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Beschreibung: Hydrological Processes, Volume 0, Issue ja, -Not available-.

Beschreibung: Soil moisture dynamics have a significant effect on overland flow generation. Catchment aspect is one of the major controlling factors of overland flow and soil moisture behaviour. A few experimental studies have been carried out in the uneven topography of the Himalayas. This study presents plot-scale experiments using portable rainfall simulator at an altitude of 1230 m above mean sea level and modelling of overland flow using observed datasets. Two plots were selected in two different aspects of Aglar watershed of lesser Himalaya; the agro-forested plot was positioned at the north aspect whereas the degraded plot was located at the south aspect of the hillslope. HS flumes and rain gauges were installed to measure the runoff at the outlet of the plot and the rainfall depth during rainfall simulation experiments. Moreover, ten soil moisture sensors were installed at upslope and downslope locations of both the plots at 5, 15, 25, 35 and 45 cm depth from ground level to capture the soil moisture dynamics. The tests were conducted at intensities of 79.8 and 75 mm/hr in agroforested plot and 82.2 and 72 mm/hr in the degraded plot during Test 1 and Test 2, respectively. The observed data indicates the presence of re-infiltration process only in the agro-forested. The high water holding capacity and the presence of re-infiltration process results in less runoff volume in the agro-forested plot compared to the degraded plot. The Hortonian overland flow mechanism was found to be the dominant overland flow mechanism as only a few layers of top soil gets saturated during all of the rainfall-runoff experiments. The runoff, rainfall and soil moisture data were subsequently used to calibrate the parameters of HYDRUS-2D overland flow module to simulate the runoff hydrograph and soil moisture. The components of hydrograph were evaluated in terms of peak discharge, runoff volume and time of concentration, the results were found to be within the satisfactory range. The goodness of fit of simulated hydrographs were more than 0.85 and 0.95 for agro-forested and degraded plot, respectively. The model produced satisfactory simulation results of soil moisture for all of the rainfall-runoff experiments. The HYDRUS-2D overland flow module was found promising to simulate the runoff hydrograph and soil moisture in plot-scale research.

Beschreibung: Despite the widely held assumption that trees negatively affect the local water budget in densely planted tree plantations, we still lack a clear understanding of the underlying processes by which canopy cover influences local soil water dynamics in more open, humid tropical ecosystems. In this study, we propose a new conceptual model that uses a combination of stable isotope and soil moisture measurements throughout the soil profile to assess potential mechanisms by which evaporation (of surface soil water and of canopy-intercepted rainfall) affects the relationship between surface soil water isotopic enrichment (lc-excess) and soil water content (SWC). Our conceptual model was derived from soil water data collected under deciduous and evergreen plants in a shade grown coffee agroforestry system in Costa Rica. Reduced soil moisture under shade trees during the “drier” season, coinciding when these trees were defoliated, was largely the result of increase soil water evaporation as indicated by the positive relationship between soil water content and lc-excess of surface soil water. In contrast, the evergreen coffee shrubs had a higher leaf area index during the “drier” season, leading to enhanced rainfall interception and a negative relationship between lc-excess and soil water content. During the wet season, there was no clear relationship between soil water content and between lc-excess of surface soil water. Greater surface soil water under coffee during the dry season may, in part, explain greater preferential flow under coffee compared to under trees in conditions of low rainfall intensities. However, with increasing rainfall intensities during the wet season there was no obvious difference in preferential flow between the two canopy covers. Results from this study indicate that our new conceptual model can be used to help disentangling the relative influence of canopy cover on local soil water isotopic composition and dynamics, yet also stresses the need for additional measurements to better resolve the underlying processes by which canopy structure influences local water dynamics.

Beschreibung: Recent advancements in analytical solutions to quantify water and solute travel time distributions (TTDs) and the related StorAge Selection (SAS) functions synthesize catchment complexity into a simplified, lumped representation. While these analytical approaches are efficient in application, they require rarely available long-term and high-frequency hydrochemical data for parameter estimation. Alternatively, integrated hydrologic models coupled to Lagrangian particle-tracking approaches can directly simulate age under different catchment geometries and complexity, but at a greater computational expense. Here, we bridge the two approaches, using a physically based model to explore the uncertainty in the estimation of the SAS function shape. In particular, we study the influence of subsurface heterogeneity, interactions between distinct flow domains (i.e., the vadose zone and saturated groundwater), diversity of flow pathways, and recharge rate on the shape of TTDs and the SAS functions. We use an integrated hydrology model, ParFlow, linked with a particle-tracking model, SLIM, to compute transient residence times (or ages) at every cell in the domain, facilitating a direct characterization of the SAS function. Steady-state results reveal that the SAS function shape shows a wide range of variation with respect to the variability in the structure of subsurface heterogeneity. Ensembles of spatially correlated realizations of hydraulic conductivity indicate that the SAS functions in the saturated groundwater have an overall weak tendency toward sampling younger ages, while the vadose zone gives a strong preference for older ages. We further show that the influence of recharge rate on the TTD is tightly dependent on the variability of subsurface hydraulic conductivity.

Beschreibung: Mountainous areas are characterized by steep slopes and rocky landforms, with hydrological conditions varying rapidly from upstream to downstream, creating variable interactions between groundwater and surface water. In this study, mechanisms of surface water–groundwater interaction within a headwater catchment of the North China Plain were assessed along the stream length and during different seasons, using hydrochemical and stable isotope data, and groundwater residence times estimated using chlorofluorocarbons (CFCs). These tracers indicate that the river is gaining, due to groundwater discharge in the headwater catchment both in the dry and rainy seasons. Residence time estimation of groundwater using CFCs data reveals that groundwater flow in the shallow sedimentary aquifer is dominated by the binary mixing of water approximating a piston flow model along two flow paths; old water, carried by a regional flow system along the direction of river flow, along with young water, which enters the river through local flow systems from hilly areas adjacent to the river valley (particularly during the rainy season). The larger mixing ratio of young water from lateral groundwater recharge and return flow of irrigation during the rainy season result in higher ion concentrations in groundwater than in the dry season. The binary mixing model showed that the ratio of young water vs. total groundwater ranged from 0.88 to 0.22 and 1.0 to 0.74 in the upper and lower reaches, respectively. In the middle reach, meandering stream morphology allows some loss of river water back into the aquifer, leading to increasing estimates of the ratio of young water (from 0.22 to 1). This is also explained by declining groundwater levels near the river, due to groundwater extraction for agricultural irrigation. The switch from a greater predominance of regional flow in the dry season, to more localized groundwater flow paths in the wet season is an important ground-surface water interaction mechanism, with important catchment management implications.

Beschreibung: Stone covers on loessial slopes can increase the time of infiltration by slowing the velocity of the overland flow, which reduces the transport of solutes, but few mechanistic models have been tested under water-scouring conditions. We carried out field experiments to test a previously proposed, physically based model of water and solute transport. The area of soil infiltration was calculated from the uncovered surface area, and Richards' equation and the kinematic wave equation were used to describe water infiltration and flow along slopes with stone covers. The transport of chemicals into the runoff from the surface soil, presumably by diffusion, and their movement in the soil profile could be described by the convection-diffusion equations of the model. The simulated and measured data correlated well. The stones on the soil surface reduced the area available for infiltration but increased the Manning coefficient, eventually leading to increased water infiltration and decreased solute loss with runoff. Our results indicated that the traditional model of water movement and solute migration could be used to simulate water transport and solute migration for stone-covered soil on loessial slopes.

Beschreibung: Finding an operational parameter vector is always challenging in the application of hydrologic models, with over-parameterisation and limited information from observations leading to uncertainty about the best parameter vectors. Thus, it is beneficialto find every possible behavioural parameter vector. This paper presents a new methodology, called the Patient Rule Induction Method for Parameter Estimation (PRIM-PE), to define where the behavioural parameter vectors are located in the parameter space. The PRIM-PE was used to discover all regions of the parameter space containing an acceptable model behaviour. This algorithm consists of an initial sampling procedure to generate a parameter sample that sufficiently represents the response surface witha uniform distribution within the "good-enough" region (i.e. performance better than a predefined threshold), and a rule induction component (PRIM) which is then used to define regions in the parameter space in which the acceptable parameter vectors are located. To investigate its ability in different situations, the methodology is evaluated using four test problems. The PRIM-PE sampling procedure was also compared against a Markov Chain Monte Carlo (MCMC) sampler known as the Differential Evolution Adaptive Metropolis (DREAM ZS ) algorithm. Finally, a spatially distributed hydrological model calibration problem with two settings (a 3-parameter and a 23-parameter calibration problem) was solved using the PRIM-PE algorithm. The results show that the PRIM-PE method captured the good-enough region in the parameter space successfully using 8 and 107 boxes for the 3-parameter and the 23-parameter problem, respectively. This good-enough region can be used in a global sensitivityanalysis to provide a broad range of parameter vectors that produce acceptable model performance. Moreover, for a specific objective function and model structure, the size of the boxes can be used as a measure of equifinality.

Beschreibung: The Soil and Water Assessment Tool (SWAT) has been widely used and thoroughly tested in many places in the world. The application of the SWAT model has pointed out that two of the major weaknesses of SWAT are related to the non-spatial reference of the Hydrologic Response Unit (HRU) concept and to the simplified groundwater concept, which contribute to its low performance in baseflow simulation and its inability to simulate regional groundwater flow. This study modified the groundwater module of SWAT to overcome the above limitations. The modified groundwater module has two aquifers. The local aquifer, which is the shallow aquifer in the original SWAT, represents a local groundwater flow system. The regional aquifer, which replaces the deep aquifer of the original SWAT, represents intermediate and regional groundwater flow systems. Groundwater recharge is partitioned into local and regional aquifer recharges. The regional aquifer is represented by a Multi-Cell Aquifer (MCA) model. The regional aquifer is discretized into cells using the Thiessen polygon method, where centers of the cells are locations of groundwater observation wells. Groundwater flow between cells is modeled using Darcy's Law. Return flow from cell to stream is conceptualized using a non-linear storage-discharge relationship. The SWAT model with the modified aquifer module, the so-called SWAT-MCA, was tested in two basins (Wipperau and Neetze) with porous aquifers in a lowland area in Lower Saxony, Germany. Results from the Wipperau basin show that the SWAT-MCA model is able (1) to simulate baseflow in a lowland area (where baseflow is a dominant source of streamflow) better than the original model, and (2) to simulate regional groundwater flow, shown by the simulated groundwater levels in cells, quite well.

Beschreibung: Stable isotopes analyses of oxygen ( 18 O) and hydrogen ( 2 H) of lake water were used to estimate the effect of evaporation ( E ) on the water quality of four shallow lakes in the Amapá State coast – Amazon/Brazil. These lakes, with different size and hydrologic conditions, were sampled during the course of the 2015/2016 El-Niño (record-breaking warming/drought in the Amazon rainforest). Hydrometeorological and water quality parameters were simultaneously performed to the isotopic sampling. The results showed that the evaporation process and the water quality can be explained by climate season and distances from the Atlantic Ocean. Lake evaporation losses ranged from ≈0-22% during the wet season in April/2016 and ≈35.7% during the dry season in November/2015. As expected, the evaporation of lake water was greater during the dry season, but it was higher for lakes farther away from the Atlantic Ocean compared to more coastal lakes due to tidal preponderance and the influence of major river channels. The more inland estuarine lakes showed a lower level of salinity (0.00-0.03ppt) compared to those closer to the Atlantic Ocean (0.01-0.08ppt). The El Niño phenomenon, with a lower precipitation in the Amazon basin, may initiate salinization of lakes closer to the Atlantic Ocean. Furthermore, strong mean seasonal variations of evaporation (0.06≤E≤0.22) and other hydrologic parameters were observed (precipitation, water temperature and water depth), with significant effects on the water quality such as salinity, dissolved oxygen, chlorophyll (p〈0.05). We conclude that the occurrence of the extreme climatic events can disrupt the biogeochemical and hydrological balance of these aquatic ecosystems and salinization of lakes closer to the Atlantic Ocean.

Beschreibung: In an aquifer system with complex hydrogeology, mixing of groundwater with different ages could occur associated with various flow pathways. In this study, we applied different groundwater age estimation techniques (lumped parameter model, and numerical model) to characterize groundwater age distributions and the major pathways of nitrate contamination in the Gosan agricultural field, Jeju Island. According to the lumped parameter model, groundwater age in the study area could be explained by the binary mixing of the young groundwater (4–33 years) and the old water component (〉60 years). The complex hydrogeologic regimes and local heterogeneity observed in the study area (multi-layered aquifer, well leakage hydraulics) were particularly well reflected in the numerical model. The numerical model predicted that the regional aquifer of Gosan responded to the fertilizer applications more rapidly (mean age: 9.7–22.3 years) than as estimated by other models. Our study results demonstrated that application and comparison of multiple age estimation methods can be useful to understand better the flow regimes and the mixing characteristics of groundwater with different ages (pathways), hence, to reduce the risk of improper groundwater management plan arising from the aquifer heterogeneity.

Beschreibung: Recession flows of a basin provide valuable information about its storage-discharge relationship as during recession periods discharge occurs due to depletion of storage. Storage-discharge analysis is generally performed by plotting − dQ / dt against Q , where Q is discharge at time t . For most real world catchments, − dQ / dt vs Q show a power-law relationship of the type: − dQ / dt  =  kQ α . Because the coefficient k varies across recession events significantly, the exponent α needs to be computed separately for individual recession events. The median α can then be considered as the representative α for the basin. The question that arises here is: what are the basin characteristics that influence the value of  α ? Studies based on a small number of basins (up to 50 basins) reveal that α has good relationship with several basin characteristics. However, whether such a relationship is universal remains an important question, since a universal relationship would allow prediction of the value of α for any ungauged basin. To test this hypothesis, here we study data collected from a relatively large number of basins (358 basins) in USA, and examine the influence of 35 different physio-climatic characteristics on α . We divide the basins into two groups based on their longitudes and test the relationship between α and basin characteristics separately for the two groups. The results indicate that α is not identically influenced by different basin characteristics for the two datasets. This may suggest that the power-law exponent α of a region is determined by the way local physio-climatic forces have shaped the landscape.

Beschreibung: Long-term hydrological data are key to understanding catchment behaviour and for decision making within water management and planning. Given the lack of observed data in many regions worldwide, such as Central America, hydrological models are an alternative for reproducing historical streamflow series. Additional types of information – to locally observed discharge – can be used to constrain model parameter uncertainty for ungauged catchments. Given the strong influence that climatic large-scale processes exert on streamflow variability in the Central-American region, we explored the use of climate variability knowledge as process constraints to constrain the simulated discharge uncertainty for a Costa Rican catchment, assumed to be ungauged. To reduce model uncertainty we first rejected parameter relationships that disagreed with our understanding of the system. Then, based on this reduced parameter space, we applied the climate-based process constraints at long-term, inter-annual and intra-annual time scales. In the first step we reduced the initial number of parameters by 52%, and then we further reduced the number of parameters by 3% with the climate constraints. Finally, we compared the climate-based constraints to a constraint based on global maps of low-flow statistics. This latter constraint proved to be more restrictive than those based on climate variability (further reducing the number of parameters by 66% compared to 3%). Even so, the climate-based constraints rejected inconsistent model simulations that were not rejected by the low-flow statistics constraint. When taken all together, the constraints produced constrained simulation uncertainty bands, and the median simulated discharge followed the observed time series to a similar level as an optimised model. All the constraints were found useful in constraining model uncertainty for an – assumed to be – ungauged basin. This shows that our method is promising for modelling long-term flow data for ungauged catchments on the Pacific side of Central America, and that similar methods can be developed for ungauged basins in other regions where climate variability exerts a strong control on streamflow variability.

