechanism to allow uncertainties to influence investment decisions. This raises the question as to whether it might better place a cost on uncertainty, and use this to drive more data collection or improved models, before making investment decisions concerning for example mitigation strategies. This article is protected by copyright. All rights reserved.

Beschreibung: This study was undertaken to evaluate the effects of climatic variability on inter-annual variations in each component of evapotranspiration ( ET ) and the total ET in a temperate coniferous forest in Japan. We conducted eddy covariance flux and meteorological measurements for 7 years and parameterized a one-dimensional multi-layer biosphere-atmosphere model (Kosugi et al., 2006) that partitions ET to transpiration ( Tr ), wet-canopy evaporation ( E wet ), and soil evaporation ( E soil ). The model was validated with the observed flux data. Using the model, the components of ET were estimated for the 7 years. Annual precipitation, ET , Tr , E wet , and E soil over the 7 years were 1536 ± 334 mm, 752 ± 29 mm, 425 ± 37 mm, 219 ± 34 mm, and 108 ± 10 mm, respectively. The maximum inter-annual fluctuation of observed ET was 64 mm with a coefficient of variance (CV) of 2.7%, in contrast to relatively large year-to-year variations in annual rainfall (CV = 20.1%). Tr was related to the vapor pressure deficit, incoming radiation, and air temperature with relatively small inter-annual variations (CV = 8.2%). E soil (CV = 8.6%) was related mainly to the vapor pressure deficit. E wet was related to precipitation with large inter-annual variations (CV = 14.3%) because of the variability in precipitation. The variations in E wet were counterbalanced by the variations in Tr and E soil , producing the small inter-annual variations in total ET . This article is protected by copyright. All rights reserved.

Beschreibung: Backflow, the temporary reversal of discharge at the outlet of a lake, is an important mechanism controlling flow and transport in many connected river-lake systems. This study used statistical methods to examine long-term variations and primary causal factors of backflow from the Yangtze River to a laterally connected, large floodplain lake (Poyang Lake, China). Additionally, the effects of backflow on the lake hydrology were explored using a physically based hydrodynamic model and a particle-tracking model. Although backflow into Poyang Lake occurs frequently, with an average of 16 backflow events per year, and varies greatly in magnitude between years, statistical analysis indicates that both the frequency and magnitude of backflow reduced significantly during 2001-2010 relative to the previous period of 1960-2000. The ratio of Poyang Lake catchment inflows to Yangtze River discharge can be used as an indication of the daily occurrence of backflow, which is most likely to occur during periods when this ratio is lower than 5%. Statistical analysis also indicates that the Yangtze River discharge is the main controlling factor of backflow during July-October, rather than catchment inflows to the lake. Hydrodynamic modeling reveals that, in general, backflow disturbs the normal northward water flow direction in Poyang Lake, and transports mass ~20 km southward into the lake. The effects of backflow on flow direction, water velocities and water levels propagate to virtually its upstream extremity. The current study represents a first attempt to explore backflow and causal factors for a highly dynamic lake-floodplain system. An improved understanding of Poyang Lake backflow is critical for guiding future strategies to manage the lake, its water quality and ecosystem value, given proposals to modify the lake-river connectivity. This article is protected by copyright. All rights reserved.

Beschreibung: This paper reports the first results on δ 18 O and δ 2 H analysis of precipitations, cave drip waters, and groundwaters from sites in Mallorca (Balearic Islands, western Mediterranean), a key region for paleoclimate studies. Understanding the isotopic variability and the sources of moisture in modern climate systems is required to develop speleothem isotope-based climate reconstructions. The stable isotopic composition of precipitation was analyzed in samples collected between March 2012 and March 2013. The values are in the range reported by GNIP Palma station. Based on these results, the local meteoric water line δ 2 H = 7.9 (±0.3) δ 18 O + 10.8 (±2.5) was derived, with slightly lower slope than GMWL. The results help tracking two main sources of air masses affecting the study sites: rain events with the highest δ 18 O values (〉 –5 ‰) originate over the Mediterranean Sea, whereas the more depleted samples (〈 –8 ‰) are sourced in the North Atlantic region. The back trajectory analysis and deuterium excess values, ranging from 0.4 to 18.4 ‰, further support our findings. To assess the isotopic variation across the island, water samples from eight caves were collected. The δ 18 O values range between –6.9 and –1.6 ‰. With one exception (Artà), the isotopic composition of waters in caves located along the coast (Drac, Vallgornera, Cala Varques, Tancada, and Son Sant Martí) indicates Mediterranean-sourced moisture masses. By contrast, the drip water δ 18 O values for inland caves (Campanet, ses Rates Pinyades) or developed under a thick (〉50 m) limestone cap (Artà) exhibit more negative values. A well-homogenized aquifer supplied by rainwaters of both origins is clearly indicated by groundwater δ 18 O values, which show to be within 2.4 ‰ of the unweighted arithmetic mean of –7.4 ‰. Although limited, the isotopic data presented here constitute the baseline for future studies using speleothem δ 18 O records for western Mediterranean paleoclimate reconstructions. This article is protected by copyright. All rights reserved.

Beschreibung: Extremely high precipitation occurs in the Southern Alps of New Zealand, associated with both orographic enhancement and synoptic-scale weather processes. In this study, we test the hypothesis that atmospheric rivers (ARs) are a key driver of floods in the Southern Alps of New Zealand. Vertically-integrated water vapour and horizontal water vapour transport, and atmospheric circulation, are investigated concurrently with major floods on the Waitaki River (a major South Island river). Analysis of the largest eight annual maximum floods between 1979 and 2012 indicates that all are associated with ARs. Geopotential height fields reveal that these ARs are located in slow eastward moving extra-tropical cyclones, with high pressure to the northeast of New Zealand. The confirmation of ARs as a contributor to Waitaki flooding indicates the need for their further exploration to better understand South Island hydrometeorological extremes. This article is protected by copyright. All rights reserved.

Beschreibung: Land cover has been increasingly recognized as an important factor affecting hydrologic processes at the basin and regional level. Therefore, improved understanding of how land cover change affects hydrologic systems is needed for better management of water resources. The objective of this study is to investigate the effects of land cover change on the duration and severity of high and low flows by using the Soil Water Assessment Tool (SWAT) model, Bayesian Model Averaging (BMA) and copulas. Two basins dominated by different land cover in the Ohio River basin are used as study area in this study. Two historic land covers from the 1950s and 1990s are considered as input to the SWAT model, thereby investigating the hydrologic high and low flow response of different land cover conditions of these two basins. The relationships between the duration and severity of both low and high flow are defined by applying the copula method; changes in the frequency of the duration and severity are investigated. The results show that land cover changes affect both the duration and severity of both high and low flows. An increase in forest area leads to a decrease in the duration and severity during both high and low flows, but its impact is highest during extreme flows. The results also show that the land cover changes have had significant influences on changes in the joint return periods of duration and severity of low and high flows. While this study sheds light on the role of land cover change on severity and duration of high and low flow conditions, more studies using various land cover conditions and climate types are required in order to draw more reliable conclusions in future. This article is protected by copyright. All rights reserved.

Beschreibung: The mountain headwater Bow River at Banff, Alberta, Canada was subject to a large flood in June 2013, over which considerable debate has ensued regarding its probability of occurrence. It is therefore instructive to consider what information long term streamflow discharge records provide about environmental change in the Upper Bow River basin above Banff. Though protected as part of Banff National Park, since 1885, the basin has experienced considerable climate and land cover changes, each of which has the potential to impact observations, and hence the interpretations of flood probability. The Bow River at Banff hydrometric station is one of Canada's longest operating reference hydrological basin network stations and so has great value for assessing changes in flow regime over time. Furthermore, the station measures a river that provides an extremely important water supply for Calgary and irrigation district downstream and so is of great interest for assessing regional water security. These records were examined for changes in several flood attributes and to determine whether flow changes may have been related to landscape change within the basin as caused by forest fires, conversion from grasslands to forest with fire suppression, and regional climate variations and/or trends. Floods in the Upper Bow River are generated by both snowmelt and rain-on-snow (ROS) events, the latter type which include floods events generated by spatially and temporally large storms such as occurred in 2013. The two types of floods also have different frequency characteristics. Snowmelt and ROS flood attributes were not correlated significantly with any climate index or with burned area except that snowmelt event duration correlated negatively to the Pacific Decadal Oscillation. While there is a significant negative trend in all floods over the past 100 years, when separated based on generating process, neither snowmelt floods nor large ROS floods associated with mesoscale storms show any trends over time. Despite extensive changes to the landscape of the basin and in within the climate system, the flood regime remains unchanged, something identified at smaller scales in the region but never at larger scales.

Beschreibung: A theoretical, dimensionless rainfall-runoff model was used to simulate the discharge of Wulongdong spring in western Hubei Province, South China. The single parameter (time constant τ) in the model is easy to obtain by fitting the recession rate of the observed hydrographs. The model was scaled by simply matching the total annual flow volume of the model to the observed value. Annual distribution of actual evapotranspiration was embedded in the model input to calculate the accumulated deficit of soil moisture before each rain event. Hourly precipitation input data performed better than daily data, defining τ of 0.85 days, and returning a Nash-Sutcliffe efficiency of 0.89 and the RMSE of 0.07. This model offers an effective way to simulate the discharge of karst springs that respond sensitively to rainfall events. The model parameters of a successful simulation can be used to estimate the recharge area and indicate the intrinsic response time of the basin.

Beschreibung: The change of hydrological regimes may cause impacts on human and natural system. Therefore, investigation of hydrologic alteration induced by climate change is essential for preparing timely proper adaptation to the changes. This study employed 24 climate projections from the Coupled Model Intercomparison Project Phase 5 (CMIP5) under Representative Concentration Pathway (RCP) 4.5 scenario. The climate projections were downscaled at a station-spacing for seven Korean catchments by a statistical downscaling method that preserves a long-term trend in climate projections. Using an ensemble of future hydrologic projections simulated by three conceptual rainfall-runoff models (GR4J, IHACRES, and Sacramento models), we calculated Hydrologic Alteration Factors (HAFs) to investigate degrees of variations in Indicators of Hydrologic Alteration (IHAs) derived from the hydrologic projections. The results showed that the seven catchments had similar trend in terms of the HAFs for the 24 IHAs. Given that more frequent severe floods and droughts were projected over Korean catchments, sound water supply strategies are definitely required to adapt to the alteration of streamflow. A wide range of HAFs between rainfall-runoff models for each catchment was detected by large variations in the magnitude of HAFs with the hydrologic models and the difference could be the hydrologic prediction uncertainty. There were no-consistent tendency in the order of HAFs between the hydrologic models. In addition, we found that the alterations of hydrologic regimes by climate change are smaller as the size of catchment is larger.

Beschreibung: The lower stretch of the Vistula is the most ice-jammed river section on the North European Plain. Since 1982, the structure of hanging dams has been studied by means of a mechanical non-core sampler. In this article, a selected of field research results of the hanging dams structure and the degree of filling of the cross-section with ice obtained during surface ice jam events in the years 1995-2014 are presented, along with an explanation of their causes. Surface ice jams occurred during spring snowmelt surges and ice breakups and also during freeze-up and ice covered periods. Their main cause was changes in the river flow and were also affected by anthropogenic sources. A characteristic feature of the analysed cross-sections was the considerable share of the underhanging ice dam's firm accumulation with ice floes, when the cross-section would be filled with ice in excess of 70%. In most cases, due to low river discharge, there was no substantial flooding damage. This article is protected by copyright. All rights reserved.

Beschreibung: Uruguay has stimulated the development of its forest sector since the promulgation of Forest Law N° 15 939 in December of 1987. Nevertheless, the substitution of natural grasslands with forest plantations for industrial use has raised concerns regarding hydrological processes of groundwater recharge and water consumption involving evapotranspiration. The purpose of this study is to assess the effects of this substitution approach on water resources. Input data were collected from two small experimental watersheds of roughly 100-200 hectares located in western Uruguay. The watersheds are characterized by Eucalyptus Globulus ssp. Maidenni and natural grasslands for cattle use. Total rainfall, stream discharge, rainfall redistribution, soil water content and groundwater level data were collected. Groundwater recharge was estimated from water table fluctuations and from groundwater contributions to base flows. Seasonal and annual water budgets were computed from October of 2006 to September of 2014 to evaluate changes in the hydrological processes. The data show a decrease in annual specific discharge of roughly 17% for mean hydrological years and no conclusive effects on annual groundwater recharge in the forested watershed relative to the reference pasture watershed. Reduced annual specific discharge is equivalent to the mean annual interception. The computed actual annual evapotranspiration is consistent with international catchment measurements. Reduction rates vary seasonally and according to accumulated rainfall and its temporary distribution. The degree of specific discharge decline is particularly high for drier autumns and winters (32 to 28%) when the corresponding rainfall varies from 275 to 400 mm. These results are of relevance for water resources management efforts, as water uses downstream can be affected. These findings, based on a study period dominated by anomalous wet springs and summers and by dry autumns and winters, oppose earlier results based on 34 years of rainfall and discharge data drawn from Uruguayan large basins. Copyright © 2016 John Wiley & Sons, Ltd.

Beschreibung: River water temperature is a key physical variable controlling several chemical, biological and ecological processes. Its reliable prediction is a main issue in many environmental applications, which however is hampered by data scarcity, when using data-demanding deterministic models, and modeling limitations, when using simpler statistical models. In this work we test a suite of models belonging to air2stream family (Toffolon and Piccolroaz, 2015), which are characterized by a hybrid formulation that combines a physical derivation of the key equation with a stochastic calibration of parameters. The air2stream models rely solely on air temperature and streamflow, and are of similar complexity as standard statistical models. The performances of the different versions of air2stream in predicting river water temperature are compared with those of the most common statistical models typically used in the literature. To this aim, a dataset of 38 Swiss rivers is used, which includes rivers classified into four different categories according to their hydrological characteristics: low-land natural rivers, lake outlets, snow-fed rivers, and regulated rivers. The results of the analysis provide practical indications regarding the type of model that is most suitable to simulate river water temperature across different time scales (from daily to seasonal) and for different hydrological regimes. A model intercomparison exercise suggests that the family of air2stream hybrid models generally outperforms statistical models, while cross-validation conducted over a 30-year period indicates that they can be suitably adopted for long-term analyses.

