Scale Hydrologic Processes Research Articles

Understanding dominant hydrological processes and mechanisms of water flow in a semi-arid mountainous catchment of the Cape Fold Belt

Improving our understanding of streamflow characteristics, water storage, and dominant flowpaths in mountainous regions is important as mountains play a vital role in delivering water to lowlands, particularly in semi-arid areas. This work characterized water sources, flowpaths, and streamflow characteristics in the semi-arid, mountainous Kromme catchment in Eastern Cape Province of South Africa. Precipitation, shallow and deep groundwater levels, and streamflow data were analysed to identify patterns that indicate the occurrence and/or dominance of certain processes, responses, and flowpaths. Results of the study demonstrated how the catchment responds to rainfall events across seasons and rainfall intensities. Steep and rocky areas that make up much of the catchment contributed to significant flood peaks after high-intensity storms. Quick and slow responses in flow after rainfall events indicated the dominance of both surface and subsurface flowpaths respectively. Furthermore, surface and subsurface flows were significant in recharging the floodplain alluvial aquifer as well as maintaining streamflow during dry periods. Average annual runoff coefficients were low (0.09), which implied large evapotranspiration (ET) withdrawals from dominant flowpaths and/or storage in inactive groundwater. The Kromme catchment has a sizeable floodplain with large alluvial aquifers, which make significant contributions to catchment storage and outflows. Overall, the catchment streamflow was sustained by baseflow (for ∼50% of the time). Recession patterns suggested that the channel receives flow from different storages with the alluvial and bedrock aquifers as main contributors. Flow contributions had different rates with maximum recession periods up to 22 days, indicative of interflow dominance and floodplain drainage. Throughout the monitoring period, the river system was gaining flow at the different monitored sites during both low and high flow conditions. The channel was also gaining from the mountain bedrock through tributary flows and from the alluvial aquifer. A conceptual model of flowpaths and processes at the catchment scale is presented to improve the understanding of catchment scale hydrological processes in a semi-arid meso-scale mountainous environment.

Toward a Multi‐Representational Approach to Prediction and Understanding, in Support of Discovery in Hydrology

AbstractKey to model development is the selection of an appropriate representational system, including both the representation of what is observed (the data), and the formal mathematical structure used to construct the input‐state‐output mapping. These choices are critical, because they completely determine the questions we can ask, the nature of the analyses and inferences we can perform, and the answers we can obtain. Accordingly, a representation that is suitable for one kind of investigation might be limited in its ability to support some other kind. Arguably, how different representational approaches affect what we can learn from data is poorly understood. This paper explores three representational strategies as vehicles for understanding how catchment scale hydrological processes vary across hydro‐geo‐climatologically diverse Chile. Specifically, we test a lumped water‐balance model (GR4J), a data‐based dynamical systems model (LSTM), and a data‐based regression tree model (Random Forest). Insights were obtained regarding system memory encoded in data, spatial transferability by use of surrogate attributes, and informational deficiencies of the data set that limit our ability to learn an adequate input‐output relationship. As expected, each approach exhibits specific strengths, with LSTM providing the best characterization of dynamics, GR4J being the most robust under informationally deficient conditions, and Random Forest regression‐tree method being most supportive of interpretation. Overall, the contrasting nature of the three approaches suggests the value of adopting a multi‐representational framework to more fully extract information from the data and, by doing so, find information that better facilities the goals of robust prediction and improved understanding, ultimately supporting enhanced scientific discovery.

Satellite based Budyko framework reveals the human imprint on long-term surface water partitioning across India