Beschreibung: Karst areas and their catchments pose a great challenge for protection because fast conduit flow results in low natural attenuation of anthropogenic contaminants. Studies of the hydrochemistry of karst sources and river solutes are an important tool for securing and managing water resources. A study of the geochemical downriver evolution of the Wiesent River and its tributaries, located in a typical karst terrain, revealed unexpected downstream decreases of nitrate with maximum mean values of 30 mg L –1 at the source to minimum values of 18 mg L –1 near the river mouth. This trend persisted over the length of the river even though increased agricultural activities are evident in the downstream section of the catchment. This pattern is caused by fertilizer inputs via diffusive and fast conduits flow from karst lithology in the upstream area that may have reached the river's source even from beyond the hydrological catchment boundaries. Further downstream, these influences became diluted by tributary inputs that drain subcatchments dominated by claystone and sandstone lithologies that increased potassium and sulfate concentrations. In addition, with the exception of inputs in the source area, groundwater contributions along the river course were found to be minor. Our findings indicate that bedrock geology remains the dominant control on the major ion chemistry of the Wiesent River, and that agricultural influences are strongest near the headwaters despite increased land-use further downstream, due to long-term storage and accumulation in karst aquifers. This feature may not be unique to the Wiesent River system, as carbonates cover significant portions of the Earth's surface and subsequent work in other river systems could establish whether such patterns are ubiquitous worldwide.

Beschreibung: A continuous SCS CN method that considers time-varied SCS CN values was developed based on the original SCS CN method with a revised soil moisture accounting approach to estimate runoff depth for long-term discontinuous storm events. The method was applied to spatially distributed long-term hydrologic simulation of rainfall-runoff flow with an underlying assumption for its spatial variability using a GIS-based spatially distributed Clark's unit hydrograph method (Distributed-Clark; hybrid hydrologic model), which is a simple few parameter runoff routing method for input of spatiotemporally varied runoff depth, incorporating conditional unit hydrograph adoption for different runoff precipitation depth-based direct runoff flow convolution. Case studies of spatially distributed long-term (total of 6 years) hydrologic simulation for four river basins using daily NEXRAD quantitative precipitation estimates (QPEs) demonstrate overall performances of Nash-Sutcliffe efficiency ( E NS ) 0.62, coefficient of determination ( R 2 ) 0.64, and percent bias ( PBIAS ) 0.33% in direct runoff and E NS 0.71, R 2 0.72, and PBIAS 0.15% in total streamflow for model result comparison against observed streamflow. These results show better fit (improvement in E NS of 42.0% and R 2 of 33.3% for total streamflow) than the same model using spatially averaged gauged rainfall. Incorporation of logic for conditional initial abstraction in a continuous SCS CN method, which can accommodate initial runoff loss amounts based on previous rainfall, slightly enhances model simulation performance; both E NS and R 2 increased by 1.4% for total streamflow in a 4-year calibration period. A continuous SCS CN method-based hybrid hydrologic model presented in this study is, therefore, potentially significant to improved implementation of long-term hydrologic applications for spatially distributed rainfall-russnoff generation and routing, as a relatively simple hydrologic modeling approach for the use of more reliable gridded types of QPEs.

Beschreibung: In the northern Great Plains, melting snow is a primary driver of spring flooding, but limited knowledge of the magnitude and spatial distribution of snow water equivalent (SWE) hampers flood forecasting. Passive microwave remote sensing has the potential to enhance operational river flow forecasting, but is not routinely incorporated in operational flood forecasting. We compare satellite passive microwave estimates from the Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR-E) to NOAA Office of Water Prediction (OWP) airborne gamma radiation snow survey and U.S. Army Corps of Engineers (USACE) ground snow survey SWE estimates in the northern Great Plains from 2002 to 2011. AMSR-E SWE estimates compare favorably with USACE SWE measurements in the low relief, low vegetation study area (mean difference = -3.8 mm, root mean squared difference (RMSD) = 34.7 mm), but less so with OWP airborne gamma SWE estimates (mean difference = -9.5 mm, RMSD = 42.7 mm). An error simulation suggests that up to half of the error in the former comparison is potentially due to sub-pixel scale SWE variability, limiting the maximum achievable RMSD between ground and satellite SWE to approximately 26-33 mm in the Northern Great Plains. The OWP gamma vs. AMSR-E SWE comparison yields larger error than the point-scale USACE vs. AMSR-E comparison, despite a larger measurement footprint (5-7 km 2 vs. a few square centimeters, respectively), suggesting that there are unshared errors between the USACE and OWP gamma SWE data.

Beschreibung: Groundwater is not a sustainable resource, unless abstraction is balanced by recharge. Identifying the sources of recharge in a groundwater basin is critical for sustainable groundwater management. We studied the importance of river water recharge to groundwater in the southeastern San Joaquin Valley (24,000 km 2 , population 4 million). We combined dissolved noble gas concentrations, stable isotopes, tritium and carbon-14 analyses to analyze the sources, mechanisms and time scales of groundwater recharge. Area-representative groundwater sampling and numerical model input data enabled a stable isotope mass balance and quantitative estimates of river and local recharge. River recharge, identified by a lighter stable isotope signature, represents 47±4% of modern groundwater in the San Joaquin Valley (recharged after 1950), but only 26±4% of pre-modern groundwater (recharged before 1950). This implies that the importance of river water recharge in the San Joaquin valley has nearly doubled and is likely the result of a 40% increase in total recharge, caused by river water irrigation return flows and increased stream depletion and river recharge due to groundwater pumping. Compared to the large and long-duration capacity for water storage in the subsurface, storage of water in rivers is limited in time and volume, as evidenced by cold river recharge temperatures resulting from fast infiltration and recharge. Groundwater banking of seasonal surface water flows and expansion of managed aquifer recharge practices therefore appear to be a natural and promising method for increasing the resilience of the San Joaquin Valley water supply system.

Beschreibung: Abstract Yinchuan Basin, a semi-arid area located in Northwest China, is currently subject to increasing pressure from the altered hydrology due to the anthropogenic activities as well as increasing water demands for regional development. Sustainable water management across the region must be underpinned by a clear understanding of the factors that constrain water supply in this area. We measured the stable isotope of oxygen and hydrogen to determine the likely processes that control the interrelations among precipitation, surface water (Yellow River) and groundwater. The hydrogen and oxygen values demonstrate that two primary hydrochemical processes, mixing and evaporation/condensation, occurred in the Basin. Recharge proportions of precipitation and Yellow River were quantitatively evaluated based on the isotope mass balance method. The proportions of the Yellow River and atmospheric precipitation recharge are 87.7% and 12.3%, respectively. The evaporation proportions calculated with 18 O and D by Rayleigh fractional equation are close to each other, which demonstrate that evaporation intensity increases following the flow direction of the Yellow River. The findings obtained in this study are useful for recognizing the significance of Yellow River to Yinchuan Basin, and some optimal allocation schemes can be adopted for a prospective development of this reputed area in Northwest China.

Beschreibung: Spatial variability of throughfall ( TF ) isotopic composition, used as tracer input, influences isotope hydrological applications in forested watersheds. Notwithstanding, identification of the dominant canopy factors and processes that affect the patterns of TF isotopic variability remains ambiguous. Here, we examined the spatio-temporal variability of TF isotopic composition in a Japanese cypress plantation, in which intensive strip-thinning was performed, and investigated whether canopy structure at a fine resolution of canopy effect analysis is related to TF isotopic composition and how this is affected by meteorological factors. Canopy openness, as an index of canopy structure, was calculated from hemispherical photographs at different zenith angles. TF samples were collected in a 10 × 10 m experimental plot in both pre-thinning (from July to November 2010) and post-thinning (from May 2012 to March 2013) periods. Our results show that thinning resulted in a smaller alteration of input δ 18 O of gross precipitation, whereas the changes in d-excess varied in both directions. Despite the temporal stability of spatial patterns in TF amount, the spatial variability of TF isotopic composition was not temporally stable in both pre- and post-thinning periods. Additionally, after thinning, the isotopic composition of TF was best related to canopy openness calculated at the zenith angle of 7°, exhibiting three different relationships, i.e., significantly negative, significantly positive, and non-significant. Changes in meteorological factors (wind speed, rainfall intensity, and temperature) were found to affect the relationships between TF δ 18 O and canopy openness. The observed shifts in the relationships reveal different dominant factors (partial evaporation and the selection), and canopy water flowpaths control such differences. This study provides useful insights into the spatial variability of TF isotopic composition and improves our understanding of the physical processes of interception through canopy passage.

Beschreibung: Policies, measures and models geared towards flood prevention and managing surface waters benefit from high quality data on the presence and characteristics of drainage ditches. As a cost and labour effective alternative for acquiring such data through field surveys, we propose a method (i) to extract vector data representing ditch drainage networks based on local morphologic features derived from high resolution digital elevation models, and (ii) to identify possible (dis)connections in the ditch network by calculating a probability of the connectivity using a logistic regression where the predictor variables are characteristics of the ditch centre lines or derived from the DEM. Using LiDAR derived DEMs with a 1 m resolution, the method was developed and tested for a mixed agricultural residential area in north-eastern Belgium. The derived ditch segments derived had an error of omission of 8% and an error of commission of 5%. The original positional accuracy of the centre lines of the extracted ditches was 0.6 m and could be improved to 0.4 m by shifting each vertex to the position of the lowest LiDAR point located within a radius equal to the spatial resolution of the used DEM. 69% of the false disconnections in the network were identified and corrected leading to a reduction of the unconnected parts of the ditch network by 71%. The extracted and connected network approximated the reference ditch network fairly well.

Beschreibung: Use of isotopic tracers for sediment source apportionment is gaining interest with recent introduction of compound-specific stable isotope (CSSI) tracers. The method relies on linear mixing of source isotopic tracers, and deconvolution of a sediment mixture initially quantifies the contribution of sources to the mixture's tracer signature. Therefore, a correction to obtain real sediment source proportions is subsequently required. As far as we are aware, all published studies to date have used total isotopic tracer content or a proxy (e.g. soil carbon content) for this post-unmixing correction. However, as the relationship between the isotopic tracer mixture and the source mixture is different for each isotopic tracer, post-unmixing corrections cannot be carried out with one single factor. This contribution presents an isotopic tracer model structure - the concentration-dependent isotope mixing model (CD-IMM) - to overcome this limitation. Herein, we aim to clarify why the ‘conventional' approach to converting isotopic tracer proportions to source proportions using a single factor is wrong. In an initial mathematical assessment, error incurred by not using CD-IMM (NCD-IMM) in unmixing two sources with two isotopic tracers showed a complex relation as a function of relative tracer contents. Next, three artificial mixtures with different proportions of three soil sources were prepared and deconvoluted using 13 C of fatty acids using CD-IMM and NCD-IMM. Using NCD-IMM affected both accuracy (mean average error increased up to a threefold compared to the CD-IMM output) and precision (interquartile range was up to 2.5 times larger). Finally, as an illustrative example, the proportional source contribution reported in a published study was recalculated using CD-IMM. This resulted in changes in estimated source proportions and associated uncertainties. Content of isotopic tracers is seldom reported in published work concerning use of isotopic tracers for sediment source partitioning. The magnitude of errors made by miscalculation in former studies is therefore difficult to assess. With this contribution, we hope the community will acknowledge the limitations of prior approaches and use a CD-IMM in future studies.

Beschreibung: An analytical solution is presented for the slug tests conducted in a partially penetrating well in an unconfined aquifer affected from above by an unsaturated zone. The solution considers the effects of wellbore skin and oscillatory responses on underdamped slug tests. The flow in the saturated zone is described by a two-dimensional, axially symmetric governing equation, and the flow in the unsaturated zone above the water table by a linearized one-dimensional Richards' equation. The unsaturated medium properties are represented by the exponential constitutive relationships. A Laplace domain solution is derived using the Laplace and finite Fourier transform and the solution in the real-time domain is evaluated using the numerical inverse Laplace transform method. The solution derived in this study is more general and reduces to the most commonly used solutions for slug tests in their specified conditions. It is found that the unsaturated flow has a significant impact on the slug test conducted in an unconfined aquifer. The impact of unsaturated flow on such a slug test is enhanced with a larger anisotropy ratio, a shorter well screen length, a shorter distance between the well screen and the water table or larger well screen radius. The impact of unsaturated flow on slug tests decreases as the degree of penetration (the length of well screen) increases. For a fixed well screen length, the impact of unsaturated flow on slug tests decreases as the distance between the center of screen and the water table increases. A large dimensionless well screen radius (〉0.01) leads to significant effects of unsaturated flow on slug tests. The unsaturated flow reduces the oscillatory responses to underdamped slug tests. The unsaturated zone has significant impact on slug test under high-permeability wellbore skin.

Beschreibung: An understanding of surface and subsurface water contributions to streamflow is essential for accurate predictions of water supply from mountain watersheds that often serve as water towers for downstream communities. As such, this study used the end-member mixing analysis (EMMA) technique to investigate source water contributions and hydrologic flow paths of the 264 km 2 Boulder Creek Watershed, which drains the Colorado Front Range, USA. Four conservative hydrochemical tracers were used to describe this watershed as a three end-member system, and tracer concentration reconstruction suggested that the application of EMMA was robust. On average from 2009 to 2011, snowmelt and rain water from the subalpine zone and groundwater sampled from the upper montane zone contributed 54%, 22%, and 24% of the annual streamflow, respectively. These values demonstrate increased rain water and decreased snow water contributions to streamflow relative to area-weighted mean values derived from previous work at the headwater scale. Young water (2.3 ± 0.8 months) fractions of streamflow decreased from 18-22% in the alpine catchment to 8-10% in the lower elevation catchments and the watershed outlet with implications for subsurface storage and hydrological connectivity. These results contribute to a process-based understanding of the seasonal source water composition of a mesoscale watershed that can be used to extrapolate headwater streamflow generation predictions to larger spatial scales.