Beschreibung: As large, high-severity forest fires increase and snowpacks become more vulnerable to climate change across the western US, it is important to understand post-fire disturbance impacts on snow hydrology. Here, we examine, quantify, parameterize, model, and assess the post-fire radiative forcing effects on snow to improve hydrologic modeling of snow-dominated watersheds having experienced severe forest fires. Following a 2011 high-severity forest fire in the Oregon Cascades, we measured snow albedo, monitored snow and micrometeorological conditions, sampled snow surface debris, and modeled snowpack energy and mass balance in adjacent burned and unburned forest sites. For three winters following the fire, charred debris in the burned forest reduced snow albedo, accelerated snow albedo decay, and increased snowmelt rates thereby advancing the date of snow disappearance compared to the unburned forest. We demonstrate a new parameterization of post-fire snow albedo as a function of days-since-snowfall and net snowpack energy balance using an empirically-based exponential decay function. Incorporating our new post-fire snow albedo decay parameterization in a spatially-distributed energy and mass balance snow model, we show significantly improved predictions of snow cover duration and spatial variability of snow water equivalent across the burned forest, particularly during the late snowmelt period. Field measurements, snow model results, and remote sensing data demonstrate that charred forests increase the radiative forcing to snow and advance the timing of snow disappearance for several years following fire. This article is protected by copyright. All rights reserved.

Beschreibung: A spatially distributed representation of basin hydrology and transport processes in hydrologic models facilitates the identification of critical source areas and the placement of management and conservation measures. Floodplains are critical landscape features that differ from neighboring uplands in terms of their hydrological processes and functions. Accordingly, an important step in watershed modeling is the representation of floodplain and upland areas within a watershed. The aim of this study is (1) to evaluate four floodplain-upland delineation methods that use readily available topographic data (topographic wetness index, slope position, uniform flood stage, and variable flood stage) with regard to their suitability for hydrological models and (2) to introduce an evaluation scheme for the delineated landscape units. The methods are tested in three U.S. watersheds ranging in size from 334 to 629 km 2 with different climatic, hydrological and geomorphological characteristics. Evaluation of the landscape delineation methods includes visual comparisons, error matrices (i.e. cross-tabulations of delineated versus reference data), and geometric accuracy metrics. Reference data was obtained from SSURGO (Soil Survey Geographic database) and FEMA (Federal Emergency Management Agency) flood maps. Results suggest that the slope position and the variable flood stage method work very well in all three watersheds. Overall percentages of floodplain and upland areas allocated correctly were obtained by comparing delineated and reference data. Values range from 83 to 93 % for the slope position and from 80 to 95 % for the variable flood stage method. Future studies will incorporate these two floodplain-upland delineation methods into the subwatershed-based hydrologic model SWAT to improve the representation of hydrological processes within floodplain and upland areas. This article is protected by copyright. All rights reserved.

Beschreibung: There has been a great deal of research interest regarding changes in flow path/runoff source with increases in catchment area. However, there have been very few quantitative studies taking subscale variability and convergence of flow path/runoff source into account, especially in relation to headwater catchments. This study was performed to elucidate how the contributions and discharge rates of subsurface water (water in the soil layer) and groundwater (water in fractured bedrock) aggregate and change with catchment area increase, and to elucidate whether the spatial variability of the discharge rate of groundwater determines the spatial variability of stream discharge or groundwater contribution. The study area was a 5-km 2 forested headwater catchment in Japan. We measured stream discharge at 113 points and water chemistry at 159 points under base flow conditions. End-member mixing analysis (EMMA) was used to separate stream water into subsurface water and groundwater. The contributions of both subsurface water and groundwater had large variability below 1 km 2 . The contribution of subsurface water decreased markedly, while that of groundwater increased markedly, with increases in catchment area. The specific discharge of subsurface water showed a large degree of variability and decreased with catchment area below 0.1 km 2 , becoming almost constant above 0.1 km 2 . The specific discharge of groundwater showed large variability below 1 km 2 and increased with catchment area. These results indicated that the variabilities of stream discharge and groundwater contribution corresponded well with the variability of the discharge rate of groundwater. However, below 0.1 km 2 , it was necessary to consider variations in the discharge rates of both subsurface water and groundwater.

Beschreibung: In arctic and sub-arctic environments, mercury (Hg), and more specifically toxic methylmercury (MeHg) is of growing concern to local communities because of its accumulation in fish. In these regions, there is particular interest in the potential mobilisation of atmospherically-deposited Hg sequestered in permafrost that is thawing at unprecedented rates. Permafrost thaw and the resulting ground surface subsidence transforms forested peat plateaus into treeless and permafrost-free thermokarst wetlands where inorganic Hg released from the thawed permafrost and draining from the surrounding peat plateaus may be transformed to MeHg. This study begins to characterise the spatial distribution of MeHg in a peat plateau-thermokarst wetland complex, a feature that prevails throughout the wetland-dominated southern margin of thawing discontinuous permafrost in Canada's Northwest Territories. We measured pore water total Hg, MeHg, dissolved organic matter (DOM) characteristics and general water chemistry parameters to evaluate the role of permafrost thaw on the pattern of water chemistry. A gradient in vegetation composition, water chemistry and DOM characteristics followed a toposequence from the ombrotrophic bogs near the crest of the complex to the poor fens at its downslope margins. We found that pore waters in poor fens contained elevated levels of MeHg and the water draining from these features had dissolved MeHg concentrations 4.5 to 14.5 times higher than the water draining from the bogs. It was determined through analysis of historical aerial images that the poor fens in the toposequence had formed relatively recently (early 1970s) as a result of permafrost thaw. Differences between the fens and bogs are likely to be due to their differences in groundwater function and this suggests that permafrost thaw in this landscape can result in hotspots for Hg methylation that are hydrologically connected to downstream ecosystems.

Beschreibung: The objective of this work was to build a prognostic water flow model and potentially toxic elements (lead, cadmium, zinc) transport model in the unsaturated zone. Research was conducted in the catchment area of Kosnica regional wellfield, where the unsaturated zone is characterised by Fluvisol. Lower sorption capacities were determined in the first horizons for all three potentially toxic elements. Correlation coefficient of the measured and simulated values of tracer concentration is 0.58 for the AC horizon and 0.84 for the 2C/C1 horizon. Based on calibrated water flow and transport parameters, a prognostic water flow model and potentially toxic elements (lead, cadmium, zinc) transport model in the unsaturated zone was built. In case of an accidental spill of potentially toxic elements with concentrations of 1000 mg/L, the risk of contamination of the aquifer is present.

Beschreibung: Headwater storage-discharge remains one of the least understood processes, and there is renewed interest in the S-Q relation. How well can the S-Q relation be interpreted mechanistically using geometric factors? In this paper, the hillslope storage Boussinesq (hsB) and hillslope storage kinematic wave (hsKW) equation were adopted to guide the theoretical derivations. Analytical solutions were derived based on the hsKW equation for nine idealized hillslope aquifers, which were subdivided into two groups, i.e., hillslope aquifers with exponential hillslope width function (C1) and hillslope aquifers with Gaussian hillslope width function (C2). We found that analytical expressions of the S-Q relation can be derived for C1 hillslope aquifers. For more compound hillslope aquifers, i.e., C2, no explicit S-Q relation can be obtained. The whole subsurface recession after a rainstorm is simulated by applying the initial saturation condition. We found that the simulated S-Q processes can be characterized by a two-phase recession, i.e., quick and slow recession. The time (t b ) at the dividing point of the quick and slow recessions depends on the geometric factors, such as the plan and profile curvature. In the quick recession for C1, many of the S-Q curves can be described as linear or quasi-linear functions, which indicate that linear reservoir models can be applied approximately for recession simulations. However, during the slow recession phase of C1 and during the whole recession of C2, the S-Q relations are highly non-linear. Finally, we compared the hsKW and hsB models for simulating subsurface water recession after a rainstorm event in a real-world headwater catchment (G5) in China. Through comparison of the recession slope curves, we found that the simulated results of the models employing the Gaussian hillslope width function match the observed hydrograph. The results indicated that appropriate organization of the hillslope geometric factors enhances our ability to make storage-discharge predictions. This article is protected by copyright. All rights reserved.

Beschreibung: Spatial and temporal variation in wet canopy conditions following precipitation events can influence processes such as transpiration and photosynthesis, which can be further enhanced as upper canopy leaves dry more rapidly than the understory following each event. As part of a larger study aimed at improving land-surface modeling of evapotranspiration processes in wet tropical forests, we compared transpiration among trees with exposed and shaded crowns under both wet and dry canopy conditions in central Costa Rica, which has an average 4200 mm annual rainfall. Transpiration was estimated for 5 months using 43 sap flux sensors in 8 dominant, 10 midstory, and 8 suppressed trees in a mature forest stand surrounding a 40-m tower equipped with micrometeorological sensors. Dominant trees were 13% of the plot's trees and contributed around 76% to total transpiration at this site, whereas midstory and suppressed trees contributed 18% and 5%, respectively. After accounting for vapor pressure deficit and solar radiation, leaf wetness was a significant driver of sap flux, reducing it by as much as 28%. Under dry conditions, sap flux rates ( J s ) of dominant trees were similar to midstory trees and were almost double that of suppressed trees. On wet days, all trees had similarly low J s . As expected, semi-dry conditions (dry upper canopy) led to higher J s in dominant trees than midstory, which had wetter leaves, but semi-dry conditions only reduced total stand transpiration slightly and did not change the relative proportion of transpiration from dominant and midstory. Therefore, models that better capture forest stand wet-dry canopy dynamics and individual tree water use strategies, are needed to improve accuracy of predictions of water recycling over tropical forests.

Beschreibung: Snow and glacial melt processes are an important part of the Himalayan water balance. Correct quantification of melt runoff processes is necessary to understand the region's vulnerability to climate change. This paper describes in detail an application of conceptual GR4J hydrological model in the Tamor catchment in Eastern Nepal using typical elevation band and degree-day factor approaches to model Himalayan snow and glacial melt processes. The model aims to provide a simple model that meets most water planning applications. The paper contributes a model conceptualization (GR4JSG) that enables coarse evaluation of modeled snow extents against remotely sensed MODIS snow extent. Novel aspects include the glacial store in GR4JSG and examination of how the parameters controlling snow and glacial stores correlate with existing parameters of GR4J. The model is calibrated using a Bayesian Monte Carlo Markov Chain method against observed streamflow for one glaciated catchment with reliable data. Evaluation of the modelled streamflow with observed streamflow gave Nash Sutcliffe Efficiency of 0.88 and Percent Bias of 〈4%. Comparison of the modelled snow extents with MODIS gave R 2 of 0.46, with calibration against streamflow only. The contribution of melt runoff to total discharge from the catchment is 14-16% across different experiments. The model is highly sensitive to rainfall and temperature data, which suffer from known problems and biases, for example due to stations being located predominantly in valleys and at lower elevations. Testing of the model in other Himalayan catchments may reveal additional limitations. This article is protected by copyright. All rights reserved.

Beschreibung: The impact of climate change on the behaviour of intensity-duration-frequency (IDF) curves is critical to the estimation of design storms, and thus to the safe design of drainage infrastructure. The present study develops a regional time trend methodology that detects the impact of climate change on extreme precipitation from 1960 to 2010. The regional time trend linear regression method is fitted to different durations of annual maximum precipitation intensities derived from multiple sites in Ontario, Canada. The results show the relationship between climate change and increased extreme precipitation in this province. The regional trend analysis demonstrates, under nonstationary conditions arising from climate change, that the intensity of extreme precipitation increased decennially between 1.25 per cent for the 30 min storm and 1.82 per cent for the 24-hour storm. A comparison of the results with a regional Mann-Kendall test validates the found regional time-trend results. The results are employed to extrapolate the IDF curves temporally and spatially for future decades across the province. The results of the regional time trend assessment helps with the establishment of new safety margins for infrastructure design in Ontario.

Beschreibung: Changes in potential evapotranspiration and surface runoff can have profound implications for hydrological processes in arid and semiarid regions. In this study, we investigated the response of hydrological processes to climate change in Upper Heihe River Basin (UHRB) in Northwest China for the period from 1981 to 2010. We used agronomic, climatic and hydrological data to drive the SWAT (Soil and Water Assessment Tool) model for changes in potential evapotranspiration (ET 0 ) and surface runoff and the driving factors in the study area. The results showed that increasing autumn temperature increased snow melt, resulting in increased surface runoff, especially in September and October. The spatial distribution of annual runoff was different from that of seasonal runoff, with the highest runoff in Yeniugou River, followed by Babaohe River and then the tributaries in the northern of the basin. There was no evaporation paradox at annual and seasonal time scales, and annual ET 0 was driven mainly by wind speed. ET 0 was driven by relative humidity in spring, sunshine hour duration in autumn, and both sunshine hour duration and relative humility in summer. Surface runoff was controlled by temperature in spring and winter and by precipitation in summer (flood season). Although surface runoff increased in autumn with increasing temperature, it depended on rainfall in September and on temperature in October and November. This article is protected by copyright. All rights reserved.

Beschreibung: As a consequence of the remote location of the Andean páramo is knowledge on their hydrologic functioning limited, notwithstanding this alpine tundra ecosystem act as water towers for a large fraction of the society. Given the harsh environmental conditions in this region is year-round monitoring cumbersome, and it would be beneficially if the monitoring needed for the understanding of the rainfall-runoff response could be limited in time. To identify the hydrological response and the effect of temporal monitoring a nested (n = 7) hydrological monitoring network was set up in the Zhurucay catchment (7.53 km 2 ), south Ecuador. The research questions were: (1) can event sampling provide similar information in comparison to continuous monitoring, and (2) if so, how many events are needed to achieve a similar degree of information? A subset of 34 rainfall runoff events was compared to monthly values derived from a continuous monitoring scheme from December 2010 to November 2013. Land cover and physiographic characteristics were correlated with eleven hydrological indices. Results show that despite some distinct differences between event and continuous sampling, both datasets reveal similar information; more in particular the monitoring of a single event in the rainy season provides the same information as continuous monitoring, while during the dry season 10 events ought to be monitored. This article is protected by copyright. All rights reserved.