The Budyko curve, relating a catchment’s water and energy balance, provides a useful tool to analyse how physio-climatic and socio-economic characteristics may impact long-term runoff. Often a parametric form of the curve, the Tixeront-Fu’s equation, is used to represent the catchment’s long-term water partitioning behaviour. Tixeront-Fu’s parameter ω, typically derived from observed climate and runoff data, can further be related to catchments’ physio-climatic characteristics to enable understanding the main drivers of their water balance. At times, prior analyses have reported potentially conflicting controls of characteristics on ω. Based on the rationale that several hydrological processes act across varying spatio-temporal scales, we hypothesize that the impact of a physio-climatic factor on ω is driven by its broader regional setting. We test our hypothesis by developing relationships between ω and a curated database of 33 physio-climatic and socio-economic characteristics for 520 regional divisions of India. We employ two related data-space splitting algorithms: classification and regression trees (CART) and random forest (RF) to study the effects of potential controlling factors within their regional context. The algorithms diagnose a hierarchy of representative vegetation, climate, soil, land use land cover, topography and anthropogenic controls. The most important characteristics controlling ω were found to be: correlation between monthly time series of P and PET, long-term temperature, long-term precipitation, and the synchronicity of P and PET. Human influences quantified in terms of population density and the extent of cultivated area were found to be important factors influencing sub-regional scale hydrological processes. These anthropogenic factors displayed predictive importance similar to vegetation and topographic factors. Our results indicate the dominant influence of climate and its seasonality on long-term water partitioning across Indian landmasses with landscape features including human influences playing a secondary but important role.

黄河三角洲刺槐群落土壤优先流及养分分布特征

黄河三角洲湿地面临严重的水资源短缺、湿地退化、土壤盐碱化等问题，湿地土壤干旱缺水，土壤收缩产生裂隙等优先流路径，在小尺度上改变湿地内部以及湿地板块之间水文连通性，小尺度水文效应往往制约大尺度水文连通性，基于当前社会对湿地修复的迫切需求，从小尺度对水文连通研究有必要加以重视。然而，目前的研究多集中在大尺度水文连通的阐述，为进一步明确黄河三角洲湿地小尺度水文连通和养分随优先流路径运移分布情况，以此区域典型刺槐群落为研究对象，基于室外染色示踪实验和室内图像处理技术，分析土壤优先流形态和分布特征，并探究优先流和土壤养分含量之间的关系。结果表明：（1）刺槐群落染色面积比随土壤深度变化主要包括2个阶段，第一阶段：优先流和基质流相互影响作用显著，第二阶段：优先流和基质流相互作用不显著，优先流强度逐渐增强，明显存在指流现象，极少部分管流现象。（2）刺槐群落染色面积比、基质流深度和百分之五十染色深度的数值较大，优先流区染色面积比较小，基质流深度发生在土壤深10-15 cm区间，该类型植被群落水流均匀入渗深度较大，优先流和基质流相互作用程度大，水文连通性较高。（3）优先流区土壤有机碳、有机质、全氮、全磷和有效磷含量值均高于基质流区；（4）土壤优先流区染色面积比与5种养分指标均呈负相关关系，有机碳、有机质和有效磷含量受优先流路径影响显著。（5）土壤中有机碳、有机质和有效磷含量在一定程度上可以衡量土壤优先流发育水平。研究结果可为黄河三角洲湿地生态系统保护和管理提供参考。;More serious environmental problems including water shortage, wetland degradation, and soil salinization are gradually recognized in the Yellow River Delta wetland. Due to water deficiency of wetland soils, preferential pathways including soil cracks occur as soil shrinkage which changes the hydrological connectivity of inside wetlands or between wetlands at small scale, and small scale hydrological processes tend to affect the large scale hydrological cycle, depending on our urgent needs to restore wetlands, it is necessary to pay attentions to the study of hydrological connectivity research from microcosmic perspectives to promote the understanding of connotation of hydrological connectivity. However, most of the current studies focused on the hydrological connectivity from macroscopical perspectives. In order to clarify the small-scale hydrological connectivity in the Yellow River Delta, the morphology and distribution characteristics of preferential flow in soil were analyzed based on outdoor dyeing tracer experiment and indoor image processing technology. The relationship between preferential flow and soil nutrient content was explored. The results show that:(1) There are two stages in the variation of the dry coverage of Robinia pseudoacacia Linn community with soil depth. The first stage:the interaction between preferential flow and matrix flow is significant; the second stage:the interaction between preferential flow and matrix flow is not significant, and the intensity of preferential flow is gradually increasing, finger flow phenomenon and a small part of pipe flow phenomenon is observed. (2) Vertical soil-profile indexes (dry coverage, dry coverage of preferential pathway, depth of diffusion area, and depth of 50% dry coverage) reveal a strong interaction between preferential flow and matrix flow, and depth of diffusion area occurs in the range of 10-15cm in the soil, which indicates that this type of vegetation community has large depth of water infiltration flow uniformly and the hydrological connectivity is high; (3) The contents of soil organic carbon, organic matter, total nitrogen, total phosphorus and available phosphorus in preferential flow area were higher than those in matrix flow area; (4) The dye coverage ratio of preferential flow area was negatively correlated with the five nutrient indexes, and the contents of organic carbon, organic matter and available phosphorus were significantly affected by preferential pathway. (5) The content of organic carbon, organic matter and available phosphorus in soil can measure the development level of preferential flow to a certain extent. These results can provide references for wetland ecosystem protection and management in the Yellow River Delta.