Beschreibung: Soils in post-wildfire environments are often characterized by a low infiltration capacity with a high degree of spatial heterogeneity relative to unburned areas. Debris flows are frequently initiated by runoff in recently burned steeplands, making itcritical to develop and test methods for incorporating spatial variability in infiltration capacity into hydrologic models. We use Monte Carlo simulations of runoff generation over a soil with a spatially heterogenous saturated hydraulic conductivity( K s ) to derive an expression for an aerially averaged saturated hydraulic conductivity ( ) that depends on the rainfall rate, the statistical properties of K s , and the spatial correlation length scale associated with K s . The proposed method for determining is tested by simulating runoff on synthetic topography over a wide range of spatial scales. Results provide a simplified expression for an effective saturated hydraulic conductivity that can be used to relate a distribution of small scale K s measurements to infiltration and runoff generation over larger spatial scales. Finally, we use a hydrologic model based on to simulate runoff and debris flow initiation at a recently burned catchment in in the Santa Ana Mountains, CA, USA and compare results to those obtained using an infiltration model based on the Soil Conservation Service Curve Number.

Beschreibung: This study focuses on the development of an approach to document the hydrological characteristics of peatlands and understand their potential influence on runoff processes and groundwater flow dynamics. Spatial calculations were performed using geographic information systems (GIS) data in order to evaluate the distribution of peatlands according to (1) neighboring hydrogeological units, and (2) their position within the hydrographic network. The data obtained from these calculations were plotted in a multiple trilinear diagram (two ternary plots projected into a diamond-shaped diagram) that illustrates the position of a given peatland within the hydrogeological environment. The data allows for the segregation of peatlands according to groups sharing similarities as well as the identification of peatlands that are most likely to have similar hydrological functions. The approach was tested in a 19 549 km 2 region of the southern portion of the Barlow-Ojibway Clay Belt (in Abitibi-Témiscamingue, Canada) and lead to a conceptual model representing the hydrological interactions between peatlands, aquifers and surface waters. This approach allows for a GIS-based differentiation of headwater peatland complexes that are likely to interact with aquifers and to supply continuous baseflow to small streams from lowland peatland complexes of the clay plain that are isolated from surrounding aquifers but that can act as storage reservoirs within the hydrographic network. The typology is further used to discuss land management strategies aimed at preserving peatland hydrodiversity within the study region. The proposed approach relies on widely applicable hydrogeological and hydrographic criteria and provides a tool that could be used for assessing peatland hydrodiversity in other regions of the planet.

Beschreibung: Geographically isolated wetlands, those entirely surrounded by uplands, provide numerous landscape-scale ecological functions, many of which are dependent on the degree to which they are hydrologically connected to nearby waters. There is a growing need for field-validated, landscape-scale approaches for classifying wetlands based on their expected degree of hydrologic connectivity with stream networks. This study quantified seasonal variability in surface hydrologic connectivity (SHC) patterns between forested Delmarva bay wetland complexes and perennial/intermittent streams at 23 sites over a full water year (2014-15). Field data were used to develop metrics to predict SHC using hypothesized landscape drivers of connectivity duration and timing. Connection duration was most strongly related to the number and area of wetlands within wetland complexes as well as the channel width of the temporary stream connecting the wetland complex to a perennial/intermittent stream. Timing of SHC onset was related to the topographic wetness index and drainage density within the catchment. Stepwise regression modeling found that landscape metrics could be used to predict SHC duration as a function of wetland complex catchment area, wetland area, wetland number, and soil available water storage (adj-R 2 = 0.74, p 〈 0.0001). Results may be applicable to assessments of forested depressional wetlands elsewhere in the U.S. Mid-Atlantic and Southeastern Coastal Plain, where climate, landscapes, and hydrological inputs and losses are expected to be similar to the study area.

Beschreibung: Interpreting rainfall-runoff erosivity by a process-oriented scheme allows to conjugate the physical approach to soil loss estimate with the empirical one. Including the effect of runoff in the model permits to distinguish between detachment and transport in the soil erosion process. In this paper, at first a general definition of the rainfall-runoff erosivity factor REF e including the power of both event runoff coefficient Q R and event rainfall erosivity index EI 30 of the Universal Soil Loss Equation (USLE) is proposed. The REF e factor is applicable to all USLE-based models (USLE, Modified USLE (USLE-M) and Modified USLE-M (USLE-MM)) and it allows to distinguish between purely empirical models (e.g. USLE-MM) and those supported by applying theoretical dimensional analysis and self-similarity to Wischmeier and Smith scheme. This last model category includes USLE, USLE-M and a new model, named USLE-MB, that uses a rainfall-runoff erosivity factor in which a power of runoff coefficient multiplies EI 30. Using the data base of Sparacia experimental site, the USLE-MB is parameterized and a comparison with soil loss data is carried out. The developed analysis shows that USLE-MB (characterized by a Nash-Sutcliffe Efficiency Index NSEI equal to 0.73 and a root mean square error RMSE = 11.7 Mg ha -1 ) has very similar soil loss estimate performances as compared with the USLE-M (NSEI = 0.72 and RMSE = 12.0 Mg ha -1 ). However, the USLE-MB yields a maximum discrepancy factor between predicted and measured soil loss values (176) which is much lower than that of USLE-M (291). In conclusion, the USLE-MB should be preferred in the context of theoretically supported USLE type models.

Beschreibung: In this study, the Precipitation-Runoff Modeling System (PRMS) was used to simulate changes in surface-water depression storage in the 1,126-square-kilometer Upper Pipestem Creek basin located within the Prairie Pothole Region (PPR) of North Dakota, United States. The PPR is characterized by millions of small water bodies (or surface-water depressions) which provide numerous ecosystem services and are considered an important contribution to the hydrologic cycle. The Upper Pipestem PRMS model was extracted from the U.S. Geological Survey's (USGS) National Hydrologic Model (NHM), developed to support consistent hydrologic modeling across the conterminous United States (CONUS). The Geospatial Fabric (GF) database, created for the USGS NHM, contains hydrologic model parameter values derived from datasets that characterize the physical features of the entire CONUS for 109,951 hydrologic response units (HRUs). Each HRU in the GF was parameterized using aggregated surface-water depression area derived from the National Hydrography Dataset Plus (NHDPlus), an integrated suite of application-ready geospatial data sets. This paper presents a calibration strategy for the Upper Pipestem PRMS model that uses normalized lake elevation measurements to calibrate the parameters influencing simulated fractional surface-water depression storage. Results indicate that inclusion of measurements that give an indication of the change in surface-water depression storage in the calibration procedure resulted in accurate changes in surface-water depression storage in the water balance. Regionalized parameterization of the USGS NHM will require a proxy for change in surface-storage to accurately parameterize surface-water depression storage within the USGS NHM.

Beschreibung: This study adopts a fresh approach to find the similarities between river planforms in contrast to many previous studies that have presented distinguishing characteristics and thresholds. This new approach links textural and morphological attributes of bedforms, termed morpho-texture, with process and hydrological regime thereby indicated. The study was carried out in depositional features of ephemeral rivers having a range of planforms and located in the Judean Desert. High resolution terrestrial laser scanning was undertaken to extract morphological and textural (roughness) characteristics. Results strongly indicate that bars are statistically coarser grained than the adjacent thalweg or anabranches where depositional processes occur. This suggests universal processes within ephemeral rivers regardless of their planform, where rapid flow recession causes the coarse tail of the bedload to be deposited as bars while the finer fractions deposit in the thalweg and thereby prevent the development of bed armor.

Beschreibung: This study demonstrates the importance of the including and appropriately parameterizing peatlands and forestlands for basin-scale integrated surface-subsurface models in the northern boreal forest, with particular emphasis on the Athabasca River Basin (ARB). With a long-term water balance approach to the ARB, we investigate reasons why downstream mean annual stream flow rates are consistently higher than upstream, despite the sub-humid water deficit conditions in the downstream regimes. A high-resolution 3D variably-saturated subsurface and surface water flow and evapotranspiration model of the ARB is constructed based on the bedrock and surficial geology and the spatial distribution of peatlands and their corresponding eco-regions. Historical climate data were used to drive the model for calibration against 40-year long-term average surface flow and groundwater observations during the historic instrumental period. The simulation results demonstrate that at the basin-scale peatlands and forestlands can have a strong influence on the surface-subsurface hydrologic systems. In particular, peatlands in the middle and downstream regimes of the ARB increase the water availability to the surface-subsurface water systems by reducing water loss through evapotranspiration. Based on the comparison of forestland evapotranspiration between observation and simulation, the overall spatial average evapotranspiration in downstream forestlands is larger than that in peatlands and thus the water contribution to the stream flow in downstream areas is relatively minor. Therefore, appropriate representation of peatlands and forestlands within the basin-scale hydrologic model is critical to reproduce the water balance of the ARB.

Beschreibung: In this paper a recently theoretically deduced rill flow resistance equation, based on a power-velocity profile, is tested experimentally on plots of varying slopes in which mobile bed rills are incised. Initially measurements of flow velocity, water depth, cross section area, wetted perimeter and bed slope conducted in 106 reaches of rills incised on an experimental plot having a slope of 14% were used to calibrate the flow resistance equation. Then the relationship between the velocity profile parameter Γ, the channel slope and the flow Froude number, which was calibrated using the 106 rill reach data, was tested using measurements carried out in plots having slopes of 22% and 9%. The measurements carried out in the latter slope conditions confirmed that a) the Darcy-Weisbach friction factor can be accurately estimated using the proposed theoretical approach, and b) the data were supportive of the slope independence hypothesis of rill velocity stated by Govers.

Beschreibung: Preferential flow, a term that includes macropore flow, non-equilibrium flow, and finger flow, stands in well-known conflict with Richards (1931) capillary flow. Acoustic velocity experiments demonstrate that preferential flow moves independently from, faster than, and before capillary flow during gravity-driven infiltration. Viscous flow in permeable media is briefly introduced to the point where Richards' (1931) particular treatment of viscosity turns out as the hydro-mechanical bifurcation from general laminar flow. Preferential flow is expected during significant infiltration, however, spatio-temporarily limited according to the viscous-flow regime. Two ways of delineating capillary flow from viscous flow reveal minimum path widths of preferential flow in the range of about 20 μm.

Beschreibung: Shallow groundwater plays a key role in agro-hydrological processes of arid areas. Groundwater often supplies a necessary part of the water requirement of crops and surrounding native vegetation, such as groundwater-dependent ecosystems. However, the impact of water saving irrigation on cropland water balance, such as the contribution of shallow groundwater to field evapotranspiration (ET) requires further investigation. Increased understanding of quantitative evaluation of field-scale water productivity under different irrigation methods aids policy and decision making. In this study, high-resolution water table depth (WTD) and soil water content (SWC) in field maize were monitored under conditions of flood irrigation (FI) and drip irrigation (DI), respectively. Groundwater evapotranspiration (ET g ) was estimated by the combination of the water table fluctuation method and an empirical groundwater-soil-atmosphere continuum model. The results indicate that daily ET g at different growth stages varies under the two irrigation methods. Between two consecutive irrigation events of the FI site, daily ET g rate increases from zero to greater than that of the DI site. Maize under DI steadily consumes more groundwater than FI, accounting for 16.4% and 14.5% of ET a , respectively. Overall, FI recharges groundwater, while DI extracts water from shallow groundwater. The yield under DI increases compared to that under FI, with less ET a (526 mm) compared to FI (578 mm), and irrigation water productivity improves from 3.51 kg m -3 (FI) to 4.58 kg m -3 (DI) through reducing deep drainage and soil evaporation by DI. These results highlight the critical role of irrigation method and groundwater on crop water consumption and productivity. This study provides important information to aid the development of agricultural irrigation schemes in arid areas with shallow groundwater.

Beschreibung: Forests in the Southeast U.S. are predicted to experience future changes in seasonal patterns of precipitation inputs as well as more variable precipitation events. These climate change induced alterations could increase drought and lower soil water availability. Drought could alter rooting patterns and increase the importance of deep roots that access subsurface water resources. To address plant response to drought in both deep rooting and soil water utilization as well as soil drainage, we utilize a throughfall reduction experiment in a loblolly pine plantation of the Southeast U.S. to calibrate and validate a hydrological model. The model was accurately calibrated against field measured soil moisture data under ambient rainfall and validated using 30% throughfall reduction data. Using this model we then tested these scenarios: (1) evenly reduced precipitation; (2) less precipitation in summer, more in winter; (3) same total amount of precipitation with less frequent but heavier storms; and (4) shallower rooting depth under the above three scenarios. When less precipitation was received, drainage decreased proportionally much faster than evapotranspiration implying plants will acquire water first to the detriment of drainage. When precipitation was reduced by more than 30%, plants relied on stored soil water to satisfy evapotranspiration suggesting 30% may be a threshold that if sustained over the long-term would deplete plant available soil water. Under the third scenario, evapotranspiration and drainage decreased, while surface runoff increased. Changes in root biomass measured before and four years after the throughfall reduction experiment were not detected among treatments. Model simulations, however, indicated gains in evapotranspiration with deeper roots under evenly reduced precipitation and seasonal precipitation redistribution scenarios, but not when precipitation frequency was adjusted. Deep soil and deep rooting can provide an important buffer capacity when precipitation alone cannot satisfy the evapotranspirational demand of forests. How this buffering capacity will persist in the face of changing precipitation inputs, however, will depend less on seasonal redistribution than on the magnitude of reductions and changes in rainfall frequency.

Beschreibung: Many hydrological models have been calibrated and validated using hydrographs alone. Since streamflow integrates water fluxes in space, many distributed hydrological models tend to have multiple feasible descriptions of hydrological processes. This equifinality usually leads to substantial prediction uncertainty. In this study, additional constraints – namely the spatial patterns of long-term average evapotranspiration (ET), shallow groundwater level and land cover change – were used to investigate the reduction of equifinality and prediction uncertainty in the Soil and Water Assessment Tool (SWAT) in the Wami River basin in Tanzania. The additional constraints were used in the setup, parameter emulation and calibration of the SWAT model termed an “improved hydrological model” (IHM). The IHM was then compared against a classical hydrological model (CHM) which was also developed using the SWAT model but without additional constraints. In the calibration, the CHM used only the hydrograph, but the IHM used the hydrograph and the spatial pattern of long-term average ET as an additional constraint. The IHM produced a single, unique behavioural simulation, whereas the CHM produced many behavioural simulations that resulted in prediction uncertainty. The performance of the IHM with respect to the hydrograph was more consistent than that of the CHM, and the former clearly captured the mean behaviour of ET in the river basin. Therefore, we conclude that additional constraints substantially reduce equifinality and prediction uncertainty in a distributed hydrological model.