Beschreibung: We apply the process-based, distributed TOPKAPI-ETH glacio-hydrological model to glacierized catchment (19% glacierized) in the semiarid Andes of central Chile. The semiarid Andes provides vital freshwater resources to valleys in Chile and Argentina, but only few glacio-hydrological modelling studies have been conducted and its dominant hydrological processes remain poorly understood. The catchment contains two debris-free glaciers reaching down to 3900 m asl (Bello and Yeso glaciers) and one debris-covered avalanche-fed glacier reaching to 3200 m asl (Piramide Glacier). Our main objective is to compare the mass balance and runoff contributions of both glacier types under current climatic conditions. We use a unique dataset of field measurements collected over two ablation seasons combined with the distributed TOPKAPI-ETH model that includes physically-oriented parameterizations of snow and ice ablation, gravitational distribution of snow, snow albedo evolution and the ablation of debris-covered ice. Model outputs indicate that while the mass balance of Bello and Yeso glaciers is mostly explained by temperature gradients, the Piramide Glacier mass balance is governed by debris thickness and avalanches and has a clear non-linear profile with elevation as a result. Despite the thermal insulation effect of the debris cover, the mass balance and contribution to runoff from debris-free and debris-covered glaciers is similar in magnitude, mainly due to elevation differences. However, runoff contributions are distinct in time and seasonality with ice melt starting approximately four weeks earlier from the debris-covered glacier, what is of relevance for water resources management. At the catchment scale, snowmelt is the dominant contributor to runoff during both years. However, during the driest year of our simulations, ice melt contributes 42 ± 8% and 67 ± 6% of the annual and summer runoff, respectively. Sensitivity analyses show that runoff is most sensitive to temperature and precipitation gradients, melt factors and debris cover thickness. This article is protected by copyright. All rights reserved.

Beschreibung: In northern regions, river ice-jam flooding can be more severe than open-water flooding causing property and infrastructure damages, loss of human life, and adverse impacts on aquatic ecosystems. Very little has been done to assess the risk induced by ice-related floods since most risk assessments are limited to open-water floods. The specific objectives of this study is to incorporate ice-jam numerical modelling tools (e.g. RIVICE, Monte-Carlo simulation) into flood hazard and risk assessment along the Peace River at the Town of Peace River (TPR) in Alberta, Canada. Adequate historical data for different ice-jam and open-water flooding events were available for this study site and were useful in developing ice-affected stage frequency curves. These curves were then applied to calibrate a numerical hydraulic model which simulated different ice jams and flood scenarios along the Peace River at the TPR. A Monte-Carlo analysis was then carried out to acquire an ensemble of water level profiles to determine the 1:100 and 1:200 year AEP (annual exceedance probability) flood stages for the TPR. These flood stages were then used to map flood hazard and vulnerability of the TPR. Finally, the flood risk for a 200-year return-period was calculated to be an average of $27/m 2 /a ($/m 2 /a corresponds to a unit of annual expected damages or risk). This article is protected by copyright. All rights reserved.

Beschreibung: Hydrogeologic field work in remote settings is often challenging: assessing spring behavior and aquifer characteristics can be expensive in both time commitment and resources needed to assess these systems. In this study, we document the hydrology and geochemistry of 47 perennial karst springs in the Kaweah River, a mountain river basin in the Sierra Nevada, California. After preliminary hydrogeochemical characterization and grouping, select springs were continuously monitored to further assess aquifer characteristics in each group. Later, in areas without previous dye tracing work, traces were conducted to establish connections between large sinking streams and springs. The springs have a wide range of inter- and intra-spring variability in discharge and geochemistry. We assessed this variability by performing statistical comparisons of spring chemistry and principal components analysis of all measured variables. Results show that springs can be divided into two distinct groups: high elevation springs of the Mineral King Valley, and lower elevation springs throughout the rest of the basin. Continuous discharge, temperature, and specific conductivity data from four springs (two from each group) were then used to characterize the hydrograph recession behavior of springs in each group. Both groups showed statistically similar baseflow recession slopes, suggesting that both groups contain baseflow storage compartments with similar hydrogeologic properties. The biggest difference between each group is the variability in amount of water remaining in the aquifer during baseflow conditions. High elevation springs have lower baseflow discharges, relative to peak flow, than lower elevation springs, despite of the fact that more precipitation falls at higher elevation. This is likely caused by differences in the amount of soil and epikarst storage, which are related to recent geomorphic events: high elevation aquifers were glaciated as recent as 41 thousand years ago (kya), while there is no evidence that low elevation aquifers were glaciated. As a result, lower elevations have developed thicker soils, weathered bedrock, and epikarst. This article is protected by copyright. All rights reserved.

Beschreibung: Recent flood events in Canada have led to speculation that changes in flood behaviour are occurring; these changes have often been attributed to climate change. This paper examines flood data for a collection of 132 gauging stations in Canada. All of these watersheds are part of the Canadian Reference Hydrometric Basin Network (RHBN), a group of gauging stations specifically assembled to assist in the identification of the impacts of climate change. The RHBN stations are considered to have good quality data and were screened to avoid the influences of regulation, diversions, or land use change. Daily flow data for each watershed are used to derive a peaks over threshold (POT) dataset. Several measures of flood behaviour are examined based on the POT data, which afford a more in-depth analysis of flood behaviour than can be obtained using annual maxima data. Analysis is conducted for four time periods ranging from 50 to 80 years in duration; the latter period results in a much smaller number of watersheds that have data for the period. The changes in flood responses of the watersheds are summarized by grouping the watersheds by size (small, medium, and large) and also by hydrologic regime (nival, mixed and pluvial). The results provide important insights into the nature of the changes that are occurring in flood regimes of Canadian rivers, which include more flood exceedences, reduced maximum flood exceedence magnitudes for snowmelt events, and earlier flood events. This article is protected by copyright. All rights reserved.

Beschreibung: In the last decades, human activity has been contributing to climate change that is closely associated with an increase in temperatures, increase in evaporation, intensification of extreme dry and wet rainfall events, and widespread melting of snow and ice. Understanding the intricate linkage between climate warming and the hydrological cycle is crucial for sustainable management of groundwater resources, especially in a vulnerable continent like Africa. This study investigates the relationship between climate-change drivers and potential groundwater recharge (PGR) patterns across Africa for a long-term record (1960–2010). Water-balance components were simulated by using the PCR-GLOBWB model and were reproduced in both gridded maps and latitudinal trends that vary in space with minima on the Tropics and maxima around the Equator. Statistical correlations between temperature, storm occurrences, drought, and PGR were examined in six climatic regions of Africa. Surprisingly, different effects of climate-change controls on PGR were detected as a function of latitude in the last three decades (1980–2010). Temporal trends observed in the Northern Hemisphere of Africa reveal that the increase in temperature is significantly correlated to the decline of PGR, especially in the Northern Equatorial Africa. The climate indicators considered in this study were unable to explain the alarming negative trend of PGR observed in the Sahelian region, even though the Standardized Precipitation-Evapotranspiration Index (SPEI) values report a 15% drought stress. On the other hand, increases in temperature have not been detected in the Southern Hemisphere of Africa, where increasing frequency of storm occurrences determine a rise of PGR, particularly in southern Africa. Time analysis highlights a strong seasonality effect while PGR is in-phase with rainfall patterns in the summer (Northern Hemisphere) and winter (Southern Hemisphere) and out-of-phase during the fall season. This study helps to elucidate the mechanism of the processes influencing groundwater resources in six climatic zones of Africa, even though modeling results need to be validated more extensively with direct measurements in future studies. This article is protected by copyright. All rights reserved.

Beschreibung: A general method is proposed which measures the increase in uncertainty when sampling effort is reduced in sediment fingerprinting. The method gives quantitative measures of how reduced sampling of material in one of the source areas, and/or of suspended sediment in streams, increases the uncertainties in the proportions of sediment contributed from the sources. Since the proportions of sediment contributed by the source areas must add to one, standard errors of the estimated proportions cannot be used as the usual measures of uncertainty: the paper uses instead the volume of the joint 95% confidence region for the estimated proportions. The paper shows how the uncertainty in this volume changes as numbers of suspended sediment samples, and the numbers of samples collected from cropped fields, are reduced by successive steps from 24 ( 20 , in the case of cropped fields) to 16 , 12 , 8 , 4 and 2 samples. As expected, uncertainty increases rapidly as the number of samples - whether of suspended sediment or from cropped fields - is reduced drastically. The pattern of increasing uncertainty is similar both for reductions in suspended sediment sampling, and for reduced sampling from cropped areas. When the number of suspended sediment samples, and the number of samples from cropped fields, are reduced to the same values, the increase in uncertainty from fewer suspended sediment samples was always slighly greater than the increased uncertainty from the reduced sampling of cropped areas, although this finding took no account of differences in the costs of field sampling and laboratory analysis. This article is protected by copyright. All rights reserved.

Beschreibung: Measurements of δ 2 H and δ 18 O composition of pore waters in saturated and unsaturated soil samples are routinely performed in hydrological studies. A variety of in-situ and lab-based pore water extraction methods for the analysis of the stable isotopes of water now exist. While some have been used for decades (e.g. cryogenic vacuum extraction) others are relatively new, such as direct vapor equilibration or the microwave extraction technique. Despite their broad range of application, a formal and comprehensive intercomparison of soil water extraction methods for stable isotope analysis is lacking and long overdue. Here we present an intercomparison among five commonly used lab-based pore water extraction techniques (high pressure mechanical squeezing, centrifugation, direct vapor equilibration, microwave extraction, and cryogenic extraction). We applied these extraction methods to two physicochemically different soil types that were dried and rewetted with water of known isotopic composition at three different water contents. Our results showed that the extraction approach can have a significant effect on pore water isotopic composition as all methods exhibited significant deviations from the spiked reference water, depending secondarily on the soil type and soil water content. Most pronounced, cryogenic water extraction showed large deviations from the spiked reference water, whereas mechanical squeezing and centrifugation provided results closest to the spiked water for both soil types. We also compared results for each extraction method – where liquid water was obtained – on both an OA-ICOS and IRMS. Differences between these two analytical instruments were negligible for these organic compound-free waters. We suggest that users of soil water extraction approaches carefully choose an extraction technique that is suitable for the specific research question, adapted to the dominant soil type and water content of the study. This article is protected by copyright. All rights reserved.

Beschreibung: Frequent algal blooms in surface water bodies caused by nutrient loading from agricultural lands are an ongoing problem in many regions globally. Tile drains beneath poorly and imperfectly drained agricultural soils have been identified as a key pathway for phosphorus (P) transfer. Two tile drains in an agricultural field with sandy loam soil in southern Ontario, Canada were monitored over a 28-month period to quantify discharge and the concentrations and loads of dissolved reactive P (DRP) and total P (TP) in their effluent. This paper characterizes seasonal differences in runoff generation and P export in tile drain effluent and relates hydrologic and biogeochemical responses to precipitation inputs and antecedent soil moisture conditions. The generation of runoff in tile drains was only observed above a clear threshold soil moisture content (~0.49 m 3• m -3 in the top 10 cm of the soil; above field capacity and close to saturation), indicating that tile discharge responses to precipitation inputs are governed by the available soil-water storage capacity of the soil. Soil moisture content approached this threshold throughout the non-growing season (October – April), leading to runoff responses to most events. Concentrations of P in effluent were variable throughout the study but were not correlated with discharge (p 〉 0.05). However, there were significant relationships between discharge volume (mm) and DRP and TP loads (kg ha -1 ) for events occurring over the study period (R 2  〉 =0.49, p = 〈0.001). This research has shown that the hydrologic and biogeochemical responses of tile drains in a sandy loam soil can be predicted to within an order of magnitude from simple hydrometric data such as precipitation and soil moisture once baseline conditions at a site have been determined. This article is protected by copyright. All rights reserved.

Beschreibung: Gravel road surfaces can be a major source of fine sediment to streams, yet their contribution to channel reach sediment balances remains poorly documented. To quantify the input of road surface material and to compare this input to natural sediment sources at the reach-scale, suspended sediment dynamics were examined and a 16 month sediment balance was developed for a ~35 channel-width (approx. 425 m) reach of the Honna River, a medium-size, road-affected stream located in coastal British Columbia. Of the 105 +/- 33 tons of suspended material passing through the reach, 18% ± 6% was attributed to the road surface. The high availability of sediment on the road surface appears to limit hysteresis in road runoff. During rainstorms that increase streamflow, road surface material composed 0.5% to 15% of sediment inputs during relatively dry conditions from April to the end of September and 5% to 70% through wetter conditions from October to the end of March, but our data do not show evidence of major sediment accumulation on the river bed in the reach. A comparison of modeled sediment production on the road surface to observed yields from drainage channels suggests that (1) during low intensity rainfall, ditches and drainage channels may trap sediment from road runoff, which is subsequently released during events of greater intensity and/or (2) production models do not effectively describe processes, such as deposition or erosion of sediment in ditches, which control sediment transport and delivery. Our findings further emphasize the risk of unpaved roads in polluting river systems, and highlight the continued need for careful road design and location away from sensitive aquatic environments. This article is protected by copyright. All rights reserved.

Beschreibung: Radar accuracy in estimating qualitative precipitation estimation at distances larger than 120 km degrades rapidly due to increased volume coverage and beam height. The performance of the recently upgraded dual-polarized technology to the NEXRAD network and its capabilities are in need of further examination, as improved rainfall estimates at large distances would allow for significant hydrological modeling improvements. Parameter based methods were applied to radars from St. Louis (KLSX) and Kansas City (KEAX), Missouri, USA, to test the precision and accuracy of both dual- and single-polarized parameter estimations of precipitation at large distances. Hourly aggregated precipitation data from terrestrial-based tipping buckets provided ground-truthed reference data. For all KLSX data tested, an R(Z,ZDR) algorithm provided the smallest absolute error (3.7 mm hr -1 ) and root-mean-square-error (45%) values. For most KEAX data, R(ZDR,KDP) and R(KDP) algorithms performed best, with RMSE values of 37%. With approximately 100 hours of precipitation data between April and October of 2014, nearly 800 and 400 mm of precipitation was estimated by radar precipitation algorithms but was not observed by terrestrial-based precipitation gauges for KLSX and KEAX, respectively. Additionally, nearly 30 and 190 mm of measured precipitation observed by gauges were not deteced by the radar rainfall estimates from KLSX and KEAX, respectively. Results improve understanding of radar based precipitation estimates from long ranges thereby advancing applications for hydrometeorological modeling and flood forecasting. This article is protected by copyright. All rights reserved.