Coupling hydroclimate-hydraulic-sedimentation models to estimate flood inundation and sediment transport during extreme flood events under a changing climate

Extreme flood events are disastrous and can cause serious damages to society. Flood frequency obtained based on historical flow records may also be changing under future climate conditions. The associated flood inundation and environmental transport processes will also be affected. In this study, an integrated numerical modeling framework is proposed to investigate the inundation and sedimentation during multiple flood events (2,5,10, 20, 50, 100, 200-year) under future climate change scenarios in a watershed system in northern California, USA. The proposed modeling framework couples physical models of various spatial resolution: kilometers to several hundred kilometers climatic processes, hillslope scale hydrological processes in a watershed, and centimeters to meters scale hydrodynamic and sediment transport processes in a riverine system. The modeling results show that compared to the flows during historical periods, extreme events become more extreme in the 21st century and higher flows tend to be larger and smaller flows tend to be smaller in the system. Flood inundation in the study area, especially during 200-year events, is projected to increase in the future. More sediment will be trapped as the flow increases and the deposition will also increase in the settling basin. Sediment trap efficiency values are within 37.5–65.4% for the historical conditions, within 32.4–68.8% in the first half of the 21st century, and within 34.9–69.3% in the second half of the 21st century. The results highlight the impact of climate change on extreme flood events, the resulting sedimentation, and reflected the importance of incorporating the coupling of physical models into the adaptive watershed and river system management.

Mapping Water Surface Elevation and Slope in the Mississippi River Delta Using the AirSWOT Ka-Band Interferometric Synthetic Aperture Radar

AirSWOT is an airborne Ka-band synthetic aperture radar, capable of mapping water surface elevation (WSE) and water surface slope (WSS) using single-pass interferometry. AirSWOT was designed as a calibration and validation instrument for the forthcoming Surface Water and Ocean Topography (SWOT) mission, an international spaceborne synthetic aperture radar mission planned for launch in 2022 which will enable global mapping of WSE and WSS. As an airborne instrument, capable of quickly repeating overflights, AirSWOT enables measurement of high frequency and fine scale hydrological processes encountered in coastal regions. In this paper, we use data collected by AirSWOT in the Mississippi River Delta and surrounding wetlands of coastal Louisiana, USA, to investigate the capabilities of Ka-band interferometry for mapping WSE and WSS in coastal marsh environments. We introduce a data-driven method to estimate the time-varying interferometric phase drift resulting from radar hardware response to environmental conditions. A system of linear equations based on AirSWOT measurements is solved for elevation bias and time-varying phase calibration parameters using weighted least squares. We observed AirSWOT WSE uncertainty of 12 cm RMS compared to in situ water level measurements when averaged over an area of 0.5 km 2 at incidence angles below 15 ∘ . At higher incidence angles, the observed AirSWOT elevation bias is possibly due to residual phase calibration errors or radar backscatter from vegetation. Elevation profiles along the Wax Lake Outlet river channel indicate AirSWOT can measure WSS over a 24 km distance with uncertainty below 0.3 cm/km, 8% of the true water surface slope as measured by in situ data. The data analysis and results presented in this paper demonstrate the potential of AirSWOT to measure water surface elevation and slope within highly dynamic and spatially complex coastal environments.