Beschreibung: Rock fragment cover has long been an important agricultural crop production technique on the Loess Plateau, China. Although this approach plays an important role in controlling hydrological processes and preventing soil erosion, inconsistent results have been recovered in this field. In this study, we investigated the effects of rock fragment cover on infiltration, runoff, soil erosion and hydraulic parameters using rainfall simulation in the field in a semi-arid region of China. Two field plots encompassing six rock fragment coverages (0%, 10%, 20%, 25%, 30% and 40%), as well as two rock fragment positions and sizes were exposed to rainfall at a particular intensity (60 mm h –1 ). The results of this study showed that increasing the rock fragment coverage with rock fragments resting on the soil surface increased infiltration but decreased runoff generation and sediment yield. A contrasting result was found, however, when rock fragments were partially embedded into the soil surface; in this case, a positive relationship between rock fragment coverage and runoff rate as well as a non-monotonic relationship with respect to soil loss rate was recovered. The size of rock fragments also exerted a positive effect on runoff generation and sediment yield, but had a negative effect on infiltration. At the same time, both mean flow velocity and Froude number decreased with increasing rock fragment coverage regardless of rock fragment position and size, while both Manning roughness and Darcy-Weisbach friction factor were positively correlated. Results show that stream power is the most sensitive hydraulic parameter affecting soil loss. Combined with variance analysis, we concluded the order of significance of rock fragment cover variables was position followed by coverage and then size. We also quantitatively incorporated the effects of rock fragment cover on soil loss via the C and K factors in the Revised Universal Soil Loss Equation. Overall, this study will enable the development of more accurate modeling approaches and lead to a better understanding of hydrological processes under rock fragment cover conditions.

Beschreibung: The importance of riparian tree cover in reducing energy inputs to streams is increasingly recognised in schemes to mitigate climate change effects and protect fresh water ecosystems. Assessing different riparian management strategies requires catchment-scale understanding of how different planting scenarios would affect the stream energy balance, coupled with a quantitative assessment of spatial patterns of streamflow generation. Here, we use the physically-based MIKE SHE model to integrate simulations of catchment-scale runoff generation and in-stream hydraulics with a heat transfer model. This was calibrated to model the spatio-temporal distribution of hourly stream water temperature during warm low flow periods in a Scottish salmon stream. The model was explored as a “proof of concept” for a tool to investigate the effects of riparian management on high stream water temperatures that could affect juvenile Atlantic salmon. Uncertainty was incorporated in to the assessment using the Generalised Likelihood Uncertainty Estimation (GLUE) approach. Results showed that by decreasing both the warming (daylight hours) and the cooling (night-time hours) rates, forest cover leads to a reduction of the temperature range (with a delay of the time to peak by up to 2 hours) and can therefore be effectively used to moderate projected climate change effects. The modelling presented here facilitated the quantification of potential mitigating effects of alternative riparian management strategies and provided a valuable tool that has potential to be utilised as an evidence base for catchment management guidance.

Beschreibung: Lacustrine groundwater discharge (LGD) and the related water residence time are crucial parameters for quantifying lake matter budgets and assessing its vulnerability to contaminant input. Our approach utilizes the stable isotopes of water (δ 18 O, δ 2 H) and the radioisotope radon ( 222 Rn) for determining long-term average and short-term snapshots in LGD. We conducted isotope balances for the 0.5 km 2 Lake Ammelshainer See (Germany) based on measurements of lake isotope inventories and groundwater composition accompanied by good quality and comprehensive long-term meteorological and isotopic data (precipitation) from nearby monitoring stations. The results from the steady-state annual isotope balances that rely on only two sampling campaigns are consistent for both δ 18 O and δ 2 H and suggested an overall long-term average LGD rate which was used to infer the water residence time of the lake. These findings were supported by the good agreement of the simulated LGD driven annual cycles of δ 18 O and δ 2 H lake inventories with the observed lake isotope inventories. However, radon mass balances revealed lower values which might be the result of seasonal LGD variability. For obtaining further insights into possible seasonal variability of groundwater-lake interaction, stable water isotope and radon mass balances could be conducted more frequently (e.g., monthly) in order to use the derived groundwater discharge rates as input for time-variant isotope balances.

Beschreibung: The dependence of the soil water content ϑ upon the matric potential ψ is studied within a fractal approach which regards the water retention curve ( WRC ) as a sequence of well defined fractal regimes. Each of such regimes accounts for a given functional dependence ϑ  ≡  ϑ ( ψ ) , that in turn is characterized by a fractal dimension. The difference between the double (observed into sandy soils) and multi (observed into clay soils) fractal regimeis explained by recalling that, for a sandy soil, the transition from saturated to dry conditions is driven by a steep reduction of ψ . To the contrary, for a clay (where the change from the highest water contents to the smallest ones is characterized by a large range of the matric potential) the multi-fractal behavior is observed. These results are also confirmed by the analysis of experimental data. In particular, we show that the intermediate regime, generally accounting for the fractal multi-modality is, due to the sandy nature of the soil at stake, practically immaterial. Finally, we demonstrate that our model can be also regarded as the straightforward generalization of that of Millán and González-Posada (2005).

Beschreibung: Chemical hydrograph separation using electrical conductivity and digital filters are applied to quantify runoff components in the 1640 km 2 semi-arid Kaap River catchment and its sub-catchments in South Africa. A rich data set of weekly to monthly water quality data ranging from 1978 to 2012 (450 to 940 samples per site) was analysed at four sampling locations in the catchment. The data was routinely collected by South Africa's national Department of Water and Sanitation, using standard sampling procedures. Chemical hydrograph separation using Electrical Conductivity (EC) as a tracer was used as reference and a recursive digital filter was then calibrated for the catchment. Results of the two component hydrograph separation indicate the dominance of baseflow in the low flow regime, with a contribution of about 90% of total flow; however, during the wet season, baseflow accounts for 50% of total flow. The digital filter parameters were very sensitive and required calibration, using chemical hydrograph separation as a reference. Calibrated baseflow estimates ranged from 40% of total flow at the catchment outlet to 70% in the tributaries. The study demonstrates that routinely monitored water quality data, especially EC, can be used as a meaningful tracer, which could also aid in the calibration of a digital filter method and reduce uncertainty of estimated flow components. This information enhances our understanding of how baseflow is generated and contributes to streamflow throughout the year, which can aid in quantification of environmental flows, as well as to better parameterize hydrological models used for water resources planning and management. Baseflow estimates can also be useful for groundwater and water quality management.

Beschreibung: Rainfall is considered as the dominant water replenishment in desert ecosystems, and the conversion of rainfall into soil water availability plays a central role in sustaining the ecosystem function. In this study, the role of biological soil crusts (BSCs), typically formed in the revegetated desert ecosystem in the Tengger Desert of China, in converting rainfall into soil water, especially for the underlying soil moisture dynamics, was clarified by taking into account of the synthetic effects of BSCs, rainfall characteristics and antecedent soil water content on natural rainfall conditions at point scale. Our results showed that BSCs retard the infiltration process due to its higher water holding capacity during the initial stage of infiltration, such negative effect could be offset by the initial wet condition of BSCs. The influence of BSCs on infiltration amount was dependent on rainfall regime and soil depth. BSCs promoted a higher infiltration through the way of prolonged water containing duration in the ground surface, and exhibited a lower infiltration at deep soil layer, which were much more obvious under small and medium rainfall events for the BSCs area compared with the sand area. Generally, the higher infiltration at top soil layer only increased soil moisture at 0.03 m depth, in consequence, there was no water recharge for the deep soil, and thus BSCs had a negative effect on soil water effectiveness, which may be a potential challenge for the sustainability of the local deep-rooted vegetation under the site specific rainfall conditions in northwestern China.

Beschreibung: The understanding of the hydrology of plain basins may be improved by the combined analysis of rainfall-runoff records and remote sensed surface moisture data. Our work evaluates the surface moisture area (SMA) produced during rainfall-runoff events in a plain watershed of the Argentine Pampas Region, and studies which hydrological variables are related to the generated SMA. The study area is located in the upper and middle basins of the Del Azul stream, characterized by the presence of small gently hilly areas surrounded by flat landscapes. Data from nine rainfall-runoff events were analysed. MODIS surface reflectance data were processed to calculate SMA subsequent to the peak discharge (post-SMA), and previous to the rainfall events (prev-SMA), to consider the antecedent wetness. Rainfall-runoff data included: total precipitation depth (P), maximum intensity of rainfall over six hours (I6max), surface runoff registered between the beginning of the event and the day previous to the analysed MODIS scene (R), peak flow (Qp), and flood intensity (IF). In contrast with other works, post-SMA showed a negative relationship with the R. Three groups of cases were identified: (1) Events of low I6max, high prev-SMA and low R were associated with slow and weakly channelized flow over plain areas, leading to saturated overland flow (SOF), with large SMA. (2) Events of high I6max, low prev-SMA and medium to high R were rapidly transported along the gentle slopes of the basin, related to Hortonian overland flow (HOF) and low post-SMA. (3) Events of medium to high I6max and prev-SMA with medium R were related to heterogeneous input-antecedent-runoff conditions combined: local spatial conditions may have produced HOF or SOF, leading to an averaged response with medium SMA. The interactions between the geomorphology of the basin, the characteristics of the events and the antecedent conditions may explain the obtained results. This analysis is relevant for the general knowledge of the hydrology of large plains, whose functioning studies are still in their early stages.

Beschreibung: The response of the semi-alluvial clay-bed Watts Creek is assessed subject to climate change. Climate impacts are expected to have regional variability, and few studies have assessed the impacts of future climate in a small urban watershed. The 21 km 2 watershed located in Ottawa, Ontario, Canada is highly urbanised (68%) and agricultural (20%) with limited forest cover (12%). Continuous simulations were performed using the SWMHYMO lumped hydrologic modelling platform for the open water year, excluding spring freshet (April 1 st to October 31 st ). A shear stress exceedance and stream power erosion routine was added to the platform to calculate erosion potential. To account for uncertainty in the collected data, nine different field data sets were used to calibrate the model, each leading to a distinct set of calibrated parameter values. The difference between the data sets lies in the choice of the rating curves and calibration period. The 2041-2080 precipitation outputs of the fourth version of the Canadian Regional Climate Model (CanRCM4) ran under Representative Concentration Pathways (RCPs) 4.5 and 8.5 at the MacDonald Cartier International Airport were downscaled using quantile matching and then used as input to the continuous hydrologic model. For each set of calibrated parameters, a cumulative effective work index (CWI) based on the reach-averaged shear stress was calculated for Watts Creek using both the historic (1967 – 2007) and projected future (2041-2080) flows, using a bed material critical shear stress for entrainment of 3.7 Pa. This Results suggest an increase of 75% (resp. 139%) under RCP4.5 (resp. RCP8.5) in CWI compared to historic conditions for the average measured bed strength. The work index increase is driven by an increased occurrence of above-threshold events, and more importantly by the increased frequency of large events. The predicted flow regime under climate change would significantly alter the erosion potential and stability of Watts Creek.

Beschreibung: Preferential subsurface flow paths known as water tracks are often the principal hydrological pathways of headwater catchments in permafrost areas, exerting an influence on slope physical and biogeochemical processes. In polar deserts, where water resources depend on snow redistribution, water tracks are mostly found in hydrologically active areas downslope from snowdrifts. Here we measured the flow through seeping water track networks and at the front of a perennial snowdrift, at Ward Hunt Island in the Canadian High Arctic. We also used stable isotope analysis to determine the origin of this water, which ultimately discharges into Ward Hunt Lake. These measurements of water track hydrology indicated a glacio-nival runoff regime, with flow production mechanisms that included saturation overland flow (return flow) in a low sloping area, throughflow or pipe-like flow in most seepage locations, and infiltration excess overland flow at the front of the snowdrift. Each mechanism delivered varying proportions of snowmelt and ground water, and isotopic compositions evolved during the melting season. Unaltered snowmelt water contributed to 〉90% of total flow from water track networks early in the season, and these values fell to 〈 5% towards the end of the melting season. In contrast, infiltration excess overland flow from snowdrift consisted of a steady percentage of snowmelt water in July (mean of 69%) and August (71%). The water seeping at locations where no snow was left in August 2015 was isotopically enriched, indicating a contribution of the upper, ice-rich layer of permafrost to late summer discharge during warmer years. Air temperature is the main driver of snowmelt, but the effect of slope aspect on solar radiation best explained the diurnal discharge variation at all sites. The water tracks in this polar desert are part of a patterned ground network, which increases connectivity between the principal water sources (snowdrifts) and the bottom of the slope. This would reduce soil-water interactions and solute release, thereby favoring the low nutrient status of the lake.

Beschreibung: The El Niño Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO) are two important climate oscillations that affect hydrological processes at global and regional scales. However, few studies have attempted to identify their single and combined influences on water discharge variability at multiple time scales. In this study, we examine temporal variation in water discharge from the Yangtze River into the sea and explore the influence of the ENSO and the PDO on multi-scale variations in water discharge over the last century. The results of the wavelet transform analysis of the water discharge series show significant periodic variations at the inter-annual time scale of 2- to 8-yr and the decadal time scale of 15- to 17-yr. Water discharge tended to be higher during the La Niña/PDO cold phase and lower during the El Niño/PDO warm phase. The results of the cross wavelet spectrum and wavelet coherence analyses confirm the relationship between the inter-annual (i.e., 2- to 8-yr) and decadal (i.e., 15- to 17-yr) periodicities in water discharge with the ENSO and the PDO, respectively. As an important large-scale climate background, the PDO can modulate the influence of the ENSO on water discharge variability. In general, the warm PDO enhances the influence of El Niño events and the cold PDO enhances the influence of La Niña events. Our study is helpful in understanding the influencing mechanism of climate change on hydrological processes and provides an important scientific guideline for water resource prediction and management.

Beschreibung: Reserves of fresh groundwater on atoll islands are extremely fragile due to climatic and anthropogenic stresses. Of major concern is the quantity of water to be available in the coming decades under the influence of variable rainfall patterns, rising sea level, environmental conditions, and expected population growth that depends on groundwater resources. In this study, a three-dimensional numerical modeling approach using the SEAWAT modeling code is used to estimate freshwater lens volume fluctuation for four representative islands in the Republic of Maldives in response to long-term changes in rainfall, sea level rise, and anthropogenic stresses such as groundwater pumping, and short-term impacts from tsunami-induced marine over-wash events. This work is divided into two papers. This first paper presents numerical model set-up and calibration, and the effect of future rainfall patterns and sea level rise (SLR) on fresh groundwater reserves. The second paper focuses on marine overwash events. The results of simulated future freshwater lens volume presented in the first study contribute to efficient groundwater resources planning and management for the Maldives in the upcoming decades. Freshwater lenses in small atoll islands (area 〈 0.6 km 2 ) are shown to have a strong variability trends in the upcoming decades with expected reduction in lens volume between 11-36% due to SLR. In contrast, freshwater lenses in larger atoll islands (area 〉 1.0 km 2 ) are shown to have less variability to changing patterns with expected reduction in lens volume between 8-26% due to SLR. Study results can provide water resource managers with valuable findings for consideration in water security measures.

Beschreibung: This study presents a Lagrangian description of advection dominated field-scale transport of inert solutes, namely the statistics of advective solute travel time and trajectory, in non-stationary random groundwater flow fields. The vertical flow field is generated by the effects of changes in total stress applied in fluid-saturated heterogeneous deformable porous media. The Lagrangian statistics are developed directly from the velocity statistics based on the application of stochastic methodology such as the representation theorem and Fourier-Stieltjes integral representation. The focus of our numerical evaluation is placed on the influence of the soil compressibility on the Lagrangian statistics. The prediction on the statistics of solute travel time and trajectory may serve as the basic input to the environmental risk assessment.