Beschreibung: Different hydrological models of sediment entrainment, infiltration, overland flow production, soil loss, landslide occurrence, among others, need rainfall events data. The Minimum Inter-event Time (MIT) used to separate rainfall events affects their properties. Therefore, it is highly important their correct definition. Various event definitions are commonly used in hydrology and a high variety of MIT is applied. In this work an easy method is proposed for deciding the proper MIT in a certain place when dealing with hourly rainfall data series. This new methodology is based on the scale invariance properties of rainfall, analysed with both multifractal and Self-Organized Criticality theories. The similarity between some parameters from both analyses allows picking out the more appropriate MIT for the hourly rainfall data sets analysed, in Malaga (Southern Spain) and Bullileo (Central Chile). The method can be applied for different rainfall time resolutions according to the purpose of the model that needs the rain event information. This article is protected by copyright. All rights reserved.

Beschreibung: The operational time distribution (OTD) defines the time for bed-load sediment spent in motion, which is needed to characterize the random nature of sediment transport. This study explores the influence of bed clusters and size gradation on OTD for non-uniform bed-loads. First, both static and mobile bed armoring experiments were conducted in laboratorial flumes to monitor the transport of mixed sand/gravel sediments. Only in the mobile armoring experiment did apparent bed clusters develop, due to stable feeding and a longer transport period. Secondly, a generalized subordinated advection (GSA) model was applied to quantify the observed dynamics of tracer particles. Results show that for the static armor layer (without sediment feed), the best-fit OTD assigns more weight to the large displacement of small particles, likely due to the size-selective entrainment process. The capacity coefficient in the GSA model, which affects the width of the OTD, is space dependent only for small particles whose dynamics can be significantly affected by larger particles and whose distribution is more likely to be space dependent in a mixed sand and gravel system. However, the OTD for the mobile armor layer (with sediment recirculation) exhibited longer tails for larger particles. This is because the trailing edge of larger particles is more resistant to erosion, and their leading front may not be easily trapped by self-organized bed clusters. The strong interaction between particle-bed may cause the capacity coefficient to be space-dependent for bed-load transport along mobile armor layers. Therefore, the combined laboratory experiments and stochastic model analysis show that the OTD may be affected more by particle-bed interactions (such as clusters) than by particle-particle interactions (e.g., hiding and exposing), and that the GSA model can quantify mixed-size sand/gravel transport along river beds within either static or mobile armor layers.

Beschreibung: An imaging-based automated Large-Scale Particle Image Velocimetry (LSPIV) system for flash floods monitoring is developed and deployed in a mountainous stream in the Longchi Catchment, Chengdu, China. This system is built from a low-cost Raspberry Pi board-level computer with a camera module, which can acquire continuous images/videos automatically at programmed intervals. The Minimum Quadratic Difference (MQD) algorithm tracks surface patterns as flow tracers to estimate the distribution of surface velocities. Meanwhile, a stereo imaging-based ‘virtual pole’ method has been developed to reconstruct the three-dimensional topography with a stereo digital camera, and cross-sectional bathymetry can be generated without manual surveying. The varying water stage and water surface gradient, which are critical parameters that affect image rectification and surface velocity measurements, can also be directly resolved by applying the two imaging modules together. Discharge can then be estimated with the velocity-area method through selected cross sections. A flash flood occurred between July 24-25, 2014 is selected for analysis. Water surface level reconstructed from image processing was validated with marked water levels and a good agreement was found with a root mean square error of 3.7 cm. Discharge recorded during the flood recession process ranged from 3.5 m 3 /s to 27 m 3 /s approximately. The rating curve obtained can be well described by a power function and the linear regression suggested a Manning's n roughness coefficient of 0.18 of one specific cross section. Some limitations of the presented LSPIV system are also put forward and possible solutions are provided for future improvements. With these proposed upgrades, the system can provide valuable datasets of flash floods in steep mountainous streams, which are critically needed for improving our understanding and modeling of many hydrological processes associated with flood generation, propagation and erosion, as well as for real-time forecasting.

Beschreibung: Quantifying streamflow sources within remote, data scarce, Boreal catchments remains a significant challenge due to limited accessibility and complex, flat topography. The coupled use of hydrometric and isotopic data has previously been shown to facilitate quantification of streamflow sources, but application has generally been limited to small basins and short time scales. A lumped flow-isotope model was used to estimate contributing streamflow sources (soil, ground, and wetland water) over a four year period in two large nested headwater catchments (Sapochi and Odei Rivers) in northern Manitoba, Canada. On average, the primary streamflow source was estimated as soil water (60%) in the Sapochi River, and groundwater (54%) in the Odei River. A strong seasonal influence was observed: soil water was the primary streamflow source in summer, changing to groundwater and wetlands during the winter. Interannual variability in streamflow sources was strongly linked to the presence or absence of late summer rainfall. The greatest uncertainties in source quantification were identified during the spring freshets and high precipitation events, and hence, simulations may be improved through explicit representation of the soil freeze/thaw process and data collection during this period. Assessment of primary streamflow components and qualitative uncertainty estimation using coupled isotope-flow modelling is an effective method for first order identification of streamflow sources in data sparse remote headwaters. This article is protected by copyright. All rights reserved.

Beschreibung: Rivers of South and Southeast Asia disgorge large suspended sediment loads, reflecting exceptionally high rates of erosion promoted by natural processes (tectonic and climatic) and anthropogenic (land-use change) activities that are characteristic of the region. While particulate carbon and nitrogen fluxes have been characterized in some large Asian rivers, less is known about the headwater systems where much sediment and organic material are initially mobilized. This study, conducted in the 74-km 2 Mae Sa Experimental Catchment in northern Thailand, shows that the Sa River is an important source for particulate organic carbon (POC) and particulate organic nitrogen (PON) transported to larger river systems and downstream reservoirs. However, the yields during three years of investigation varied greatly: 5.0-22.3 Mg POC km -2 y -1 and 0.48-2.02 Mg PON km -2 y -1 . The 22.3 Mg POC km -2 y -1 yield is the highest reported for any river on the Asian continent. Stream samples collected during 12 storms showed that almost 3% of the total suspended solid load was particulate organic carbon 0.7 µm to 2.0 mm in size. This percentage is higher than other values for most large rivers on the continent. Further, we documented a strong pulse hysteretic behavior in the stream, whereby peak fluxes of POC and PON were often delayed (anticlockwise hysteresis) or accelerated (clockwise hysteresis) relative to stream flow peaks (or complex), complicating the prediction of storm-based or annual particulate carbon and nitrogen fluxes. Stream turbidity and total suspended sediment were reasonable proxies for POC and PON concentrations, while stream discharge was not a good predictor variable. Observed C:N ratios for measured particulate samples ranged from 3 to 83, with the high-end values likely associated with fresh (non-decomposed) vegetative material, greater than 2 mm in diameter. The C:N ratio (weighted based on three sediment sizes) for 12 events ranged from 7.5-15.3. These modest values reflect the relatively low C:N ratios for small size fractions (0.7-0.63 µm) that comprise 50-90% of the TTS load in the events. Overall, organic material 〈0.63 µm contributed about 75% of the total POC load and 80% of the PON load. The annual C:N ratio for the river is approximately 10-11. Collectively, our findings indicate the occasionally high yields make the Sa River—and potentially other similar headwater rivers—a hot spot for POC and PON transported to downstream water bodies. Complex hysteresis patterns and high year-to-year variability hinders our ability to calculate and predict these yields without continuous, automated monitoring of discharge and turbidity.

Beschreibung: Depression storage (DS) is the maximum storage of precipitation and runoff in the soil surface at a given slope. The DS is determined by soil roughness which in agricultural soils is largely affected by tillage. The direct measurement of DS is not straightforward because of the natural permeability of the soil. Therefore, DS has generally been estimated from 2D/3D empirical relationships and numerical algorithms based on roughness indexes and height measurements of the soil surface, respectively. The objective of this work was to evaluate the performance of some 2D models for DS, using direct and reliable measurements of DS in an agricultural soil as reference values. The study was carried out in experimental microplots where DS was measured in 6 situations resulting from the combination of 3 types of tillage carried out parallel and perpendicular to the main slope. Those data were used as reference to evaluate 4 empirical models and a numerical method. Longitudinal altitudinal profiles of the relief were obtained by a laser profilometer. Infiltration measurements were carried out before and after tillage. The DS were largely affected by tillage and its direction. Highest values of DS are found on rougher surfaces mainly when macroforms cut off the dominant slope. The empirical models had a limited performance while the numerical method was the most effective, even so, with an important variability. I In addition, a correct hydrological managements should take into account that each type of soil tillage affects infiltration rate differently. This article is protected by copyright. All rights reserved.

Beschreibung: Water transpired by trees has long been assumed to be sourced from the same subsurface water stocks that contribute to groundwater recharge and streamflow. However, recent investigations using dual water stable isotopes have shown an apparent ecohydrological separation between tree-transpired water and stream water. Here we present evidence for such ecohydrological separation in two tropical environments in Puerto Rico where precipitation seasonality is relatively low and where precipitation is positively correlated with primary productivity. We determined the stable isotope signature of xylem water of 30 mahogany ( Swietenia spp.) trees sampled during two periods with contrasting moisture status. Our results suggest that the separation between transpiration water and groundwater recharge/streamflow water might be related less to the temporal phasing of hydrologic inputs and primary productivity, and more to the fundamental processes that drive evaporative isotopic enrichment of residual soil water within the soil matrix. The lack of an evaporative signature of both groundwater and streams in the study area suggests that these water balance components have a water source that is transported quickly to deeper subsurface storage compared to waters that trees use. A Bayesian mixing model used to partition source water proportions of xylem water showed that groundwater contribution was greater for valley-bottom, riparian trees than for ridge-top trees. Groundwater contribution was also greater at the xeric site than at the mesic-hydric site. These model results (1) underline the utility of a simple linear mixing model, implemented in a Bayesian inference framework, in quantifying source water contributions at sites with contrasting physiographic characteristics, and (2) highlight the informed judgment that should be made in interpreting mixing model results, of import particularly in surveying groundwater use patterns by vegetation from regional to global scales. This article is protected by copyright. All rights reserved.

Beschreibung: We investigate the problem of balancing model complexity and input data requirements in snow hydrology. For this purpose, we analyze the performance of two models of different complexity in estimating variables of interest in snow hydrology applications. These are snow depth, snowpack bulk snow density, SWE and snow-melt runoff. We quantify the differences between data and model prediction using 18 years of measurements from an experimental site in the French Alps (Col de Porte, 1325 m AMSL). The models involved in this comparison are a one-layer temperature-index model (HyS) and a multilayer model (Crocus). Results show that the expected loss in performance in the one-layer temperature-index model with respect to the multilayer model is low when considering snow depth, SWE and bulk snow density. As for runoff, the comparison returns less clear indications for identification of a balance. In particular, differences between the models’ prediction and data with an hourly resolution are higher when considering the Crocus model than the HyS model. However, Crocus is better at reproducing sub-daily cycles in this variable. In terms of daily runoff, the multilayer physically-based model seems to be a better choice, while results in terms of cumulative runoff are comparable. The better reproduction of daily and sub-daily variability of runoff suggests that use of the multilayer model may be preferable for this purpose. Variation in performance is discussed as a function of both the calibration solution chosen and the time of year. This article is protected by copyright. All rights reserved.

Beschreibung: Estimating accurate spatial distribution of precipitation is important for understanding the hydrologic cycle and various hydro-environmental applications. Satellite-based precipitation data have been widely used to measure the spatial distribution of precipitation over large extents, but an improvement in accuracy is still needed. In this study, three different merging techniques (Conditional Merging, Geographical Differential Analysis, and Geographical Ratio Analysis) were used to merge precipitation estimations from Communication, Ocean, and Meteorological Satellite (COMS) Rainfall Intensity (RI) data and ground-based measurements. Merged products were evaluated with varying rain gauge network densities and accumulation times. The results confirmed that accuracy of detecting quantitative rainfall was improved as the accumulation time and network density increased. Also, the impact of spatial heterogeneity of precipitation on the merged estimates were investigated. Our merging techniques reproduced accurate spatial distribution of rainfall by adopting the advantages of both gauge and COMS estimates. The efficacy of the merging techniques was particularly pronounced when the spatial heterogeneity of hourly rainfall, quantified by variance of rainfall, was greater than 10 mm 2 /accumulation time 2 . Among the techniques analyzed, Conditional Merging performed the best, especially when the gauge density was low. This study demonstrates the utility of the COMS RI product, which has a shorter latency time (1 hour) and higher spatio-temporal resolution (hourly, 4 km by 4 km) than other widely used satellite precipitation products, in estimating precipitation using merging techniques with ground-based point measurements. The outcome has important implications for various hydrologic modeling approaches, especially for producing near real-time products. This article is protected by copyright. All rights reserved.

Beschreibung: The importance of conceptualising the dynamics of storage-driven saturation area connectivity in runoff generation has been central to the development of TOPMODEL and similar low parameterised rainfall-runoff models. In this contribution, we show how we developed a 40 year hydrometric data base to simulate storage-discharge relationships in the Girnock catchment in the Scottish Highlands using a simple conceptual model. The catchment is a unique fisheries reference site where Atlantic salmon populations have been monitored since 1966. The modelling allowed us to track storage dynamics in hillslopes, the riparian zone and groundwater, and explicitly link non-linear changes of streamflows to landscape storage and connectivity dynamics. This provides a fundamental basis for understanding how the landscape and riverscape are hydrologically connected and how this regulates in-stream hydraulic conditions that directly influence salmonids. We use the model to simulate storage and discharge dynamics over the 40 year period of fisheries records. The modelled storage-driven connectivity provides an ecohydological context for understanding the dynamics in stream flow generation which determine habitat hydraulics for different life stages of salmon population. This new, long-term modelling now sets this variability in the riverscape in a more fundamental context of the inter-relationships between storage in the landscape and stream flow generation. This provides a simple, robust framework for future ecohydrological modelling at this site, which is an alternative to more increasingly popular but highly parameterised and uncertain commercial ecohydrological models. It also provides a wider, novel context that is a prerequisite for any model-based scenario assessment of likely impacts resulting from climate or land use change. This article is protected by copyright. All rights reserved.