Beschreibung: Changes in climate may significantly affect how sediment moves through watersheds into harbors and channels that are dredged for navigation or flood control. Here we applied a hydrologic model driven by a large suite of climate change scenarios to simulate both historical and future sediment yield and transport in two large, adjacent watersheds in the Great Lakes region. Using historical dredging expenditure data from the US Army Corps of Engineers (USACE) we then developed a pair of statistical models that link sediment discharge from each river to dredging costs at the watershed outlet. While both watersheds show similar slight decreases in streamflow and sediment yield in the near-term, by mid-century they diverge substantially. Dredging costs are projected to change in opposite directions for the two watersheds; we estimate that future dredging costs will decline in the St. Joseph River by 8-16% by mid-century but increase by 1-6% in the Maumee River. Our results show that the impacts of climate change on sediment yield and dredging may vary significantly by watershed even within a region, and that agricultural practices will play a large role in determining future streamflow and sediment loads. We also show that there are large variations in responses across climate projections that cause significant uncertainty in sediment and dredging projections.

Beschreibung: Polders are one of the most common artificial hydrological entities in the plain river network regions of China. Due to enclosed dikes, manual drainage and irrigation intake operations, polders have had a significant impact on the hydrological processes of these areas. Distributed hydrological models are effective tools to understand and reproduce the hydrological processes of a watershed. To date, however, few models are able to simulate the drainage and irrigation intake interactions of polders at a watershed-scale. This study develops a modified version of the Soil and Water Assessment Tool (SWAT) model, which is designed to better represent polders (SWATpld). The SWATpld model simulates drainage and irrigation intake processes by calculating the excess water storage in the inner rivers and irrigation schedule for paddy rice in the polder. Both SWAT and SWATpld models were tested for the Liyang Watershed. SWATpld outperformed SWAT in simulating the daily discharge and intake of the experimental polder and predicting the monthly peak flow at the outlet of the Liyang Watershed, which suggests that the modified model simulates the hydrological responses of the study watershed with polder operations more realistically than the original SWAT model does. Further evaluation at various locations and in various climate conditions would increase the confidence of this model.

Beschreibung: Marine overwash events are among the most serious short-term threats to groundwater supply of small coral islands. During such events, seawater can inundate small islands partially or completely, causing salinization of the aquifer. A comprehensive knowledge of freshwater lens recovery is essential for water planners on these islands. In this study, a numerical modeling approach is used to quantify recovery of the freshwater lens on four islands of the Maldives after a tsunami-induced overwash event similar to that experienced from the Indian Ocean earthquake in December 2004. The islands vary in size (0.2 km 2 to 10.1 km 2 ) and span the climatic regions of the Maldives. A tested three-dimensional SEAWAT groundwater model for each island is used to simulate the recovery process. Recharge rates from historical rainfall data and from global climate models are imposed on each island during the post-overwash recovery period. The effect of groundwater pumping on lens recovery also is examined. Results show abrupt decrease in fresh groundwater volumes for each island, followed by recovery that is significantly influenced by island size and recharge patterns. Overall, salinization is more widespread on small islands (〈 1 km 2 ), but recovery is more rapid than for large islands. Between 50% and 90% of lens recovery occurs after two years for small islands (〈 1 km 2 ), whereas only 35% and 55% for large islands. Imposing pumping rates required to sustain the local population lengthened the recovery time between 5% and 15%, with smaller islands having the higher percentage. However, the governing factor on recovery time is the spatial extent of land surface inundation by the overwash event, with wave height and duration of the event having a negligible impact. A strong relationship exists between required recovery time and island surface area, thereby providing a method to determine recovery time for other atoll islands not investigated in this study with similar geologic structure. Our results can be used to aid in managing water resources during the post-overwash period.

Beschreibung: Coastal peatlands represent an interface between marine and terrestrial ecosystems; their hydrology is affected by salt and fresh water inflow alike. Previous studies on bog peat have shown that pore water salinity can have an impact on the saturated hydraulic conductivity (K s ) of peat because of chemical pore dilation effects. In this study, we aimed at quantifying the impact of higher salinities (up to 3.5 % NaCl) on K s of fen peat. Two experiments employing a constant-head upward-flow permeameter and differing in measurement and salinity change duration were conducted. Additionally, a third experiment to determine the impact of water salinity on the release of dissolved organic carbon (DOC) of the studied peat type was carried out. The results show a decrease of K s with time, which does not depend on the water salinity but is differently shaped for different peat types. We assume pore clogging due to a conglomerate of physical, chemical and biological processes, which rather depend on water movement rate and time than on water salinity. However, an increased water salinity did increase the DOC release. We conclude that salinity-dependent behavior of K s is a function of peat chemistry and that for some peat types salinity may only affect the DOC release without having a pronounced impact on water flow.

Beschreibung: Drainage networks are the basis for segmentation of watersheds, an essential component in hydrological modelling, biogeochemical applications, and resource management plans. With the rapidly increasing availability of topographic information as Digital Elevation Models (DEMs), there have been many studies on DEM-based drainage network extraction algorithms. Most of traditional drainage network extraction methods require pre-processing of the DEM in order to remove ‘spurious’ sink, which can cause unrealistic results due to removal of real sinks as well. The Least Cost Path (LCP) algorithm can deal with flow routing over sinks without altering data. However, the existing LCP implementations can only simulate either single flow direction or multiple flow direction over terrain surfaces. Nevertheless, terrain surfaces in the real world are usually very complicated including both convergent and divergent flow patterns. The Triangular Form based Multiple flow algorithm (TFM), one of the traditional drainage network extraction methods, can estimate both single flow and multiple flow patterns. Thus, in this paper, it is proposed to combine the advantages of the LCP algorithm and the TFM algorithm in order to improve the accuracy of drainage network extraction from the DEM. The proposed algorithm is evaluated by implementing a data-independent assessment method based on four mathematical surfaces and validated against ‘true’ stream networks from aerial photograph, respectively. The results show that when compared to other commonly used algorithms, the new algorithm provides better flow estimation and is able to estimate both convergent and divergent flow patterns well regarding the mathematical surfaces and the real world DEM.

Beschreibung: Groundwater beneath the alluvial plain of the Indus River, Pakistan, is reported to be widely polluted by arsenic (As) and to adversely affect human health. In 79 groundwaters reported here from the lower Indus River plain, in southern Sindh Province, concentrations of As exceeded the WHO guideline value of 10 μg/L in 38%, with 22% exceeding 50 μg/L, Pakistan's guideline value. The As pollution is caused by microbially-mediated reductive dissolution of sedimentary iron-oxyhydroxides in anoxic groundwaters; oxic groundwaters contain 〈 10 μg/L of As. In the upper Indus River plain, in Punjab Province, localised As pollution of groundwater occurs by alkali desorption as a consequence of ion-exchange in groundwater, possibly supplemented by the use for irrigation of groundwater that has suffered ion-exchange in the aquifer and so has values 〉 0 for residual sodium carbonate. In the field area in southern Sindh, concentrations of Mn in groundwater exceed 0.4 mg/L in 11% of groundwaters, with a maximum of 0.7 mg/L, as a result of reduction of sedimentary manganese oxides. Other trace elements pose little or no threat to human health. Salinities in groundwaters range from fresh to saline (EC up to 6 mS/cm). High salinities result from local inputs of waste-water from unsewered sanitation, but mainly from evaporation/evapotranspiration of canal water and groundwater used for irrigation. The process does not concentrate As in the groundwater owing to sorption of As to soils. Ion-exchange exerts a control on concentrations of Na, Ca, and B, but not on As. High values of Cl/Br mass ratios (most » 288, the marine value) reflect the pervasive influence on groundwater of sewage-contaminated water from irrigation canals through seepage loss and deep percolation of irrigation water, with additional, well-specific, contributions from unsewered sanitation.

Beschreibung: In this study we investigated rainfall, runoff, and sediment transport dynamics (414 runoff events and 231 events with sediment information) of a humid mountain badland area—the Araguás catchment (Central Pyrenees, Spain)—from October 2005 to September 2016. Use of this long-term database allows characterization of the hydrological response, which consist of low-magnitude/high-frequency events and high-magnitude/low-frequency events, and identification of seasonal dynamics and rainfall-runoff thresholds. Our results indicate that the Araguás catchment, similarly to other humid badlands, had high hydrological responsiveness (mean annual runoff coefficient: 0.52), a non-linear relationship of rainfall with runoff (common in Mediterranean environments), and seasonal hydrological and sedimentological dynamics. We created and validated a multivariate regression model to characterize the hydrological variables (stormflow and peak discharge) and sedimentological variables (mean and maximum suspended sediment concentrations and total suspended sediment load). In summer and at the beginning of autumn, the response was mainly related to rainfall intensity, suggesting a predomination of Hortonian flows. In contrast, in spring and winter, the responses were mainly related to the antecedent conditions (previous rainfall and baseflow), suggesting the occurrence of saturated excess flow processes, and the contribution of neighboring vegetated areas. The multivariate analysis also showed that total sediment load (TL) is better predicted by a multivariate regression model that integrates pre-event, rainfall, and runoff variables. In general, our models provided more accurate predictions of small-magnitude/high-frequency events than high-magnitude/low-frequency events. This study highlights the high inter- and intra-annual variability response in humid badland areas and that long-term records are needed to reduce the uncertainty of hydrological and sedimentological response in Mediterranean badland areas.

Beschreibung: Precipitation is a major component of the hydrologic cycle in arid desert areas. To date, however, few studies have been conducted on investigating the isotope characteristics and moisture sources of precipitation in arid desert environments. The Alxa Desert Plateau is a critical arid desert area in North China. This study is the first to analyse the stable isotopic composition of precipitation to identify the sources of atmospheric moisture over this plateau. Our results show that the δD and δ 18 O values of precipitation across the plateau change greatly at both daily and monthly timescales, and exhibit seasonal variations. Among the main meteorological parameters, atmospheric temperature is the most predominant factor controlling the isotopic composition and the δD-δ 18 O relationship of local precipitation. Analyses of the precipitation isotopes with the HYSPLIT model reveal that (1) the westerly and polar moisture sources are the dominant controls on summer and winter precipitation and (2) the evaporation of local lake water significantly affects winter precipitation even though it only represents a small amount. Based on the isotope data of 2013-2016 precipitation, a local meteoric water line (LMWL) is derived: δD = (8.20 ± 0.22)·δ 18 O + (8.15 ± 2.16)‰ for the study site. Compared to the global meteoric water line (GMWL), the LMWL has a greater slope and lower d-excess. This can be explained by admixing of atmospheric moisture resulting from the evaporation of local lake water. Based on this LMWL, we are able to trace that groundwater of the Badain Jaran Desert originates from the surrounding mountains with altitudes of 〈4000 m. The newly derived LMWL shows that the recharge altitudes of desert groundwater are overestimated on the basis of the previous LMWLs. This study not only provides insights into the hydrological cycle but also offers guidance for water resource management in arid desert areas of China. Additionally, this study provides techniques that can be applied to the analyses of precipitation isotopes in similar arid regions of the world.

Beschreibung: Groundwater discharges in the western Canadian oil sands region impact river water quality. Mapping groundwater discharges into rivers in the oil sands region is important to target water quality monitoring efforts and to ensure injected wastewater and steam remain sequestered rather than eventually resurfacing. Saline springs comprised of Pleistocene-aged glacial meltwater exist in the region, but their spatial distribution has not been mapped comprehensively. Here we show that formation waters discharge into three major rivers as they flow through the Athabasca Oil Sands Region adjacent to many active oil sands projects. These discharges increase river chloride concentrations from river headwaters to downstream reaches by factors of ~23 in the Christina River, ~4 in the Clearwater River, and ~5 in the Athabasca River. Our survey provides further evidence for the substantial impact of formation water discharges on river water quality, even though they comprise less than ~2 % of total streamflow. Geochemical evidence supporting formation water discharges as the leading control on river salinity include increases in river chloride concentrations, Na/(Na+Ca) ratios, Cl/(Cl+SO 4 ) ratios, and decreases in in 87 Sr/ 86 Sr ratios; each mixing trend is consistent with saline groundwater discharges sourced from Cretaceous or Devonian aquifers. These regional subsurface-to-surface connections signify that injected wastewater or steam may potentially resurface in the future, emphasizing the critical importance of mapping groundwater flow paths to understand present-day streamflow quality and to predict the potential for injected fluids to resurface.

Beschreibung: The American cranberry (Vaccinium macrocarpon Ait.) is an important part of the cultural heritage and economy of southeastern Massachusetts, yet water quality concerns and wetland protection laws challenge its commercial production. Here, we report inputs and outputs of water, nitrogen (N), and phosphorus (P) for a 2.12-ha cranberry bed over a two-year period from 2013 to 2015. Precipitation, floodwater, irrigation, and groundwater accounted for, on average, 40%, 37%, 15%, and 8%, respectively, of all hydrological inputs to the cranberry bed. Minor annual variation in surface water discharge (~90 mm yr -1 or 2%) contrasted with large decreases in net nutrient export, from 16.3 to 9.3 kg ha -1 yr -1 for total (dissolved + particulate) nitrogen (TN) and from 3.27 to 1.31 kg ha -1 yr -1 for total phosphorus (TP). Annual variation in net TN and TP export was tied to decreases in spring and summer nutrient export from years 1 to 2. Elevated spring and summer net TN and TP export was consistent with the combined effects of fertilizer management, soil biogeochemistry, and hydrology. For instance, the relatively high spring TN export in year 1 was associated with coincident increases in soil temperature and rainfall. A second factor was the timing of aerial application of fertilizer, which was applied 1-d prior to a major summer storm (i.e., 5th largest storm in 88 years) and responsible for up to 15% and 10% of the annual TN and TP export, respectively. Nutrient budgets, which included water and fertilizer inputs combined with water, fruit, and vegetative biomass outputs, were consistent with the burial of plant material, or P bound to iron and aluminum hydroxyl-oxides, consisting of 21.5 kg N ha -1 yr -1 and 7.38 kg P ha -1 yr -1 . Field measurements indicated that burial would increase TN and TP in the shallow (0-5 cm) rooting zone by 14% and 6%, respectively, which seemed plausible based on the relatively young age of the bed (4-5 yr) and new root growth patterns in Vaccinium plants.

Beschreibung: Perennial pools are common natural features of peatlands and their hydrological functioning and turnover may be important for carbon fluxes, aquatic ecology and downstream water quality. Peatland restoration methods such as ditch blocking result in many new pools. However, little is known about the hydrological function of either pool type. We monitored six natural and six artificial pools on a Scottish blanket peatland. Pool water levels were more variable in all seasons in artificial pools having greater water level increases and faster recession responses to storms than natural pools. Pools overflowed by a median of 9 and 54 times pool volume per year for natural and artificial pools respectively but this varied widely because some large pools had small upslope catchments and vice versa . Mean peat water-table depths were similar between natural and artificial pool sites but much more variable over time at the artificial pool site, possibly due to a lower bulk specific yield across this site. Pool levels and pool-level fluctuations were not the same as those of local water tables in the adjacent peat. Pool level time-series were much smoother, with more damped rainfall or recession responses than those for peat water tables. There were strong hydraulic gradients between the peat and pools, with absolute water tables often being 20-30 cm higher or lower than water levels in pools only 1-4 m away. However, as peat hydraulic conductivity was very low (median of 1.5×10 -5 and 1.4×10 -6 cm s -1 at 30 and 50 cm depths at the natural pool site) there was little deep subsurface flow interaction. We conclude that: 1) for peat restoration projects, a larger total pool surface area is likely to result in smaller flood peaks downstream, at least during summer months, because peatland bulk specific yield will be greater; and 2) surface and near-surface connectivity during storm events and topographic context, rather than pool size alone, must be taken into account in future peatland pool and stream chemistry studies.

Beschreibung: The hydrology of near-surface glacier ice remains a neglected aspect of glacier hydrology despite its role in modulating meltwater delivery to downstream environments. To elucidate the hydrological characteristics of this near-surface glacial “weathering crust”, we describe the design and operation of a bespoke capacitance-based piezometer that enables rapid, economical deployment across multiple sites and provides an accurate, high-resolution record of near-surface water level fluctuations. Piezometers were employed at ten northern hemisphere glaciers, and through the application of standard bail-recharge techniques, we derive hydraulic conductivity ( K ) values from 0.003 to 3.519 m d -1 , with a mean of 0.185 ± 0.019 m d -1 . These results are comparable to those obtained in other discrete studies of glacier near-surface ice, and for firn, and indicate that the weathering crust represents a hydrologically inefficient aquifer. Hydraulic conductivity correlated positively with water table height but negatively with altitude and cumulative short-wave radiation since the last synoptic period of either negative air temperatures or turbulent energy flux dominance. The large range of K observed suggests complex interactions between meteorological influences and differences arising from variability in ice structure and crystallography. Our data demonstrate a greater complexity of near-surface ice hydrology than hitherto appreciated, and support the notion that the weathering crust can regulate the supraglacial discharge response to melt production. The conductivities reported here, coupled with typical supraglacial channel spacing, suggest that meltwater can be retained within the weathering crust for at least several days. This has implications not only for the accuracy of predictive meltwater runoff models, but we also argue for biogeochemical processes and transfers that are strongly conditioned by water residence time and the efficacy of the cascade of sediments, contaminants, microbes and nutrients to downstream ecosystems. Since continued atmospheric warming will incur rising snowline elevations and glacier thinning, the supraglacial hydrological system may assume greater importance in many mountainous regions and, consequently, detailing weathering crust hydraulics represents a research priority since the flow-path it represents remains poorly constrained.

Beschreibung: Determining the groundwater contribution of nonpoint source pollution at a watershed scale is a challenging issue. In this study, we utilized a top-down approach to characterize representative groundwater response units (GRUs) based on land use, and landscape position (e.g., upland, sideslope, floodplain) in the 275-km 2 Clear Creek, Iowa watershed. Groundwater monitoring wells were then established along downslope transects in representative GRUs. This unique combination of top-down/ bottom-up approaches allowed us to estimate groundwater pollutant loads at the watershed scale with minimal monitoring. For the 2015 study period, results indicated that more groundwater recharge occurred in the floodplain (404 mm) compared to the uplands or sideslopes (281 and 165 mm, respectively), irrespective of land use. Recharge in the floodplains consisted of 37% of the annual precipitation, while upland wells averaged 26% and sideslopes averaged 15% of the annual precipitation. Less recharge was found to occur beneath perennial grass compared to row crop and urbanized areas. Baseflow discharge accounted for 69% of the total NO 3 -N exported from the Clear Creek watershed, with row crop areas contributing approximately 95% of the annual load. Orthophosphorus (OP) yields were approximately 0.72 kg/ha beneath urban and suburban areas, three times higher than row crop or perennial areas. Urban and suburban areas accounted for 21.4% of groundwater OP and chloride loads in the watershed compared to only 8.5% of the land area. Overall, the groundwater load allocation model for baseflow nutrient discharge to Clear Creek can be used to target future NPS load reduction strategies at the watershed scale. The use of GRUs can pinpoint better areas of concern for controlling nutrient loads.

Beschreibung: The Loess Plateau has been experiencing large scale land use/cover changes (LUCC) over the past 50 years. It is well known about the significant decreasing trend of annual streamflow and sediment load in the catchments in this area. However, how surface runoff and sediment load behaved in response to LUCC at flood events remained a research question. We investigated a total of 371 flood events from 1963 to 2011 in a typical medium-size catchment within the Plateau in order to understand how LUCC affected the surface runoff generation and sediment load and their behaviors based on the analysis of return periods. The results showed that the mean annual surface runoff and sediment load from flood events accounted for 49.6% and 91.8% of their mean annual totals. The reduction of surface runoff and associated sediment yield in floods explained about 85.0% and 89.2% of declines in the total annual streamflow and sediment load, respectively. The occurrences of flood events and peak sediment concentrations greater than 500 kg/m 3 showed a significantly downward trend, yet the counterclockwise loop events still dominated the flood event processes in the catchment. The results suggest that LUCC over the past 50 years resulted in significant changes in the water balance components and associated soil erosion and sediment transportation in the catchment. This was achieved mainly by reducing surface runoff and sediment yield during floods with return period of less than 5 years. Runoff-sediment load behavior during the extreme events with greater than 10-year return periods has not changed. Outcomes from this study are useful in understanding the eco-hydrological processes and assisting the sustainable catchment management and land use planning on the Loess Plateau, and the methodologies are general and applicable to similar areas worldwide.

Beschreibung: The effects of soil water content (SWC) on the formation of runoff in grass swales draining into a storm sewer system were studied in two 30-m test swales with trapezoidal cross-sections. Swale 1 was built in a loamy fine-sand soil, on a slope of 1.5% and Swale 2 was built in a sandy loam soil, on a slope of 0.7%. In experimental runs, the swales were irrigated with two flow rates reproducing runoff from block rainfalls with intensities approximately corresponding to two-month and three-year events. Runoff experiments were conducted for SWC ini (initial SWC) ranging from 0.18 to 0.43 m 3 m -3 . For low SWC ini , the runoff volume was greatly reduced by up to 82%, but at high SWC ini , the volume reduction was as low as 15%. The relative swale flow volume reductions decreased with increasing SWC ini and, for the conditions studied, indicated a transition of the dominating swale functions from runoff dissipation to conveyance. Runoff flow peaks were reduced proportionally to the flow volume reductions, in the range from 4-55%. The swale outflow hydrograph lag-times varied from 5 to 15 minutes, with the high values corresponding to low SWC ini . Analysis of swale inflow/outflow hydrographs for high SWC ini allowed estimations of the saturated hydraulic conductivities as 3.27 and 4.84 cm/h, in Swales 1 and 2, respectively. Such estimates differed from averages (N=9) of double-ring infiltrometer measurements (9.41 and 1.78 cm/h, respectively). Irregularities in swale bottom slopes created bottom surface depression storage of 0.35 and 0.61 m 3 , for Swales 1 and 2, respectively, and functioned similarly as check-berms contributing to runoff attenuation. The experimental findings offer implications for drainage swale planning and design: (i) SWC ini strongly affect swale functioning in runoff dissipation and conveyance during the early phase of runoff, which is particularly important for design storms and their antecedent moisture conditions, and (ii) concerning the longevity of swale operation, Swale 1 remains fully functional even after almost 60 years of operation, as judged from its attractive appearance, good infiltration rates (3.27cm/h), and high flow capacity.

Beschreibung: The response time (lag time) between rainfall input and runoff output in headwater catchments is a key parameter for flood prediction. Lag times are expected to be controlled by runoff processes both on hillslopes and in channels. To demonstrate these effects on peak lag times within a 4.5-km 2 catchment, we measured stream water levels at up to 16 channel locations at 1-min intervals and compared the lag times with topographic indices describing the length and gradient of the hillslope and channel flowpath. We captured storm events with total precipitation of 38–198 mm and maximum hourly precipitation intensity of 9–90 mm/h. There were positive relationships between lag time and flowpath length as well as the ratio of the flowpath length and the square root of the gradient of channels for the most intense storms, demonstrating that channel flowpaths generally defined the variation in lag times. Topographic analysis showed that hillslope flowpath lengths were similar among locations, while channel flowpath length increased almost one order of magnitude with a 100-fold increase in catchment area. Thus, the relative importance of hillslope flowpath decreased with increasing catchment area. Our results indicate that the variation in lag times is small when hillslopes are sufficiently wet; thus, catchment-scale variation in lag times can be explained almost entirely by channel processes. Detailed topographic channel information can improve prediction of flood peak timing, while hillslopes can be treated as homogeneous during large flood events.

Beschreibung: In temperate humid catchments, evapotranspiration returns more than half of the annual precipitation to the atmosphere, thereby determining the balance available to recharge groundwaters and support stream flow and lake levels. Changes in evapotranspiration rates and therefore catchment hydrology could be driven by changes in land use or climate. Here we examine the catchment water balance over the past 50 y for a catchment in southwest Michigan covered by cropland, grassland, forest, and wetlands. Over the study period about 27% of the catchment has been abandoned from row-crop agriculture to perennial vegetation and about 20% of the catchment has reverted to deciduous forest, and the climate has warmed by 1.14 ° C. Despite these changes in land use, precipitation and stream discharge, and by inference catchment-scale evapotranspiration, have been stable over the study period. The remarkably stable rates of evapotranspirative water loss from the catchment across a period of significant land cover change suggest that rainfed annual crops and perennial vegetation do not differ greatly in evapotranspiration rates, and this is supported by measurements of evapotranspiration from various vegetation types based on soil water monitoring in the same catchment. Compensating changes in the other meteorological drivers of evaporative water demand besides air temperature—wind speed, atmospheric humidity, and net radiation—are also possible, but cannot be evaluated due to insufficient local data across the 50-y period. Regardless of the explanation, this study shows that the water balance of this landscape has been resilient in the face of both land cover and climate change over the past 50 y.

Beschreibung: A suite of instruments was deployed in a coastal wetland ecosystem in the Albemarle estuarine system, North Carolina (USA), to characterize wind-driven transport of saltwater through a constructed (man-made) channel. Flow velocity, electrical conductivity and stage were measured in a representative channel over a two-month period from May to July 2014, during which four wind tides were observed. Collected data show that thousands of metric tons of salt were advected through the channel into coastal wetlands during each event, which lasted up to four days. The results reveal that as much as 36% of advected salts accumulated in the wetlands, suggesting that the cumulative effects of these events on the health of coastal wetlands in the Albemarle system may be substantial due to the abundance of constructed channels and the frequency of wind-driven tidal events. This study is the first to quantify wind-driven salt fluxes through constructed channels in coastal wetland settings.

Beschreibung: Riverine solute versus discharge (C–Q) relationships provide information about the magnitude and dynamics of material fluxes from landscapes. We analysed long-term patterns of C–Q relationships for 44 rivers in Florida across a suite of geogenic, nutrient, and organic solutes and investigated land cover, watershed size, and surficial geology as controls on these patterns. Solute concentrations generally exhibited far less variability than did discharge, with coherent solute-specific behaviours repeated across watersheds. Geogenic solutes generally diluted with increasing discharge, whereas organic solutes generally enriched; patterns for nutrients were highly variable across watersheds, but on average exhibited chemostasis. Despite strong evidence of both geologic and land cover controls on solute flow-weighted concentrations, these variables were poor predictors of C–Q slopes (β) or relative coefficients of variation (CVC:CVQ). CVC:CVQ generally increased with watershed size, and wetland area appeared to influence C–Q patterns for base cations and organic solutes. Perhaps most importantly, we observed significant slope breaks in C–Q association in approximately half of our observations, challenging the generality of using single power functions to describe catchment solute export patterns. For all solutes except phosphorus (P), C–Q slopes decreased above statistically identified breaks (slopes for P increased), with breaks consistently at or near median flow (i.e., 50% flow exceedance probability). This common pattern significantly impacts solute load estimates; failing to account for slope breaks overestimates nitrate and total organic carbon loads as much as 125% and underestimates P loads as much as 35%. In addition to challenging generic power-law characterization of C–Q relationships for these coastal plain rivers, and exploring the load estimate consequences thereof, our study supports emerging insights about watershed hydrochemical behaviours across a wide array of solutes. Copyright © 2017 John Wiley & Sons, Ltd.

Beschreibung: Partitioning evapotranspiration (ET) into evaporation (E) and transpiration (T) in wetlands is important for understanding the hydrological processes in wetlands and the contribution of wetland ET to local and regional water cycling and for designing effective wetland management strategies. Stable water isotopes are useful in the application of ET partitioning through the evaluation of the isotopic compositions of E (δE), T (δT), and ET (δET) obtained from observation or modelling methods. However, this approach still suffers from potentially large uncertainties in terms of estimating the isotopic endmembers. In this study, we modified the traditional isotope-based ET partitioning methods to include leaf-level biological constraints to separately estimate the relative contributions of T from Scirpus triqueter and Phragmites australis and the relative contributions of E from the standing surface water in a semiarid marsh wetland in northeastern China. The results showed that although the δT values of S. triqueter and P. australis were rather similar, the mean δT values of the 2 species were different from the values of δE, making it possible to distinguish the relative contributions of E and T through the use of isotopes. The simulation of leaf water using a non-steady-state model indicated obvious deviations in leaf water enrichment (δLb) from isotopic steady states for both species, especially during early mornings and evenings when relative humidity was highest. The isotopic mass balance showed that E accounted for approximately 60% of ET, and T from S. triqueter and P. australis each contributed approximately 20% to ET; this implied that the transpiration of one reed was equivalent to that of 5.25 individuals of S. triqueter. Using the estimated ratio of T to ET and the measured leaf transpiration, the total ET was estimated to be approximately 10 mm day−1. Using the NSS-Tr method, the estimated ET was higher than the water loss calculated from the water level gauge. This indicated that the river water and surrounding groundwater were the sources of the marsh wetland, with a supply rate of 8.3 mm day−1. Copyright © 2017 John Wiley & Sons, Ltd.

Beschreibung: Following the publication of the above article in Hydrological Processes 31(24): 4250–4268; November 2017, the author has identified the need to change the email address orginally detailed as diogo.costa@arch.usask.ca to diogo.costa@usask.ca. The author would like to apologize for any inconvenience caused. Copyright © 2018 John Wiley & Sons, Ltd.