Beschreibung: Water percolation and flow processes in subsurface geologic media play an important role in determining the water source for plants and the transport of contaminants or nutrients, which is essential for water resource management and the development of measures for pollution mitigation. During June 2013, the dynamics of the rainwater, soil water, subsurface flows and groundwater in a shallow Entisol on sloping farmland were monitored using a hydrometric and isotopic approach. The results showed that effective mixing of rainwater and soil water occurred in hours. The rebound phenomenon of δ D profiles in soils showed that most isotope-depleted rainwater largely bypassed the soil matrix when the water saturation in the soil was high. Preferential-flow, which was the dominant water movement pattern in the vadose zone, occurred through the whole soil profile, and infrequent piston-flow was mainly found at 20-40 cm in depth. The interflow in the soil layer, composed of 75.2% rainwater, was only generated when the soil profile had been saturated. Underflow in the fractured mudrock was the dominant flow type in this hillslope, and outflow was dominated by base flow (groundwater flow) with a mean contribution of 76.7%. The generation mechanism of underflow was groundwater ridging, which was superimposed upon preferential-flow composed mainly of rainwater. The quick mixing process of rainwater and soil water and the rapid movement of the mixture through preferential channels in the study soil, which shows a typical bimodal pore size distribution, can explain the prompt release of pre-event water in subsurface flow. Water sources of subsurface flows at peak discharge could be affected by the antecedent soil water content, rain characteristics and antecedent groundwater levels. This article is protected by copyright. All rights reserved.

Beschreibung: In glacier fed rivers melting of glacier ice sustains streamflow during the driest times of the year, especially during drought years. Anthropogenic and ecologic systems that rely on this glacial buffering of low flows are vulnerable to glacier recession as temperatures rise. We demonstrate the evolution of glacier melt contribution in watershed hydrology over the course of a 184-year period from 1916-2099 through the application of a coupled hydrological and glacier dynamics model to the Hood River Basin in Northwest Oregon, U.S.A. We performed continuous simulations of glaciological processes (mass accumulation and ablation; lateral flow of ice; heat conduction through supra-glacial debris) which are directly linked with seasonal snow dynamics as well as other key hydrologic processes (e.g., evapotranspiration; subsurface flow). Our simulations show that historically, the contribution of glacier melt to basin water supply was up to 79% at upland water management locations. We also show that supraglacial debris cover on the Hood River glaciers modulates the rate of glacier recession and progression of dry season flow at upland stream locations with debris covered glaciers. Our model results indicate that dry season (July-Sept.) discharge sourced from glacier melt started to decline early in the 21st century following glacier recession that started early in the 20th century. Changes in climate over the course of the current century will lead to 14-63% (18-78%) reductions in dry season discharge across the basin for IPCC emission pathway RCP4.5 (RCP8.5). The largest losses will be at upland drainage locations of water diversions that were dominated historically by glacier melt and seasonal snowmelt. The contribution of glacier melt not only varies greatly in space, but also in time. It displays a strong decadal scale fluctuations that are super-imposed on the effects of a long-term climatic warming trend. This decadal variability results in reversals in trends in glacier melt which underscore the importance of long time series of glacio-hydrologic analyses for evaluating the hydrological response to glacier recession. This article is protected by copyright. All rights reserved.

Beschreibung: Understanding the dynamics of spatial and temporal variability of soil moisture at the regional scale and daily interval, respectively, has important implications for remote sensing calibration and validation missions as well as environmental modelling applications. The spatial and temporal variability of soil moisture was investigated in an agriculturally dominated region using an in-situ soil moisture network located in central Saskatchewan, Canada. The study site evaluated three depths (5, 20, 50 cm) through 139 days producing a high spatial and temporal resolution data set, which were analyzed using statistical and geostatistical means. Processes affecting standard deviation at the 5 cm depth were different from the 20 cm and 50 cm depths. Deeper soil measurements were well correlated through the field season. Further analysis demonstrated that lag time to maximum correlation between soil depths increased through the field season. Temporal autocorrelation was approximately twice as long at depth compared to surface soil moisture as measured by the e -folding frequency. Spatial correlation was highest under wet conditions caused by uniform rainfall events with low coefficient of variation. Overall soil moisture spatial and temporal variability was explained well by rainfall events and antecedent soil moisture conditions throughout the Kenaston soil moisture network. It is expected that the results of this study will support future remote sensing calibration and validation missions, data assimilation, as well as hydrologic model parameterization for use in agricultural regions.

Beschreibung: Changes in hydrologic flowpaths have important impacts on the timing, magnitude, and hydrochemistry of runoff during snowmelt in forested catchments, but how flowpaths are affected by variation in winter climate and the irregular presence of soil frost remains poorly understood. The depth and extent of soil frost may be expected to increase as snowpack decreases or develops later due to climate change. In this study we used end-member mixing analysis (EMMA) to determine daily contributions of snow, forest floor soil water, and groundwater to stream runoff during snowmelt under different soil frost regimes resulting from interannual and elevational variation at the Hubbard Brook Experimental Forest in New Hampshire, USA. We observed greater routing of runoff through forest floor flowpaths during early snowmelt in 2011, when the snowpack was deep and soil frost was minimal, compared to the early snowmelt in 2012 under conditions of deep and extensive soil frost. The results indicate that widespread soil frost that penetrated the depth of the forest floor decreased the flow signal through the shallowest subsurface flowpaths, but did not reduce overall infiltration of melt waters, as the contribution from the snow-precipitation end-member was similar under both conditions. These results are consistent with development of granular soil frost which permits vertical infiltration of melt waters, but either reduces lateral flow in the forest floor or prevents the solute exchange that would produce the typical chemical signature of shallow subsurface flowpaths in streamwater.

Beschreibung: Irregular wetting, water repellency and preferential flow are well-documented properties of coastal sandy podzols, though little is known about the effect of fire on unsaturated zone processes in this environment. This study investigates water repellency at and below the soil surface in two coastal sandy podzols following bushfire. Water Drop Penetration Time (WDPT) tests were applied to burned and unburned soils at a high dune field site in South East Queensland, Australia. It was found that the mean WDPT of the burned soil was four times that of the unburned soil but both soils were largely non-repellent. Post-fire repellency peaked below the surface in a patchy layer, in contrast to the laterally extensive layer reported in other studies, while high organic matter content in the soil did not appear to significantly influence repellency post-burn. Non-parametric statistics were used to quantify the high spatial variability in water repellency, which was ultimately insufficiently captured by atypically large (n = 1000 drop) datasets. This study confirms the presence of naturally occurring repellency and patchy infiltration in sandy soils while demonstrating that conclusively describing the influence of fire is challenging in a soil with heterogeneous infiltration characteristics. With respect to this uncertainty, it appears that fire does not increase soil water repellency such that infiltration and runoff processes due to fire-induced water repellency would differ post-burn.

Beschreibung: Projected changes in rainfall seasonality and interannual variability are expected to have severe impacts on arid and semi-arid tropical vegetation, which is characterized by a fine-tuned adaptation to extreme rainfall seasonality. To study the response of these ecosystems and the related changes in hydrological processes to changes in the amount and seasonality of rainfall, we focused on the caatinga biome, the typical seasonally dry forest in semi-arid Northeast Brazil. We selected four sites across a gradient of rainfall amount and seasonality and analyzed daily rainfall and biweekly NDVI (Normalized Difference Vegetation Index) data for hydrological years 2000 to 2014. Rainfall seasonal and interannual statistics were characterized by recently proposed metrics describing duration, timing, and intensity of the wet season and compared to similar metrics of NDVI time series. The results show that the caatinga tends to have a more stable response with longer and less variablegrowing seasons (3.1 ± 0.1 months) compared to the rainfall wet seasons (2.0 ± 0.5 months). The ecosystem ability to buffer the interannual variability of rainfall is also evidenced by the stability in the timing of the growing season compared to the wet season, which results in variable delays (ranging from 0 to 2 months) between the peak of the rainfall season and the production of leaves by the ecosystem. The analyses show that the shape and size of the related hysteresis loops in the rainfall-NDVI relations are linked to the buffering effects of soil moisture and plant growth dynamics. Finally, model projections of vegetation response to different rainfall scenarios reveal the existence of a maximum in ecosystem productivity at intermediate levels of rainfall seasonality, suggesting a possible trade-off in the effects of intensity (i.e., amount) and duration of the wet season on vegetation growth and related soil moisture dynamics and transpiration rates.

Beschreibung: The transferability of hydrologic models is of ever increasing importance for making improved hydrologic predictions and testing hypothesized hydrologic drivers. Here, we present an investigation into the variability and transferability of the recently introduced Catchment Connectivity Model (CCM) [ Smith et al ., 2013]. The CCM was developed following extensive experimental observations identifying the key drivers of streamflow in the Tenderfoot Creek Experimental Forest (TCEF) [ Jencso et al ., 2009; Jencso et al ., 2010], with the goal of creating a simple model consistent with internal observations of catchment hydrologic connectivity patterns. The model was applied across seven catchments located within TCEF to investigate spatial variability and transferability of model performance and parameterization. The results demonstrated that the model resulted in historically good fits (based on previous studies at the sites) to both hydrograph fit and internal water table dynamics (corroborated with experimental observations). The impact of a priori parameter limits were also examined. It was observed that enforcing field-based limits on model parameters resulted in slight reductions to streamflow hydrograph fits, but significant improvements to model process fidelity (as hydrologic connectivity), as well as moderate improvement in the transferability of model parameterizations from one catchment to the next.

Beschreibung: Groundwater resources are typically the main fresh water source in arid and semi-arid regions. Natural recharge of aquifers is mainly based on precipitation; however, only heavy precipitation events (HPEs) are expected to produce appreciable aquifer recharge in these environments. In this work we used daily precipitation and monthly water level time series from different locations over a Mediterranean region of Southeastern Spain to identify the critical threshold value to define HPEs that lead to appreciable aquifer recharge in this region. Wavelet and trend analyses were used to study the changes in the temporal distribution of the chosen HPEs (≥20 mm day -1 ) over the observed period 1953-2012 and its projected evolution by using eighteen downscaled climate projections over the projected period 2040-2099. The used precipitation time series were grouped in ten clusters according to similarities between them assessed by using Pearson correlations. Results showed that the critical HPEs threshold for the study area is 20 mm day -1 . Wavelet analysis showed that observed significant seasonal and annual peaks in global wavelet spectrum in the first sub-period (1953-1982) are no longer significant in the second sub-period (1983-2012) in the major part of the ten clusters. This change is due to the reduction of the mean HPEs number, which showed a negative trend over the observed period in nine clusters and was significant in five of them. However, the mean size of HPEs showed a positive trend in six clusters. A similar tendency of change is expected over the projected period. The expected reduction of the mean HPEs number is two times higher under the high climate scenario (RCP8.5) than under the moderate scenario (RCP4.5). The mean size of these events is expected to increase under the two scenarios. The groundwater availability will be affected by the reduction of HPEs number which will increase the length of no aquifer recharge periods (NARP) accentuating the groundwater drought in the region.

Beschreibung: The need to understand and simulate hydrological phenomena and their interactions, and the impact of anthropogenic and climate changes on natural environments have promoted the study of evaporation from bare soils in arid climates. In closed Altiplano basins, such as those encountered in arid and hyper arid basins in northern Chile, evaporation from shallow groundwater is the main source of aquifer depletion and thus, its study is crucial for water resources management. The objective of this work is to understand the mechanisms of evaporation in saline soils with shallow water tables, in order to better quantify evaporation fluxes and improve our understanding of the water balance in these regions. To achieve this objective, a model that couples fluid flow with heat transfer was developed and calibrated using column experiments with saline soils from the Huasco salt flat basin, Chile. The model enables determination of both liquid and water vapor fluxes, as well as the location of the evaporation front. Experimental results showed that salt transport inside the soil profile modified the water retention curve, highlighting the importance of including salt transport when modeling the evaporation processes in these soils. Indeed, model simulations only agreed with the experimental data when the effect of salt transport on water retention curves was taken into account. Model results also showed that the evaporation front is closer to the soil surface as the water table depth reduces. Therefore, the model allows determining the groundwater level depth that results in disconnection of liquid fluxes in the vadose zone. A sensitivity analysis allowed understanding the effect of water-flux enhancements mechanisms on soil evaporation. The results presented in this study are important as they allow quantifying the evaporation that occurs in bare soils from Altiplano basins, which is typically the main water discharge in these closed basins.

Beschreibung: Recent studies using water stable isotopes (δ 18 O and δ 2 H) have suggested an ecohydrological separation of water flowing to streams or recharging groundwater and water used by trees, known as the “two water worlds” (TWW) hypothesis. In this study, we measured water isotopic composition in precipitation (open field and throughfall, i.e. LMWL) and the mobile water compartment (i.e. stream and soil solution), bulk soil water and xylem water over a period of 1.5 years in two headwater catchments: NF, covered with old growth native evergreen forest ( Aetoxicon punctatum , Laureliopsis phillipiana and Eucriphya cordifolia ); and EP, covered with 4 and 16 years old Eucalyptus nitens stands. Our results show that precipitation, stream and soil solution plot approximately along the local meteorical water line (LMWL), while xylem waters from all studied tree species plot below the LMWL, supporting the TWW hypothesis. However, we also found evidence of ecohydrological connectivity during the wet season, likely controlled by the amount of antecedent precipitation. These observations hold for all investigated tree species. On both sites, a different precipitation source for stream and xylem water was observed. However, in EP bulk soil showed a similar precipitation source as xylem water from both E. nitens stands. This suggests that E. nitens may use water that is recharging the bulk soil compartment. We conclude that under a rainy temperate climate the TWW hypothesis is temporal and does not apply during wet seasons.

Beschreibung: The Mackenzie River, Canada's longest and largest river system, provides the greatest Western Hemisphere discharge to the Arctic Ocean. Recent reports of declining flows have prompted concern because (1) this influences Arctic Ocean salinity, stratification, and polar ice; (2) a major tributary, the Peace River, has large hydroelectric projects and further dams are proposed; and (3) the system includes the extensive and biodiverse Peace-Athabasca, Slave and Mackenzie deltas. To assess hydrological trends over the past century that could reflect climate change, we analyzed historic patterns of river discharges. We expanded the data series by infilling for short gaps; calculating annual discharges from early summer-only records (typical r 2  〉 0.9); coordinating data from sequential hydrometric gauges (requiring r 2  〉 0.8); and advancing the data to 2013. For trend detection, Pearson correlation provided similar outcomes to non-parametric Kendall τ and Spearman ρ tests. There was no overall pattern for annual flows of the most southerly Athabasca River (1913-2013) while the adjacent, regulated Peace River displayed increasing flows (1916-2013, p  〈 0.05). These rivers combine to form the Slave River, which did not display an overall trend (1917-2013). The more northerly, free-flowing Liard River, is the largest tributary and displayed increasing annual flows (1944-2013, p  〈 0.01, ~3.5% per decade) due to increasing winter, spring and summer flows, and annual maximum and minimum flows also increased. Following from the tributary contributions, the Mackenzie River flows gradually increased (Fort Simpson 1939-2013, p  〈 0.05, ~1.5% per decade), but the interannual patterns for the Liard and other rivers were correlated with the Pacific Decadal Oscillation (PDO), complicating the pattern. This conclusion of increasing river flows to the Arctic Ocean contrasts with some prior reports, based on shorter time series. The observed flow increase is consistent with increasing discharges of the large Eurasian Arctic drainages, suggesting a common northern response to climate change. Analyses of historic trends are strengthened with lengthening records and with the PDO influence we recommend century-long records for northern rivers.