Beschreibung: Polders are one of the most common artificial hydrological entities in the plain river network regions of China. Due to enclosed dikes, manual drainage, and irrigation intake operations, polders have had a significant impact on the hydrological processes of these areas. Distributed hydrological models are effective tools to understand and reproduce the hydrological processes of a watershed. To date, however, few models are able to simulate the drainage and irrigation intake interactions of polders at a watershed scale. This study develops a modified version of the Soil and Water Assessment Tool (SWAT) model, which is designed to better represent polders (SWATpld). The SWATpld model simulates drainage and irrigation intake processes by calculating the excess-water storage in the inner rivers and irrigation schedule for paddy rice in the polder. Both SWAT and SWATpld models were tested for the Liyang watershed. SWATpld outperformed SWAT in simulating the daily discharge and intake of the experimental polder and predicting the monthly peak flow at the outlet of the Liyang watershed, which suggests that the modified model simulates the hydrological responses of the study watershed with polder operations more realistically than the original SWAT model does. Further evaluation at various locations and in various climate conditions would increase the confidence of this model. Copyright © 2018 John Wiley & Sons, Ltd.

Beschreibung: The rainfall–run-off convolution integral is analytically solved for several models for the elementary hydrograph. These solutions can be combined with available rainfall frequency analyses to predict flood flows along streams for different recurrence intervals, using no free parameters for gauged streams and one estimable parameter for ungauged streams. Extreme discharge magnitudes at gauged sites can be typically estimated within a factor of two of actual records, using no historical data on extreme flows. The flow predictions reproduce several important characteristics of the flood phenomenon, such as the slope of the regression line between observed extreme flows and basin area on the conventional logQ versus logA plot. Importantly, for the models and data sets investigated, the storm duration of greatest significance to flooding was found to approximate the intrinsic transport timescale of the particular watershed, which increases with basin size. Thus, storms that deliver extraordinary amounts of rainfall over a particular time interval will most greatly activate basins whose time constants approximately equal that interval. This theoretical finding is supported by examination of the regional hydrological response to the massive storms of September 14, 2008, and April 28–30, 2017, which caused extraordinary record flooding of basins of about 5–100 km2 and 500–4,000 km2, respectively, but produced few records in basins that were larger or smaller than those ranges. Copyright © 2018 John Wiley & Sons, Ltd.

Beschreibung: Finding an operational parameter vector is always challenging in the application of hydrologic models, with over-parameterization and limited information from observations leading to uncertainty about the best parameter vectors. Thus, it is beneficial to find every possible behavioural parameter vector. This paper presents a new methodology, called the patient rule induction method for parameter estimation (PRIM-PE), to define where the behavioural parameter vectors are located in the parameter space. The PRIM-PE was used to discover all regions of the parameter space containing an acceptable model behaviour. This algorithm consists of an initial sampling procedure to generate a parameter sample that sufficiently represents the response surface with a uniform distribution within the “good-enough” region (i.e., performance better than a predefined threshold) and a rule induction component (PRIM), which is then used to define regions in the parameter space in which the acceptable parameter vectors are located. To investigate its ability in different situations, the methodology is evaluated using four test problems. The PRIM-PE sampling procedure was also compared against a Markov chain Monte Carlo sampler known as the differential evolution adaptive Metropolis (DREAMZS) algorithm. Finally, a spatially distributed hydrological model calibration problem with two settings (a three-parameter calibration problem and a 23-parameter calibration problem) was solved using the PRIM-PE algorithm. The results show that the PRIM-PE method captured the good-enough region in the parameter space successfully using 8 and 107 boxes for the three-parameter and 23-parameter problems, respectively. This good-enough region can be used in a global sensitivity analysis to provide a broad range of parameter vectors that produce acceptable model performance. Moreover, for a specific objective function and model structure, the size of the boxes can be used as a measure of equifinality. Copyright © 2018 John Wiley & Sons, Ltd.

Beschreibung: This study focuses on the development of an approach to document the hydrological characteristics of peatlands and understand their potential influence on runoff processes and groundwater flow dynamics. Spatial calculations were performed using geographic information systems data in order to evaluate the distribution of peatlands according to (a) neighbouring hydrogeological units and (b) their position within the hydrographic network. The data obtained from these calculations were plotted in a multiple trilinear diagram (two ternary plots projected into a diamond-shaped diagram) that illustrates the position of a given peatland within the hydrogeological environment. The data allow for the segregation of peatlands according to groups sharing similarities as well as the identification of peatlands that are most likely to have similar hydrological functions. The approach was tested in a 19,549 km2 region of the southern portion of the Barlow-Ojibway Clay Belt (in Abitibi-Témiscamingue, Canada) and lead to a conceptual model representing the hydrological interactions between peatlands, aquifers, and surface waters. This approach allows for a geographic information systems-based differentiation of headwater peatland complexes that are likely to interact with aquifers and to supply continuous baseflow to small streams from lowland peatland complexes of the clay plain that are isolated from surrounding aquifers but that can act as storage reservoirs within the hydrographic network. The typology is further used to discuss land management strategies aimed at preserving peatland hydrodiversity within the study region. The proposed approach relies on widely applicable hydrogeological and hydrographic criteria and provides a tool that could be used for assessing peatland hydrodiversity in other regions of the planet. © 2018 John Wiley & Sons, Ltd.

Beschreibung: Despite the widely held assumption that trees negatively affect the local water budget in densely planted tree plantations, we still lack a clear understanding of the underlying processes by which canopy cover influences local soil water dynamics in more open, humid tropical ecosystems. In this study, we propose a new conceptual model that uses a combination of stable isotope and soil moisture measurements throughout the soil profile to assess potential mechanisms by which evaporation (of surface soil water and of canopy-intercepted rainfall) affects the relationship between surface soil water isotopic enrichment (lc-excess) and soil water content. Our conceptual model was derived from soil water data collected under deciduous and evergreen plants in a shade grown coffee agroforestry system in Costa Rica. Reduced soil moisture under shade trees during the “drier” season, coinciding when these trees were defoliated, was largely the result of increase soil water evaporation as indicated by the positive relationship between soil water content and lc-excess of surface soil water. In contrast, the evergreen coffee shrubs had a higher leaf area index during the “drier” season, leading to enhanced rainfall interception and a negative relationship between lc-excess and soil water content. During the wet season, there was no clear relationship between soil water content and between lc-excess of surface soil water. Greater surface soil water under coffee during the dry season may, in part, explain greater preferential flow under coffee compared with under trees in conditions of low rainfall intensities. However, with increasing rainfall intensities during the wet season, there was no obvious difference in preferential flow between the two canopy covers. Results from this study indicate that our new conceptual model can be used to help disentangling the relative influence of canopy cover on local soil water isotopic composition and dynamics, yet also stresses the need for additional measurements to better resolve the underlying processes by which canopy structure influences local water dynamics. Copyright © 2018 John Wiley & Sons, Ltd.

Beschreibung: Recent advancements in analytical solutions to quantify water and solute travel time distributions (TTDs) and the related StorAge Selection (SAS) functions synthesize catchment complexity into a simplified, lumped representation. Although these analytical approaches are efficient in application, they require rarely available long-term and high-frequency hydrochemical data for parameter estimation. Alternatively, integrated hydrologic models coupled to Lagrangian particle-tracking approaches can directly simulate age under different catchment geometries and complexity, but at a greater computational expense. Here, we bridge the two approaches, using a physically based model to explore the uncertainty in the estimation of the SAS function shape. In particular, we study the influence of subsurface heterogeneity, interactions between distinct flow domains (i.e., the vadose zone and saturated groundwater), diversity of flow pathways, and recharge rate on the shape of TTDs and the SAS functions. We use an integrated hydrology model, ParFlow, linked with a particle-tracking model, SLIM, to compute transient residence times (or ages) at every cell in the domain, facilitating a direct characterization of the SAS function. Steady-state results reveal that the SAS function shape shows a wide range of variation with respect to the variability in the structure of subsurface heterogeneity. Ensembles of spatially correlated realizations of hydraulic conductivity indicate that the SAS functions in the saturated groundwater have an overall weak tendency toward sampling younger ages, whereas the vadose zone gives a strong preference for older ages. We further show that the influence of recharge rate on the TTD is tightly dependent on the variability of subsurface hydraulic conductivity. Copyright © 2018 John Wiley & Sons, Ltd.

Beschreibung: Lacustrine groundwater discharge (LGD) and the related water residence time are crucial parameters for quantifying lake matter budgets and assessing its vulnerability to contaminant input. Our approach utilizes the stable isotopes of water (δ18O, δ2H) and the radioisotope radon (222Rn) for determining long-term average and short-term snapshots in LGD. We conducted isotope balances for the 0.5-km2 Lake Ammelshainer See (Germany) based on measurements of lake isotope inventories and groundwater composition accompanied by good quality and comprehensive long-term meteorological and isotopic data (precipitation) from nearby monitoring stations. The results from the steady-state annual isotope balances that rely on only two sampling campaigns are consistent for both δ18O and δ2H and suggested an overall long-term average LGD rate that was used to infer the water residence time of the lake. These findings were supported by the good agreement of the simulated LGD-driven annual cycles of δ18O and δ2H lake inventories with the observed lake isotope inventories. However, radon mass balances revealed lower values that might be the result of seasonal LGD variability. For obtaining further insights into possible seasonal variability of groundwater–lake interaction, stable water isotope and radon mass balances could be conducted more frequently (e.g., monthly) in order to use the derived groundwater discharge rates as input for time-variant isotope balances. Copyright © 2018 John Wiley & Sons, Ltd.

Beschreibung: The El Niño Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO) are two important climate oscillations that affect hydrological processes at global and regional scales. However, few studies have attempted to identify their single and combined influences on water discharge variability at multiple timescales. In this study, we examine temporal variation in water discharge from the Yangtze River into the sea and explore the influence of the ENSO and the PDO on multiscale variations in water discharge over the last century. The results of the wavelet transform analysis of the water discharge series show significant periodic variations at the interannual timescale of 2 to 8 years and the decadal timescale of 15 to 17 years. Water discharge tended to be higher during the La Niña–PDO cold phase and lower during the El Niño–PDO warm phase. The results of the cross wavelet spectrum and wavelet coherence analyses confirm the relationship between the interannual (i.e., 2 to 8 years) and decadal (i.e., 15 to 17 years) periodicities in water discharge with the ENSO and the PDO, respectively. As an important large-scale climate background, the PDO can modulate the influence of the ENSO on water discharge variability. In general, the warm PDO enhances the influence of El Niño events, and the cold PDO enhances the influence of La Niña events. Our study is helpful in understanding the influencing mechanism of climate change on hydrological processes and provides an important scientific guideline for water resource prediction and management. Copyright © 2018 John Wiley & Sons, Ltd.

Beschreibung: Spatial variability of throughfall (TF) isotopic composition, used as tracer input, influences isotope hydrological applications in forested watersheds. Notwithstanding, identification of the dominant canopy factors and processes that affect the patterns of TF isotopic variability remains ambiguous. Here, we examined the spatio-temporal variability of TF isotopic composition in a Japanese cypress plantation, in which intensive strip thinning was performed and investigated whether canopy structure at a fine resolution of canopy effect analysis is related to TF isotopic composition and how this is affected by meteorological factors. Canopy openness, as an index of canopy structure, was calculated from hemispherical photographs at different zenith angles. TF samples were collected in a 10 × 10 m experimental plot in both pre-thinning (from July to November 2010) and post-thinning (from May 2012 to March 2013) periods. Our results show that thinning resulted in a smaller alteration of input δ18O of gross precipitation, whereas the changes in deuterium excess varied in both directions. Despite the temporal stability of spatial patterns in TF amount, the spatial variability of TF isotopic composition was not temporally stable in both pre- and post-thinning periods. Additionally, after thinning, the isotopic composition of TF was best related to canopy openness calculated at the zenith angle of 7°, exhibiting three different relationships, that is, significantly negative, significantly positive, and nonsignificant. Changes in meteorological factors (wind speed, rainfall intensity, and temperature) were found to affect the relationships between TF δ18O and canopy openness. The observed shifts in the relationships reveal different dominant factors (partial evaporation and the selection), and canopy water flowpaths control such differences. This study provides useful insights into the spatial variability of TF isotopic composition and improves our understanding of the physical processes of interception through canopy passage. Copyright © 2018 John Wiley & Sons, Ltd.

Beschreibung: Coastal peatlands represent an interface between marine and terrestrial ecosystems; their hydrology is affected by salt and fresh water inflow alike. Previous studies on bog peat have shown that pore water salinity can have an impact on the saturated hydraulic conductivity (Ks) of peat because of chemical pore dilation effects. In this study, we aimed at quantifying the impact of higher salinities (up to 3.5% NaCl) on Ks of fen peat. Two experiments employing a constant-head upward-flow permeameter and differing in measurement and salinity change duration were conducted. Additionally, a third experiment to determine the impact of water salinity on the release of dissolved organic carbon (DOC) of the studied peat type was carried out. The results show a decrease of Ks with time, which does not depend on the water salinity but is differently shaped for different peat types. We assume pore clogging due to a conglomerate of physical, chemical, and biological processes, which rather depend on water movement rate and time than on water salinity. However, an increased water salinity did increase the DOC release. We conclude that salinity-dependent behaviour of Ks is a function of peat chemistry and that for some peat types, salinity may only affect the DOC release without having a pronounced impact on water flow. Copyright © 2018 John Wiley & Sons, Ltd.

Beschreibung: The importance of riparian tree cover in reducing energy inputs to streams is increasingly recognized in schemes to mitigate climate change effects and protect freshwater ecosystems. Assessing different riparian management strategies requires catchment-scale understanding of how different planting scenarios would affect the stream energy balance, coupled with a quantitative assessment of spatial patterns of streamflow generation. Here, we use the physically based MIKE SHE model to integrate simulations of catchment-scale run-off generation and in-stream hydraulics with a heat transfer model. This was calibrated to model the spatio-temporal distribution of hourly stream water temperature during warm low flow periods in a Scottish salmon stream. The model was explored as a “proof of concept” for a tool to investigate the effects of riparian management on high stream water temperatures that could affect juvenile Atlantic salmon. Uncertainty was incorporated into the assessment using the generalized likelihood uncertainty estimation approach. Results showed that by decreasing both the warming (daylight hours) and the cooling (night-time hours) rates, forest cover leads to a reduction of the temperature range (with a delay of the time to peak by up to 2 hr) and can therefore be effectively used to moderate projected climate change effects. The modelling presented here facilitated the quantification of potential mitigating effects of alternative riparian management strategies and provided a valuable tool that has potential to be utilized as an evidence base for catchment management guidance. Copyright © 2018 John Wiley & Sons, Ltd.