Beschreibung: Abstract: Over the past sixty years, road deicers (i.e., road salt) have been applied to roadways in high latitudes to improve road conditions in winter weather. However, the dissolution of road deicers in highway runoff creates waters with high concentrations of sodium, which can mobilize soil metals via soil cation exchange reactions. While several studies have detailed the interactions of road salt-rich solutions and surface and ground waters, less attention has been given to how local hydrologic flowpaths can impact the delivery of these solutions to near-road soils. Between 2013 and 2014, soil water samples were collected from a roadside transect of lysimeter nests in Pittsburgh, Pennsylvania (USA). Soil water samples were analyzed for metal concentrations and resulting data used to examine cation dynamics. While patterns in soil water calcium and magnesium concentrations follow patterns in soil water sodium concentrations, additional processes influence patterns in soil water potassium concentrations. Specifically, we observe the highest calcium and magnesium concentrations in the deepest lysimeters, suggesting divalent cations are mobilized to, and potentially accumulate in, deeper soil horizons. In contrast, soil water potassium concentrations do not follow this pattern. Additionally, in all examined elements (Ca, Mg, K, Na, Cl), the timing of concentration peaks appear be influenced by a combination of both distance from the roadside, and sampling depth. These relationships not only suggest that multiple soil water flowpaths interact with our study transect, but also confirm that road salt plumes persist and migrate following the road salting season. Characterizing the interactions of sodium-rich solutions and roadside soil cation pools clarifies our understanding of metal dynamics in the roadside environment. A deeper understanding of these processes is necessary to effectively restore and manage watersheds as high total dissolved solid solutions (e.g., road deicing melt, unconventional natural gas brines, and marginal irrigation water) continue to influence hydrological systems. This article is protected by copyright. All rights reserved.

Beschreibung: Hydrological connectivity describes the physical coupling (linkages) of different elements within a landscape regarding (sub-) surface flows. A firm understanding of hydrological connectivity is important for catchment management applications, for e.g. habitat and species protection, and for flood resistance and resilience improvement. Thinking about (geomorphological) systems as networks can lead to new insights, which has also been recognised within the scientific community, seeing the recent increase in the use of network (graph) theory within the geosciences. Network theory supports the analysis and understanding of complex systems by providing data structures for modelling objects and their linkages, and a versatile toolbox to quantitatively appraise network structure and properties. The objective of this study was to characterise and quantify overland flow connectivity dynamics on hillslopes in a humid sub-Mediterranean environment by using a combination of high-resolution digital-terrain models, overland flow sensors and a network approach. Results showed that there are significant differences between overland flow connectivity on agricultural areas and semi-natural shrubs areas. Significant positive correlations between connectivity and precipitation characteristics were found. Significant negative correlations between connectivity and soil moisture were found, most likely due to soil water repellency and/or soil surface crusting. The combination of structural networks and dynamic networks for determining potential connectivity and actual connectivity proved a powerful tool for analysing overland flow connectivity. This article is protected by copyright. All rights reserved.

Beschreibung: This paper introduces three web-based databases relating to hydrological and water resources studies in the United States of America with the intention of promoting public awareness of their existence. These open access databases are the products of efforts taken by federal and state governments to make government-owned data more accessible to the general public. The megadata sets of climate, surface water, groundwater, and water quality contained in these databases provide hydrologists and environmental scientists with unprecedented opportunities to conduct research across multiple regions and on different scales.

Beschreibung: Assessing catchment runoff response remains a key research frontier due to limitations in current observational techniques to fully characterize water source areas and transit times in diverse geographical environments. Here, we report a study that combines empirical data with modelling to identify dominant runoff processes in a sparsely monitored humid tropical catchment. The analysis integrated isotope tracers into conceptual rainfall-runoff models of varying complexity (from five to eleven calibrated parameters) that are able to simulate discharge and tracer concentrations and track the evolving age of stream water exiting the catchment. The model structures can be seen as competing hypotheses of catchment functioning and were simultaneously calibrated against uncertain streamflow gaugings and a two-year daily isotope rainfall-runoff record. Comparison of the models was facilitated using global parameter sensitivity analysis and the resulting effect on calibration. We show that a variety of tested model structures reproduced water and tracer dynamics in stream, but the simpler models failed to adequately reproduce both. The resulting water age distributions of the tested models varied significantly with little similarity between the stream water age and stored water age distributions. The sensitivity analysis revealed that only some of the more complex models (from eight parameters) could be better constrained to infer more plausible water age distributions and catchment storage estimates. These models indicated that the age of water stored in the catchment is generally older compared to the age of water fluxes, with evapotranspiration age being younger compared to streamflow. However, the water age distributions followed a similar temporal behaviour dominated by climatic seasonality. Stream water ages increased during the dry season (greater than 1 year) and decreased with increased streamflow (a few weeks old) during the wet season. We further show that the ratios of the streamwater age to stored water age distribution and the water age distribution of actual evapotranspiration to the stored water age distribution from constrained models could potentially serve as useful hydrological indicators of catchment functioning. This article is protected by copyright. All rights reserved.

Beschreibung: An understanding of heat transport and water flow in unsaturated soils experiencing freezing and thawing is important when considering hydrological and thermal processes in cold regions. Macropores, such as cracks, roots, and animal holes, provide efficient conduits for enhanced infiltration, resulting in a unique distribution of water content. However, the effects of macropores on soil freezing and thawing with infiltration have not been well studied. A one-directional soil-column freezing and thawing experiment was conducted using unsaturated sandy and silt loams with different sizes and numbers of macropores. During freezing, macropores were found to retard the formation of the frozen layer, depending on their size and number. During thawing, water flowed through macropores in the frozen layer and reached the underlying unfrozen soil. However, infiltrated water sometimes refroze in a macropore. The ice started to form at near inner wall of the macropore, grew to the center, and blocked flow through the macropore. The blockage ice in the macropore could not melt until the frozen layer disappeared. Improving a soil freezing model to consider these macropore effects is required. This article is protected by copyright. All rights reserved.

Beschreibung: A detailed study using environmental tracers such as chloride (Cl - ) and tritium ( 3 H), deuterium ( 2 H) and oxygen ( 18 O) isotopes was performed in an alluvial coastal aquifer in two contrasting environments (urban and agricultural). These environmental tracers combined with a high-resolution multi-level sampling approach were used to estimate groundwater residence time and recharge patterns and to validate the hydrogeochemical conceptual model already proposed in previous studies. δ 18 O and δ 2 H combined with Cl - data proved that the hypersaline groundwater present in the deepest part of the aquifer was sourced from the underlying hypersaline aquitard via an upward flux. Both chemical and isotopic data were employed to calibrate a density-dependent numerical model based on SEAWAT 4.0, where 3 H and Cl - were helped quantifying solutes transport within the modelled aquifer. Model results highlighted the differences on estimated recharge in the two contrasting environments, with the urban one exhibiting concentrated recharge due to preferential infiltration associated to the storm water drains network, while scarce local recharge characterized the agriculture setting. In the urban field site, is still possible to recognize at 9 m b.g.l. the input of the atmospheric anthropogenic 3 H generated by testing of thermonuclear weapons, while in the agricultural field site, the 3 H peak has been washed out at 6 m b.g.l. since the groundwater circulation is restricted only to the upper fresh part of the aquifer, drained by the reclamation system. The presented approach that combined high-resolution field monitoring, environmental tracers and numerical modelling, resulted effective in validating the conceptual model of the aquifer salinization.

Beschreibung: Many studies have empirically confirmed the relationship between urbanization and changes to the hydrologic cycle and degraded aquatic habitats. While much of the literature focuses on extent and configuration of impervious area as a causal determinant of degradation, in this article I do not attribute causes of decreased watershed storage on impervious area a priori . Rather, adapting the concept of variable source area (VSA) and its relationship to incremental storage to the particular conditions of urbanized catchments, I develop a statistically-robust linear regression-based methodology to detect evidence of VSA-dominant response. Using the physical and meteorological characteristics of the catchments as explanatory variables, I then use logistic regression to statistically analyze significant predictors of the VSA classification. I find that the strongest predictor of VSA-type response is the percent of undeveloped area in the catchment. Characteristics of developed areas, including total impervious area, percent developed open space, and the type of drainage infrastructure do not add to the explanatory power of undeveloped land in predicting VSA-type response. Within only developed areas, I find that total impervious area (TIA) and percent developed open space both decrease the odds of a catchment exhibiting evidence of VSA-type response and the effect of developed open space is more similar to that of TIA than undeveloped land in predicting VSA response. Different types of stormwater management infrastructure, including combined sewer systems (CSS) and infiltration, retention, and detention infrastructure are not found to have strong statistically significant effects on probability of VSA-type response. VSA-type response is also found to be stronger during the growing season than the dormant season. These findings are consistent across a national cross-section of urbanized watersheds, a higher resolution dataset of Baltimore Metropolitan Area watersheds, and a subsample of watersheds confirmed not to be served by (CSS). This article is protected by copyright. All rights reserved.

Beschreibung: Soil erosion by water is a pressing environmental problem caused and suffered by agriculture in Mediterranean environments. Soil conservation practices can contribute to alleviating this problem. The aim of this study is to gain more profound knowledge of the effects of conservation practices on soil losses by linking crop management and soil status to runoff and sediment losses measured at the outlet of a catchment during seven years. The catchment has 27.42 ha and is located in a commercial farm in southern Spain, where a package of soil conservation practices is an essential component of the farming system. The catchment is devoted to irrigated annual crops with maize-cotton-wheat as the primary rotation. Mean annual rainfall-induced runoff coefficient was 0.14 and mean annual soil loss was 2.4 Mg ha -1 y -1 . Irrigation contributed to 40% of the crop water supply, but the amount of runoff and sediment yield that it generated was negligible. A Principal Components Analysis showed that total soil loss is determined by the magnitude of the event (rainfall and runoff depths, duration) and by factors related to the aggressiveness of the events (rainfall intensity and preceding soil moisture). A third component showed the importance of crop coverage to reduce sediment losses. Cover crops grown during autumn and early winter and crop residues protecting the soil surface enhanced soil conservation notably. The role of irrigation to facilitate growing cover crops in Mediterranean environments is discussed.

Beschreibung: In the small Ringelbach research catchment, where studies on the water cycle components in a granitic mountainous environment have been conducted since 1976, the water-saturated areas that are hydraulically connected to the outlet play a major role in the streamflow generation, as it is here that complex interactions between atmosphere, surface and ground waters take place. During baseflow recession periods, which may last several months between two groundwater recharge events, the atmospheric inputs of water and energy on these contributing areas only explain the streamflow fluctuations observed around the master recession curve, which defines the groundwater contribution: fluctuating above it in the case of precipitation input on these areas, below it in the case of evaporation output from these areas. Streamflow may therefore largely deviate from the master recession curve in the case of long, hot, dry spells. Detailed mapping has shown that their variable extent is well related to baseflow by a loglinear curve. On the other hand, a synthetic master recession curve, well fitted by a 2 nd -order hyperbolic function, has been obtained from numerous pure recession periods. Both based on these two curves, a simple procedure and a simple model have been used to (i) validate the hypothesis that the connected saturated areas are the only permanent variable contributing areas and (2) simulate the daily streamflow volumes over long baseflow recession periods by a water balance of the aquifer below these areas only. The storm runoff ratio for small to moderate rainfall events is indeed corresponding to the catchment saturated fraction at that time. The volume of daily streamflow oscillations is indeed corresponding to the evaporation at the potential rate from the saturated areas only. In both cases, streamflow naturally tends towards the master recession curve after the end of any atmospheric perturbation. Introducing these findings into TOPMODEL led to significantly improved simulation results during baseflow recession periods. The master recession curve may therefore be considered as a dynamic equilibrium curve. Together with the relationship between saturated extent and baseflow, it provides the main characteristics necessary to understand and model the interactions at this complex interface and the resulting daily streamflow variations during baseflow recession periods in this type of catchment.

Beschreibung: The potential for dynamic storage (Kirchner J, 2009, Water Resources Research 45: doi:10.1029/2008WR006912) to serve as a metric of basin behaviour was assessed using data from five drainage basins with headwaters on the thick sand and gravel deposits of the Oak Ridges Moraine in southern Ontario, Canada. Dynamic storage was directly correlated with the ratio of variability of δ 2 H in streamflow relative to that in precipitation. This ratio has previously been shown to be inversely related to basin mean transit time ( MTT ), suggesting an inverse relationship between dynamic storage and MTT for the study basins. Dynamic storage was also directly correlated with interannual variability in stream runoff, baseflow and baseflow:runoff ratio, implying that basins with smaller dynamic storage have less interannual variability in their streamflow regimes. These preliminary results suggest that dynamic storage may serve as a readily-derived and useful metric of basin behaviour for inter-basin comparisons. This article is protected by copyright. All rights reserved.

Beschreibung: Many studies have focused on the amount of stemflow in different forests and for different rainfall events but few studies have focused on how stemflow intensity varies during events or the infiltration of stemflow into the soil. Stemflow may lead to higher water delivery rates at the base of the tree compared to throughfall over the same area and fast and deeper infiltration of this water along roots and other preferential flow pathways. In this study, stemflow amounts and intensities were measured and blue dye experiments were conducted in a mature coniferous forest in coastal British Columbia to examine double funnelling of stemflow. Stemflow accounted for only 1% of precipitation and increased linearly with event total precipitation. Funneling ratios ranged from less than 1 to almost 20; smaller trees had larger funneling ratios. Stemflow intensity generally was highest for periods with high intensity rainfall later in the event. The maximum stemflow intensities were higher than the maximum precipitation intensities. Dye tracer experiments showed that stemflow infiltrated primarily along roots and was found more frequently at depth than near the soil surface. Lateral flow of stemflow was observed above a dense clay layer for both the throughfall and stemflow experiments. Stemflow appeared to infiltrate deeper (122 cm) than throughfall (85 cm) but this difference was in part due to site specific differences in maximum soil depth. However, the observed high stemflow intensities combined with preferential flow of stemflow may lead to enhanced subsurface stormflow. This suggests that even though stemflow is only a very minor component of the water balance, it may still significantly affect soil moisture, recharge and runoff generation.

Beschreibung: The paper deals with the hydro-chemical analysis performed in order to reveal processes, sources, paths and timing of the runoff generation in an experimental catchment representative of the hilly, terrigenous and forested watershed in the Mediterranean humid eco-region of southern Italy. The analysis is based on the data recorded at the outlet of the catchment during the 2013-2014. A mixing law procedure was applied on discharge (Q) and Electrical conductivity (EC) data, by using the Q-EC end-members previously collected at selected groundwater, sub-surficial and surficial stations. In this way, we found four bound curves delimiting fields in a Q-EC plot, each with hydro-chemograph value ranges. At annual time scale, the analysis revealed a seasonal behavior of the hydrological response, different for the wet period, when the aquifer is recharging and the dry periods, when the aquifer is discharging, despite frequent summer rain showers. At event time scale, the catchment seems to show the behavior of a typical Hydro-geomorphic threshold system. We interpreted this behavior as due to a progressive addition of water from distinctive components (i.e., deep aquifer, riparian corridor, hillslope and hollow), each with originally different mechanisms of runoff production (i.e., groundwater, groundwater ridging, saturation-excess, infiltration-excess and soil pipe exfiltration) and response time. During the event, the contributing areas enlarge upward the riparian corridors and the zero order basins, where the above components become superposed and the mechanisms interact more and more. We hypothizise that the threshold values between different states of the system are defined by the intersections of the boundary curves on the Q-EC plot. Different patterns in the Q-EC hysteretic cycles are prevalently related to the pre-event soil saturation and groundwater contributions to storm flow and recharge mechanisms.

Beschreibung: Interaction between groundwater and surface water in watersheds has significant impacts on water management and water rights, nutrient loading from aquifers to streams, and in-stream flow requirements for aquatic species. Of particular importance are the spatial patterns of these interactions. This study explores the spatio-temporal patterns of groundwater discharge to a river system in a semi-arid region, with methods applied to the Sprague River Watershed (4,100 km 2 ) within the Upper Klamath Basin in Oregon, USA. Patterns of groundwater-surface water interaction are explored throughout the watershed during the 1970-2003 time period using a coupled SWAT-MODFLOW model tested against streamflow, groundwater level, and field-estimated reach-specific groundwater discharge rates. Daily time steps and coupling are used, with groundwater discharge rates calculated for each model computational point along the stream. Model results also are averaged by month and by year to determine seasonal and decadal trends in groundwater discharge rates. Results show high spatial variability in groundwater discharge, with several locations showing no groundwater/surface water interaction. Average annual groundwater discharge is 20.5 m 3 /s, with maximum and minimum rates occurring in September-October and March-April, respectively. Annual average rates increase by approximately 0.02 m 3 /s per year over the 34-year period, negligible compared to the average annual rate, although 70% of the stream network experiences an increase in groundwater discharge rate between 1970 and 2003. Results can assist with water management, identifying potential locations of heavy nutrient mass loading from the aquifer to streams, and ecological assessment and planning focused on locations of high groundwater discharge.

Beschreibung: A methodology based on long-term dynamical downscaling to analyze climate change effects on watershed-scale precipitation during a historical period is proposed in this study. The reliability and applicability of the methodology were investigated based on the long-term dynamical downscaling results. For an application of the proposed methodology, two study watersheds in Northern California were selected: the Upper Feather River watershed (UFRW) and the Yuba River watershed (YRW). Then, precipitation was reconstructed at 3 km spatial resolution and hourly intervals over the study watershed for 141 water-years from October 1, 1871 through September 30, 2012 by dynamically downscaling a long-term atmospheric reanalysis dataset, 20CRv2 by means of a regional climate model. The reconstructed precipitation was compared against observed precipitation, in order to assess the applicability of the proposed methodology for the reconstruction of watershed-scale precipitation and to validate this methodology. The validation shows the reconstructed precipitation is in good agreement with observation data. Moreover, the differences between the reconstructed precipitation and the corresponding observations do not significantly change through the historical period. After the validation, climate change analysis was conducted based on the reconstructed precipitation. Through this analysis it was found that basin-average precipitation has increased significantly over both of the study watersheds during the historical period. An upward trend in monthly basin-average precipitation is not significant in wet months except February while it is significant in dry months of the year. Furthermore, peak values of basin-average precipitation are also on an upward trend over the study watersheds. The upward trend in peak basin-average precipitation is more significant during a shorter duration.

Beschreibung: Brush removal is widely practiced as a tool for increasing groundwater recharge, but its efficacy depends greatly on the way in which the removed species interact with the hydrological system relative to the vegetation replacing it. We examined the effects of Ashe juniper removal in the recharge zone of the Edwards Aquifer, Texas, USA, a karst aquifer. The study was conducted in an Ashe juniper ( Juniperus ashei ) - live oak ( Quercus fusiformis ) woodland on a hill slope composed of rocky, shallow soils over fractured limestone bedrock. Ashe juniper is a native species that has been encroaching grasslands and savannas over the past century. In September 2008, a plot was cleared of 90% of its juniper trees. Tree transpiration, predawn water potentials, and vegetation cover across the cleared plot and an adjacent reference site were measured from May 2009 to December 2011. Stand-level tree transpiration from May 2009 to March 2010 was diminished by a severe summer drought in 2009, from which trees were slow to recover. Subsequently, tree transpiration was 5-10x higher in the woodland compared to the clearing. For all of 2011, also a drought year, tree transpiration in the woodland exceeded precipitation inputs, indicating a high capacity for water storage at the study site. However, site differences for oak trees were generally larger than for juniper trees. While juniper removal accounted for a 431 mm y -1 difference in tree transpiration between sites, vegetation cover in the clearing increased from 42% to 90% over two years, suggesting that understory growth was increasingly compensating for the loss of juniper transpiration. We conclude that the removal of a relatively shallow-rooted tree, when replaced with herbaceous vegetation and low shrubs, has little effect on deep recharge. By contrast, successive years of precipitation extremes may be more effective increasing recharge by lowering the water transport capacity of trees in the aftermath of severe drought.

Beschreibung: Accurate estimates of seasonal evapotranspiration (ET) at different temporal and spatial scales are essential for understanding the biological and environmental determinants of ecosystem water balance in arid regions and the patterns of water utilization by the vegetation. For this purpose, remote sensing ET estimates of a Patagonian desert in Southern Argentina were verified with field measurements of soil evaporation and plant transpiration using an open top chamber. Root distribution and seasonal variation in soil volumetric water content was also analyzed. There was a high correlation between remote sensing and field measurements of ecosystem water fluxes. A substantial amount of the annual ET occurred in spring and early summer (73.4 mm) using winter rain stored in the soil profile and resulting in water content depletion of the upper soil layers. A smaller amount of annual ET was derived from few rainfall events occurring during the mid or late summer (41.4 mm). According to remote sensing, the 92.9 % of the mean annual precipitation returns to the atmosphere by transpiration or evaporation from the bare soil and by canopy interception. Only 7.1 % infiltrates to soil layers deeper than 200 cm contributing to the water table recharge. Fourier time series analysis, cross-correlation methods and multiple linear regression models were used to analyze 11 years of remote sensing data to assess determinants of water fluxes. A linear model predicts well the variables that drive complex ecosystem processes such as ET. Leaf area index and air temperature were not linearly correlated to ET because of the multiple interaction among variables resulting in time lags with ET variations and thus these two variables were not included in the linear model. Soil water content, the fraction of photosynthetic active radiation and precipitation explained 86% of the ET monthly variations. The high volumetric water content and the small seasonal variations at 200 cm depth were probably the result of little water uptake from deeper soil horizons by roots with low hydraulic conductivity.

Beschreibung: Shrink–swell soils, such as those in a Mediterranean climate regime, can cause changes in terms of hydrological and erosive responses due to the changing soil water storage conditions. Only a limited number of long-term studies have focused on the impacts on both hydrological and erosive responses and their interactions in an agricultural environment. In this context, this study aims to document the dynamics of cracks, runoff and soil erosion within a small Mediterranean cultivated catchment and to quantify the influence of crack processes on the water and sediment supplied to a reservoir located at the catchment outlet using water and sediment measurements at a cultivated field outlet as baseline. Detailed monitoring of the presence of topsoil cracks was conducted within the Kamech catchment (ORE OMERE, Tunisia), and runoff and suspended sediment loads were continuously measured over a long period of time (2005–2012) at the outlets of a field (1.32 ha) and a catchment (263 ha). Analysis of the data showed that topsoil cracks were open approximately half of the year and that the rainfall regime and water table level conditions locally control the seasonal cracking dynamics. Topsoil cracks appeared to seriously affect the generation of runoff and sediment concentrations and, consequently, sediment yields, with similar dynamics observed at the field and catchment outlets. A similar time lag in the seasonality between water and sediment delivery was observed at these two scales: although the runoff rates were globally low during the presence of topsoil cracks, most sediment transport occurred during this period associated with very high sediment concentrations. This study underlines the importance of a good prediction of runoff during the presence of cracks for reservoir siltation considerations. In this context, the prediction of cracking effects on runoff and soil erosion is a key factor for the development of effective soil and water management strategies and downstream reservoir preservation. This article is protected by copyright. All rights reserved.

Beschreibung: Geographically isolated wetlands (GIWs), defined as wetlands surrounded by uplands, provide an array of ecosystem goods and services. Within the United States, federal regulatory protections for GIWs are contingent, in part, on the quantification of their singular or aggregate effects on the hydrological, biological, or chemical integrity of waterways regulated by the Clean Water Act (CWA). However, limited tools are available to assess the downgradient effects of GIWs. We constructed a Soil and Water Assessment Tool (SWAT) model with improved representations of GIW hydrologic processes for the approximately 1,700 km 2 Pipestem Creek watershed in the Prairie Pothole Region of North Dakota, USA. We then executed a series of novel modifications on the Pipestem Creek SWAT model. We (1) redefined the model's hydrologic response unit spatial boundaries to conform to mapped GIWs and associated watershed boundaries; (2) constructed a series of new model input files to direct the simulation of GIW fill-spill hydrology and upland flows to GIWs; and (3) modified the model source code to facilitate use of the new SWAT input files and improve GIW water balance simulations. We then calibrated and verified our modified SWAT model at a daily time step from 2009 through 2013. Simulation results indicated good predictive power (the maximum Nash-Sutcliffe Efficiency statistic was 0.86) and an acceptable range of uncertainty (measured using the Sequential Uncertainty Fitting v.2 uncertainty statistics). Simulation results additionally indicated good model performance with respect to GIW water balance simulations based on literature-based descriptions of regional GIW hydrologic behavior. Our modified SWAT model represents a critical step in advancing scientific understandings of the watershed-scale hydrologic effects of GIWs and provides a novel method for future assessments in different watersheds and physiographic regions. This article is protected by copyright. All rights reserved.

Beschreibung: Cosmic-ray soil moisture sensors have the advantage of a large measurement footprint (approximately 700 m in diameter) and are able to operate continuously to provide area-averaged near-surface (top 10-20 cm) volumetric soil moisture content at the field scale. This paper presents the application of this technique at four sites in southern England over almost 3 years. Results show the soil moisture response to contrasting climatic conditions during 2011-2014, and are the first such field-scale measurements made in the UK. These four sites are prototype stations for a UK COsmic-ray Soil Moisture Observing System (COSMOS-UK), and particular consideration is given to sensor operating conditions in the UK. Comparison of these soil water content observations with the Joint UK Land Environment Simulator (JULES) 10 cm soil moisture layer shows that these data can be used to test and diagnose model performance, and indicates the potential for assimilation of these data into hydro-meteorological models. The application of these large-area soil water content measurements to evaluate remotely-sensed soil moisture products is also demonstrated. Numerous applications and the future development of a national COSMOS-UK network are discussed. This article is protected by copyright. All rights reserved.

Beschreibung: Conservation management for the water dependent desert-oasis ecotone in arid northwest China requires information on the water use of the dominant species. However, no studies have quantified their combined water use or linked species composition to ecotone transpiration. Here, the water use of three dominant shelterbelt shrubs ( Haloxylon ammodendron , Nitraria tangutorum , and Calligonum mongolicum ) within an ecotone was measured throughout the full leaf-out period for three shrub species from 30 May to 16 October 2014, with sap flow gauges using the stem heat balance approach. Species-specific transpiration was estimated by scaling up sap flow velocities measured in individual stems, to stand area level, using the frequency distribution of stem diameter and assuming a constant proportionality between sap flow velocity and basal cross-sectional area for all stems. The mean peak sap flux densities ( J sn ) for H . ammodendron , N . tangutorum , and C . mongolicum , were 40.12 g cm –2 hr –1 , 71.33 g cm –2 hr –1 , and 60.34 g cm –2 hr –1 , respectively, and the mean estimated daily area-averaged transpiration rates ( T daily ) for the same species were 0.56 mm day –1 , 0.34 mm day –1 , and 0.11 mm day –1 . The accumulative stand transpiration was approximately 140.8 mm throughout the measurement period, exceeding precipitation by as much as 42.1 mm. Furthermore, T daily of these shrubs appeared to be much less sensitive to soil moisture as compared to atmospheric drivers; and the relationship between J sn and atmospheric drivers was likely uninfluenced by soil moisture regimes in the whole profile (to 1 m depth), especially for H . ammodendron and C . mongolicum . Results indicate that these shrubs may use deep soil water recharged by capillary rise, or may directly access shallow groundwater. This study provides quantitative data offering important implications for ecotone conservation and water and land resource management.