Beschreibung: An analytical solution is presented for the slug tests conducted in a partially penetrating well in an unconfined aquifer affected from above by an unsaturated zone. The solution considers the effects of wellbore skin and oscillatory responses on underdamped slug tests. The flow in the saturated zone is described by a two-dimensional, axially symmetric governing equation, and the flow in the unsaturated zone above the water table by a linearized one-dimensional Richards' equation. The unsaturated medium properties are represented by the exponential constitutive relationships. A Laplace domain solution is derived using the Laplace and finite Fourier transform and the solution in the real-time domain is evaluated using the numerical inverse Laplace transform method. The solution derived in this study is more general and reduces to the most commonly used solutions for slug tests in their specified conditions. It is found that the unsaturated flow has a significant impact on the slug test conducted in an unconfined aquifer. The impact of unsaturated flow on such a slug test is enhanced with a larger anisotropy ratio, a shorter well screen length, a shorter distance between the well screen and the water table, or a larger well screen radius. The impact of unsaturated flow on slug tests decreases as the degree of penetration (the length of well screen) increases. For a fixed well screen length, the impact of unsaturated flow on slug tests decreases as the distance between the centre of screen and the water table increases. A large dimensionless well screen radius (〉0.01) leads to significant effects of unsaturated flow on slug tests. The unsaturated flow reduces the oscillatory responses to underdamped slug tests. The unsaturated zone has significant impact on slug test under high-permeability wellbore skin. Copyright © 2018 John Wiley & Sons, Ltd.

Beschreibung: Mountainous areas are characterized by steep slopes and rocky landforms, with hydrological conditions varying rapidly from upstream to downstream, creating variable interactions between groundwater and surface water. In this study, mechanisms of groundwater–surface water interactions within a headwater catchment of the North China Plain were assessed along the stream length and during different seasons, using hydrochemical and stable isotope data, and groundwater residence times estimated using chlorofluorocarbons. These tracers indicate that the river is gaining, due to groundwater discharge in the headwater catchment both in the dry and rainy seasons. Residence time estimation of groundwater using chlorofluorocarbons data reveals that groundwater flow in the shallow sedimentary aquifer is dominated by the binary mixing of water approximating a piston flow model along 2 flow paths: old water, carried by a regional flow system along the direction of river flow, along with young water, which enters the river through local flow systems from hilly areas adjacent to the river valley (particularly during the rainy season). The larger mixing ratio of young water from lateral groundwater recharge and return flow of irrigation during the rainy season result in higher ion concentrations in groundwater than in the dry season. The binary mixing model showed that the ratio of young water versus total groundwater ranged from 0.88 to 0.22 and 1.0 to 0.74 in the upper and lower reaches, respectively. In the middle reach, meandering stream morphology allows some loss of river water back into the aquifer, leading to increasing estimates of the ratio of young water (from 0.22 to 1). This is also explained by declining groundwater levels near the river, due to groundwater extraction for agricultural irrigation. The switch from a greater predominance of regional flow in the dry season, to more localized groundwater flow paths in the wet season is an important groundwater–surface water interactions mechanism, with important catchment management implications. Copyright © 2018 John Wiley & Sons, Ltd.

Beschreibung: Critical zone influences on hydrologic partitioning, subsurface flow paths and reactions along these flow paths dictate the timing and magnitude of groundwater and solute flux to streams. To isolate first-order controls on seasonal streamflow generation within highly heterogeneous, snow-dominated basins of the Colorado River, we employ a multivariate statistical approach of end-member mixing analysis using a suite of daily chemical and isotopic observations. Mixing models are developed across 11 nested basins (0.4 to 85 km2) spanning a gradient of climatological, physical, and geological characteristics. Hydrograph separation using rain, snow, and groundwater as end-members indicates that seasonal contributions of groundwater to streams is significant. Mean annual groundwater flux ranges from 12% to 33% whereas maximum groundwater contributions of 17% to 50% occur during baseflow. The direct relationship between snow water equivalent and groundwater flux to streams is scale dependent with a trend toward self-similarity when basins exceed 5.5 km2. We find groundwater recharge increases in basins of high relief and within the upper subalpine where maximum snow accumulation is coincident with reduced conifer cover and lower canopy densities. The mixing model developed for the furthest downstream site did not transfer to upstream basins. The resulting error in predicted stream concentrations points toward weathering reactions as a function of source rock and seasonal shifts in flow path. Additionally, the potential for microbial sulfate reduction in floodplain sediments along a low-gradient, meandering portion of the river is sufficient to modify hillslope contributions and alter mixing ratios in the analysis. Soil flushing in response to snowmelt is not included as an end-member but is identified as an important mechanism for release of solutes from these mountainous watersheds. End-member mixing analysis used in combination with high-frequency observations reveals important aspects of catchment hydrodynamics across scale. Copyright © 2018 John Wiley & Sons, Ltd.

Beschreibung: Soil bulk density (ρb) is commonly treated as static in studies of land surface dynamics. Magnitudes of errors associated with this assumption are largely unknown. Our objectives were to (a) quantify ρb effects on soil hydrologic and thermal properties and (b) evaluate effects of ρb on surface energy balance and heat and water transfer. We evaluated 6 soil properties, volumetric heat capacity, thermal conductivity, soil thermal diffusivity, water retention characteristics, hydraulic conductivity, and vapour diffusivity, over a range of ρb, using a combination of 6 models. Thermal conductivity, water retention, hydraulic conductivity, and vapour diffusivity were most sensitive to ρb, each changing by fractions greater than the associated fractional changes in ρb. A 10% change in ρb led to 10–11% change in thermal conductivity, 6–11% change in saturated and residual water content, 49–54% change in saturated hydraulic conductivity, and 80% change in vapour diffusivity. Subsequently, 3 field seasons were simulated with a numerical model (HYDRUS-1D) for a range of ρb values. When ρb increased 25% (from 1.2 to 1.5 Mg m−3), soil temperature variation decreased by 2.1 °C in shallow layers and increased by 1 °C in subsurface layers. Surface water content differed by 0.02 m3 m−3 for various ρb values during drying events but differences mostly disappeared in the subsurface. Matric potential varied by 〉100 m of water. Surface energy balance showed clear trends with ρb. Latent heat flux decreased 6%, sensible heat flux increased 9%, and magnitude of ground heat flux varied by 18% (with a 25% ρb increase). Transient ρb impacted surface conditions and fluxes, and clearly, it warrants consideration in field and modelling investigations. Copyright © 2018 John Wiley & Sons, Ltd.

Beschreibung: Ice- and snow-melted water flow over partially thawed frozen soil of cultivated slopes causes serious soil erosion, which results in soil degradation and affects productivity in Northeast China. Water flow velocity over frozen and nonfrozen soil shows importance in understanding meltwater erosion. In this work, a series of laboratory experiments were conducted to measure water flow velocity over frozen and nonfrozen soil slopes. Experiments were performed using the electrolyte trace method under the pulse boundary model, under conditions of 4 slope gradients (5°, 10°, 15°, and 20°), 3 flow rates (1, 2, and 4 L/min), and 7 sensors positioned at 0.1, 1.0, 2.0, 3.0, 4.0, 5.0, and 6.0 m away from the electrolyte injection point. Results showed that velocities over frozen soil slopes increased with flow rate and slope gradient. Flow velocities over nonfrozen soil slopes increased with flow rate and slope gradients from 5° to 15° and stabilized at 15°. Flow velocities over frozen soil slopes were 30%, 54%, 71%, and 91% higher than those over nonfrozen ones at slope gradients of 5°, 10°, 15°, and 20°. Flow velocities over frozen soil slopes under different flow rates of 1, 2, and 4 L/min were approximately 52%, 59%, and 79% higher than those over nonfrozen soil, respectively. This study can help in assessing the erosion of partially thawed frozen soil by meltwater flow. Copyright © 2018 John Wiley & Sons, Ltd.

Beschreibung: Changes in climate and urban growth are the most influential factors affecting hydrological characteristics in urban and extra-urban contexts. The assessment of the impacts of these changes on the extreme rainfall–runoff events may have important implications on urban and extra-urban management policies against severe events, such as floods, and on the design of hydraulic infrastructures. Understanding the effects of the interaction between climate change and urban growth on the generation of runoff extremes is the main aim of this paper. We carried out a synthetic experiment on a river catchment of 64 km2 to generate hourly runoff time series under different hypothetical scenarios. We imposed a growth of the percentage of urban coverage within the basin (from 1.5% to 25%), a rise in mean temperature of 2.6 °C, and an alternatively increase/decrease in mean annual precipitation of 25%; changes in mean annual precipitation were imposed following different schemes, either changing rainstorm frequency or rainstorm intensity. The modelling framework consists of a physically based distributed hydrological model, which simulates fast and slow mechanisms of runoff generation directly connected with the impervious areas, a land-use change model, and a weather generator. The results indicate that the peaks over threshold and the hourly annual peaks, used as hydrological indicators, are very sensitive to the rainstorm intensity. Moreover, the effects of climate changes dominate on those of urban growth determining an exacerbation of the fast runoff component in extreme events and a reduction of the slow and deep runoff component, thus limiting changes in the overall runoff. Copyright © 2018 John Wiley & Sons, Ltd.

Beschreibung: We examined rainfall anomalies associated with the El Niño–Southern Oscillation (ENSO) in northern Sarawak, Malaysia, using the oxygen isotopic composition of rainfall. Two precipitation-sampling campaigns were conducted for isotope analysis: (a) at the Lambir Hill National Park (4.2° N, 114.0° E) from July 2004 to October 2006 and (b) at the Gunung Mulu National Park (3.9° N, 114.8° E) from January 2006 to July 2008. The records from these campaigns were merged with a previously published rainfall isotope dataset from Gunung Mulu site to create a 7-year-long record of the oxygen isotopic composition of Sarawak rainfall. The record exhibits clear intraseasonal variations (ISVs) with periods ranging from 10 to 70 days. The ISVs of 10- to 90-day band-pass filtered oxygen isotopic composition are linked to the synoptic-scale precipitation anomalies over the southern South China Sea (SCS). The lead–lag correlation map of precipitation with the filtered oxygen isotope anomalies shows that an anomalous wet condition responsible for the decrease in oxygen isotopic composition appears over the SCS in association with the passage of north-eastward propagation of the boreal summer intraseasonal oscillation (BSISO) in the summer monsoon season. The anomalous wet condition in spring is connected with eastward-propagating Madden–Julian oscillation (MJO), whereas the sustained wet condition in winter is responsible for the occurrence of the Borneo vortex (BV) over the SCS. ENSO modulates the frequency of these synoptic conditions on a seasonal and longer time scale, showing a strong correlation between the seasonal isotopic anomalies and the Southern Oscillation index. We therefore discern, from the significant correlation between the isotope anomalies and area-averaged Sarawak rainfall anomalies (R = −0.65, p 〈 0.01), that ENSO-related precipitation anomalies are linked to the seasonal modulation of the BSISO and MJO activity and BV genesis. Copyright © 2018 John Wiley & Sons, Ltd.

Beschreibung: Considering heterogeneity in porous media pore size and connectivity is essential to predicting reactive solute transport across interfaces. However, exchange with less-mobile porosity is rarely considered in surface water/groundwater recharge studies. Previous research indicates that a combination of pore-fluid sampling and geoelectrical measurements can be used to quantify less-mobile porosity exchange dynamics using the time-varying relation between fluid and bulk electrical conductivity. For this study, we use macro-scale (10 s of cm) advection–dispersion solute transport models linked with electrical conduction in COMSOL Multiphysics to explore less-mobile porosity dynamics in two different types of observed sediment water interface porous media. Modeled sediment textures contrast from strongly layered streambed deposits to poorly sorted lakebed sands and cobbles. During simulated ionic tracer perturbations, a lag between fluid and bulk electrical conductivity, and the resultant hysteresis, is observed for all simulations indicating differential loading of pore spaces with tracer. Less-mobile exchange parameters are determined graphically from these tracer time series data without the need for inverse numerical model simulation. In both sediment types, effective less-mobile porosity exchange parameters are variable in response to changes in flow direction and fluid flux. These observed flow-dependent effects directly impact local less-mobile residence times and associated contact time for biogeochemical reaction. The simulations indicate that for the sediment textures explored here, less-mobile porosity exchange is dominated by variable rates of advection through the domain, rather than diffusion of solute, for typical low-to-moderate rate (approximately 3–40 cm/day) hyporheic fluid fluxes. Overall, our model-based results show that less-mobile porosity may be expected in a range of natural hyporheic sediments and that changes in flowpath orientation and magnitude will impact less-mobile exchange parameters. These temporal dynamics can be assessed with the geoelectrical experimental tracer method applied at laboratory and field scales. Published 2018. This article is a U.S. Government work and is in the public domain in the USA.

Beschreibung: In this study, we developed the urban ecohydrology model (UEM) to investigate the role of bioretention on watershed water balance, runoff production, and streamflow variability. UEM partitions the land surface into pervious, impervious, and bioretention cell fractions. Soil moisture and vegetation dynamics are simulated in pervious areas and bioretention cells using a lumped ecohydrological approach. Bioretention cells receive runoff from a fraction of impervious areas. The model is calibrated in an urban headwater catchment near Seattle, WA, USA, using hourly weather data and streamflow observations for 3 years. The calibrated model is first used to investigate the relationship between streamflow variability and bioretention cell size that receives runoff from different values of impervious area in the watershed. Streamflow variability is quantified by 2 indices, high pulse count (HPC), which quantifies the number of flow high pulses in a water year above a threshold, and high pulse range (HPR), which defines the time over which the pulses occurred. Low values of these indices are associated with improved stream health. The effectiveness of the modelled bioretention facilities are measured by their influence on reducing HPC and HPR and on flow duration curves in comparison with modelled fully forested conditions. We used UEM to examine the effectiveness of bioretention cells under rainfall regimes that are wetter and drier than the study area in an effort to understand linkages between the degree of urbanization, climate, and design bioretention cell size to improve inferred stream health conditions. In all model simulations, limits to the reduction of HPC and HPR indicators were reached as the size of bioretention cells grew. Bioretention was more effective as the rainfall regime gets drier. Results may guide bioretention design practices and future studies to explore climate change impacts on bioretention design and management. Copyright © 2018 John Wiley & Sons, Ltd.

Beschreibung: We examined the fire-induced changes in groundwater recharge rate. This aspect is particularly important in the case of large forested areas growing over a coastal aquifer affected by saltwater intrusion. In the Ravenna coastal area (Italy), pine forests grow on coastal dune belts, overlying a sandy unconfined aquifer, which is strongly affected by marine ingression. Three groundwater profiles across the forest and perpendicular to the coastline were monitored for groundwater level, physical, and chemical parameters. The aims were to define groundwater quality, recharge rate, freshwater volume, and highlight change, which occurred after a forest fire with reference to pre-fire conditions. Analytical solutions based on Darcy Law and the Dupuit Equation were applied to calculate unconfined flow and compare recharge rates among the profiles. The estimated recharge rates increased in the partially and completely burnt areas (219 and 511 mm year−1, respectively) compared with the pristine pine forest area (73 mm year−1). Although pre-fire conditions were similar in all monitored profiles, a post-fire decrease in salinity was observed across the burnt forest, along with an increase in infiltration and freshwater lens thickness. This was attributed to decrease canopy interception and evapotranspiration caused by vegetation absence after the fire. This research provided an example of positive forest fire feedback on the quantity and quality of fresh groundwater resources in a lowland coastal aquifer affected by saltwater intrusion, with limited availability of freshwater resources. The fire provided an opportunity to evaluate a new forest management approach and consider the restoration and promotion of native dune herbaceous vegetation. Copyright © 2018 John Wiley & Sons, Ltd.