Beschreibung: The local meteoric water line (LMWL), the functional relationship between locally measured values of δ 18 O and δ 2 H in precipitation, represents the isotopic composition of water entering hydrologic systems. The degree to which the LMWL departs from the global meteoric water line (GMWL), moreover, can reveal important information about meteoric sources of water (e.g., oceanic or terrestrial) and atmospheric conditions during transport. Here we characterize the isotopic composition of precipitation within an experimental watershed in the Western US that is subject to large topographic and seasonal gradients in precipitation. Interpreting the hydrometeorologic and spatial controls on precipitation, we constructed a seasonally weighted local meteoric water line (LMWL) for southwestern Idaho that is expressed by the equation δ 2 H = 7.40*δ 18 O – 2.17. A seasonally weighted LMWL that is based on weighting isotopic concentrations by climatic precipitation volumes is novel, and we argue better represents the significant seasonality of precipitation in the region. The developed LMWL is considerably influenced by the semiarid climate experienced in southwest Idaho, yielding a slope and y-intercept lower than the GMWL (δ 2 H = 8*δ 18 O +10). Moderate to strong correlations exist between the isotopic composition of precipitation from individual events and surface meteorologic variables, specifically surface air temperature, relative humidity, and precipitation amount. A strong negative correlation exists between the annual average isotopic composition of precipitation and elevation at individual collection sites, with a lapse rate of -0.22‰/100 m.

Beschreibung: Las Vegas Valley has had a long history of groundwater development and subsequent surface deformation. InSAR interferograms have revealed detailed and complexspatial patterns of subsidence in the Las Vegas Valley area that do not coincide with major pumping regions. This research represents the first effort to use high spatial and temporal resolution subsidence observations from InSAR and hydraulic head data to inversely calibrate transmissivities ( T ), elastic and inelastic skeletal storage coefficients ( S ke and S kv ) of the developed-zone aquifer and conductance ( CR ) of the basin-fill faults for the entire Las Vegas basin. The results indicate that the subsidence observations from InSAR are extremely beneficial for accurately quantifying hydraulic parameters, and the model calibration results are far more accurate than when using only groundwater levels as observations, and just a limited number of subsidence observations. The discrepancy between distributions of pumping and greatest levels of subsidence is found to be attributed to spatial variations in clay thickness. The Eglington fault separates thicker interbeds to the northwest from thinner interbeds to the southeast and the fault may act as a groundwater-flow barrier and/or subsidence boundary, although the influence of the groundwater barrier to this area is found to be insignificant.

Beschreibung: After several years of decreased annual rainfall and water shortages, the Chilean society is demanding that forest plantations take accountability of their role in consuming scarce water resources. Evidence has shown that interception losses are considerable when determining water production in watersheds. The aim of this study was to determine if site and stand variables explain interception losses by Chilean forests for the development of an empirical model that could predict the potential impacts of forest management practices and land-use change. A total of 127 data from annual water balance plot studies in Chile were compiled to derive relationships between interception and precipitation, species composition, plantation age and other stand and site variables. The reviewed data indicated that annual interception losses are mainly explained by annual rainfall and basal area of the forest stands, with a clear difference between the northern (dryer) and southern (wetter) regions of Chile. For a wide latitudinal gradient, forest composition and age, annual interception accounted for approximately 21% of incoming precipitations. Broadleaved forest stands (including native broadleaved and eucalypt forests) generally presented higher interception losses than conifers. Interception was higher in northern zones indicating that forests have greater impacts on water resources in dryer regions. Our results were compiled in empirical models, which can be used to estimate forest interception in a latitudinal gradient in Chile and to support policy making. These results are also proposed as an approximate analogue of the changes in forest interception losses which may occur as vegetation belts shift latitudinally due to the impact of climate change.

Beschreibung: Estimates of the amount of topsoil in river sediments can help constrain sediment budgets on decadal time scales. The tracers 137 Cs and 210 Pb(ex) are used to determine the proportion of topsoil in river sediments in two Himalayan catchments, a relatively simple but effective method that could be used in many catchments in this complex mountain range for management purposes. Different results are reached apparently depending upon antecedent conditions, with a large component of topsoil in river sediments likely to be the result of rainfall that erodes hillslopes by sheet and rill processes, does not mobilise or mix with other sources of sediment such as from landslides, and does not generate high river flows to transport the topsoil downstream. These results show that sampling of tracers in sedimentary archives is essential to provide time series of topsoil input to Himalayan rivers to account for high temporal variability.

Beschreibung: The main purpose of this study is to experimentally investigate the effect of temperature on the seepage transport of suspended particles (SP) with a median diameter of 10 − 47 µm in a porous medium for various seepage velocities. The results show that the rise of temperature accelerates the irregular movements of SPs in the porous medium and reduces their migration velocity. As a result, the pore volume corresponding to the peak value of the breakthrough curves is apparently delayed, and the peak value in the effluent is decreased. The migration velocity of SPs decreases with increasing particle size, regardless of the Darcy velocity and temperature. The longitudinal dispersivity of SPs decreases slightly with increasing temperature and then remains almost unchanged. Larger particles experience more irregular movements induced by the limit of pore size, which leads to a larger dispersivity. The deposition coefficient increases with increasing temperature, especially in the case of a high seepage velocity, and then tends to be stable. The deposition coefficient for large-sized particles is higher than that for small-sized particles, which can be attributed to the restriction of large-sized particles by the narrow pores in the porous medium. The recovery rate decreases slightly with the increase of temperature until a critical value is reached, beyond which it remains almost unchanged. In summary, temperature is a significant factor affecting the transport and deposition of SPs in the porous medium, and the transport parameters such as particle velocity, dispersivity and deposition coefficient.

Beschreibung: This study aims at recognizing the mechanisms of mass transport between the karst surface and the saturated zone in a morpho-structural relief of the Mesozoic karst carbonate platform of Murgia (Puglia, Southern Italy). The large dimension of the karst aquifer, the regional scale of the flow system, the boundary condition constituted by the sea, and the lack of freshwater springs constrain to the use of wells as monitoring points and limit the study area to the recharge area comprising 986 endorheic basins. The concentrations of non-reactive tracers (nitrates) in the waters of autogenic recharge (from endorheic basins) have been modelled through the evaluation of effective infiltration, land use, and nitrogen surplus, with reference to a time window, which includes a low precipitation period followed by significant rainfall events. The comparison between the modelled nitrate concentrations and the nitrate concentrations measured in ground waters, coupled with the analysis of groundwater chemograms and records of hydraulic heads (all referred to the same time window), allows inferring the mechanism of mass transport between the karst surface and the groundwater table. The mass transport conceptual model requires the presence of the epikarst. The infiltration of significant rainfall in the endorheic basins after a low precipitation period displaces waters stored in the epikarst towards the saturated zone. Ground waters in the post-evet period show higher concentrations of nitrates, lower concentrations of Total Organic Carbon and higher Mg/Ca ratios than both those of the pre-event period and the autumn-winter recharge period. The post-event recharge from epikarst storage determines a transient hazard of groundwater pollution with a time lag from the occurrence of the heavy rainfall.

Beschreibung: Diagnosing the source of errors in snow models requires intensive observations, a flexible model framework to test competing hypotheses, and a methodology to systematically test the dominant snow processes. We present a novel process-based approach to diagnose model errors through an example that focuses on snow accumulation processes (precipitation partitioning, new snow density, and snow compaction). Twelve years of meteorological and snow board measurements were used to identify the main source of model error on each snow accumulation day. Results show that modeled values of new snow density were outside observational uncertainties in 52% of days available for evaluation, while precipitation partitioning and compaction were in error 45% and 16% of the time, respectively. Precipitation partitioning errors mattered more for total winter accumulation during the anomalously warm winter of 2014-2015, when a higher fraction of precipitation fell within the temperature range where partition methods had the largest error. These results demonstrate how isolating individual model processes can identify the primary source(s) of model error, which helps prioritize future research.

Beschreibung: Distributed hydrological models can make predictions with much finer spatial resolution than the supporting field data. They will, however, usually not have a predictive capability at model grid scale due to limitations of data availability and uncertainty of model conceptualisations. In previous publications we have introduced the Representative Elementary Scale (RES) concept as the theoretically minimum scale at which a model with a given conceptualisation has a potential for obtaining a predictive accuracy corresponding to a given acceptable accuracy. The new RES concept has similarities to the 25 year old Representative Elementary Area (REA) concept, but it differs in the sense that while REA addresses similarity between subcatchments by sampling within the catchment, RES focuses on effects of data or conceptualisation uncertainty by Monte Carlo simulations followed by a scale analysis. In the present paper we extend and generalise the RES concept to a framework for assessing the minimum scale of potential predictability of a distributed model applicable also for analyses of different model structures and data availabilities. We present three examples with RES analyses and discuss our findings in relation to Beven's alternative blueprint and environmental modelling philosophy from 2002. While Beven here addresses model structural and parameter uncertainties, he does not provide a thorough methodology for assessing to which extent model predictions for variables that are not measured possess opportunities to have meaningful predictive accuracies, or whether this is impossible due to limitations in data and models. This shortcoming is addressed by the RES framework through its analysis of the relationship between aggregation scale of model results and prediction uncertainties and for considering how alternative model structures and alternative data availability affect the results. We suggest that RES analysis should be applied in all modelling studies that aim to use simulation results at spatial scales smaller than the support scale of the calibration data.

Beschreibung: South America is experiencing rapid change in forest cover, of both native and planted forest. Forest cover loss is primarily attributable to fire, logging, and conversion of native forest to agriculture, pasture, and forest plantations, and types of change vary within and among the many diverse types of forests in South America. Major changes in forest cover and growing policy concerns underscore an urgent need for research on sustainable forest management and water ecosystem services in South America. Differences in land ownership and management objectives create tradeoffs between wood production and water ecosystem services from forests. Work is needed to quantify how forest change and management affect ecosystem services, such as wood production versus water provision. Current scientific understanding of forest management effects on water ecosystem services in South America has important limitations, including a scarcity of long-term records and few long-term integrated watershed studies. Industry, government, universities, and local communities should collaborate on integrated applied studies of forests and water. Data archiving and publically available data are required. The creation of national networks and a multi-country South America network to identify and implement common water research protocols, share results, and explore their implications would promote common and well supported policies. Hydrologists working in South America are well placed to tackle the challenges and opportunities for collaborative research that will maintain the intrinsic values and water ecosystem services provided by South America's forests.

Beschreibung: Headwater streamflows in the Rocky Mountain foothills are the key to water availability in Canadian Prairies. Headwater characteristics, however, have been and continue to be, subject to major variability and change. Here, we identify various forms of change in the annual mean streamflow and timing of the annual peak, and attempt to distinguish between the effects of multiple drivers using a generalized regression scheme. Our investigation shows that the Pacific Decadal Oscillation (PDO) is the main driver of significant monotonic trends and shifts in central tendency of annual mean streamflow in major headwaters. In parallel, the cumulative effects of non-PDO climatic drivers and human-induced land use and land management are the main causes of significant variations in the timing of the annual peak. Additional analyses show that time sequences with significant trends in annual mean streamflow and timing of the annual peak coincide with those that show significant trends in PDO or non-PDO component of the air temperature, respectively. The natural streamflow characteristics are substantially perturbed by anthropogenic river flow regulation, depending on the form of change and/or the level of regulation. Evidence suggests that the general tendency of human regulation is to reduce the severity of above- and below-average streamflow conditions; however, it may also, intensify the variability in natural streamflow characteristics under some circumstances. This has crucial importance during drier years and/or earlier annual peak timing, to which the regional water resource system is vulnerable. Our findings are important for the provision of effective regional water resource management and contribute to better understanding of the complex interactions between natural and anthropogenic drivers in coupled human-water systems.

Beschreibung: We collected soil-hydraulic property data from the literature for wildfire-affected soils, ash, and unburned soils. These data were used to calculate metrics and timescales of hydrologic response related to infiltration and surface runoff generation. Sorptivity ( S ) and wetting front potential (Ψ f ) were significantly different (lower) in burned soils compared to unburned soils, whereas field-saturated hydraulic conductivity ( K fs ) was not significantly different. The magnitude and duration of the influence of capillarity during infiltration was greatly reduced in burned soils, causing faster ponding times in response to rainfall. Ash had large values of S and K fs but moderate values of Ψ f , compared to unburned and burned soils, indicating ash has long ponding times in response to rainfall. The ratio of S 2 /K fs was nearly constant (~100 mm) for unburned soils but more variable in burned soils, suggesting that unburned soils have a balance between gravity and capillarity contributions to infiltration that may depend on soil organic matter, whereas in burned soils the gravity contribution to infiltration is greater. Changes in S and K fs in burned soils act synergistically to reduce infiltration and accelerate and amplify surface runoff generation. Synthesis of these findings identifies three key areas for future research. First, short timescales of capillary influences on infiltration indicate the need for better measurements of infiltration at times less than 1 minute to accurately characterize S in burned soils. Second, using parameter values, such as Ψ f , from unburned areas could produce substantial errors in hydrologic modeling when used without adjustment for wildfire effects, causing parameter compensation and resulting underestimation of K fs . Third, more thorough measurement campaigns that capture soil-structural changes, organic matter impacts, quantitative water repellency trends, and soil-water content along with soil-hydraulic properties could drive the development of better techniques for numerically simulating infiltration in burned areas.

Beschreibung: Arctic thaw lakes are an important source of water for aquatic ecosystems, wildlife and humans. Many recent studies have observed changes in Arctic surface waters related to climate warming and permafrost thaw, however explaining the trends and predicting future responses to warming is difficult without a stronger fundamental understanding of Arctic lake water budgets. By measuring and simulating surface and subsurface hydrologic fluxes, this work quantified the water budgets of three lakes with varying levels of seasonal drainage, and tested the hypothesis that lateral and subsurface flows are a major component of the post-snowmelt water budgets. A water budget focused only on post-snowmelt surface water fluxes (stream discharge, precipitation, and evaporation) could not close the budget for two of three lakes, even when uncertainty in input parameters was rigorously considered using a Monte Carlo approach. The water budgets indicated large, positive residuals, consistent with up to 70 % of mid-summer inflows entering lakes from lateral fluxes. Lateral inflows and outflows were simulated based on three processes; supra-permafrost subsurface inflows from basin-edge polygonal ground, and exchange between seasonally-drained lakes and their drained margins through runoff and evapotranspiration. Measurements and simulations indicate that rapid subsurface flow through highly conductive flowpaths in the polygonal ground can explain the majority of the inflow. Drained lakes were hydrologically connected to marshy areas on the lake margins, receiving water from runoff following precipitation and losing up to 38 % of lake efflux to drained margin evapotranspiration. Lateral fluxes can be a major part of Arctic thaw lake water budgets and a major control on summertime lake water levels. Incorporating these dynamics into models will improve our ability to predict lake volume changes, solute fluxes, and habitat availability in the changing Arctic.