Climate change vulnerability assessment and adaptation strategies through best management practices

There is a growing scientific consensus on climate change and its impact on catchment scale hydrologic and water quality processes. Therefore, it is not only important to understand the impact of climate change but also necessary to formulate adaptation strategies to negate the effect of climate change. To this end, we have developed a hydrologic modeling framework (Soil and Water Assessment Tool) concomitant with Analytical Hierarchical Process (AHP) to develop individual as well as composite climate vulnerability index in a Southern New Jersey Watershed. Three agricultural BMPs including cover crop, filter strip, and no till were examined to determine their ability to negate the climate change impact on water fluxes and water quality in the watershed. Downscaled projected temperature and precipitation from six General Circulation Models (GCMs) for moderate and high emission scenarios (RCP-4.5 and 8.5, respectively) from the latest suite of Coupled Model Intercomparison Project Phase 5 (CMIP5) were incorporated in to the hydrologic model.Overall, results suggested an increase in temperature and precipitation under climate change scenarios leading to rise in surface runoff, streamflow, sediment and TP load relative to baseline scenario. Highly vulnerable areas to flooding, sedimentation and eutrophication under climate change using AHP framework were standout from other types of vulnerable areas in the watershed due to presence of excess impervious surface, higher percentage of agricultural lands, and increased sediment and organic phosphorus, respectively. When agricultural BMPs were implemented in corn and soybean fields located inside critical source areas (CSAs) under climate change scenarios, BMPs showed little to no effect towards reduction of water fluxes while they reduced maximum 2% and 12% of sediment and TP, respectively, compared to no BMP under climate change scenario. Further, as the BMP were incorporated in all corn and soybean fields in the watershed, the maximum sediment and TP reduction increased to 8% and 44%, respectively. The study outcomes suggests that climate change will increase the water fluxes as well as pollution load in the watershed, however, BMPs can be used to negate the water quality impact due to climate change.

Assessing land water storage dynamics over South America

The underlying uncertainties in the prediction of freshwater evolutions in some regions can be induced by several unmitigated human actions, multi-scale climatic drivers, and dynamic physical processes. These factors have enduring hydro-ecological effects on the environments and combine to limit our understanding of large scale hydrological processes and impacts of climate on water availability. Considering the fact that several hydrogeological perturbations and disturbances have been reported during the last decade in South America (SA), a further assessment of continental land water storage is therefore warranted. In this study, a two-step regularization approach that combined the JADE (Joint Approximate Diagonalisation of Eigen matrices) algorithm and PLSR (partial least squares regression) was employed to assess GRACE (Gravity Recovery and Climate Experiment)-terrestrial water storage (TWS) over SA. Based on the Bartlett’s statistics, significant independent patterns of SST (Sea Surface Temperature) anomalies from the Pacific and Atlantic oceans were used in the PLSR scheme to model the temporal evolutions of TWS (2002-2017) over twelve prominent river basins in SA. From the JADE rotation of TWS over SA, strong inter-annual changes in TWS observed over the Amazon basin and within its floodplain corridors were identified. The unabated mass loss in Patagonia ice-field caused by warming of the climate and other GRACE-hydrological signals were also retrieved from the JADE scheme. The rainfall-TWS relationship is considerably strong (r = 0.80 at 0-2 months lag) in much of tropical SA, including the Amazon basin and highlights the influence of climate variability in the region. Medium (r=0.40) and moderately strong (r=0.60) rainfall-TWS relationship were also found to be significant (α=0.05) but with up to 4 months lag and more in some basins. During the 2010-2017 period, estimated TWS trends (α=0.05) showed a considerable fall in Orinoco (-38.48±7.90 mm/yr) and Sao Francisco (-30.84±4.17) while the strongest rise was found in Uruguay (28.28±3.49 mm/yr). As the rainfall-TWS relationship is not statistically significant (α=0.05) in some areas, the spatial distribution of trends in TWS and rainfall, especially in some arid regions, which are inconsistent confirm possible impacts resulting from complex hydrogeological processes and/or anthropogenic influence. Further, in the modelling of TWS time series using the JADE-PLSR scheme, several validation skill metrics (e.g., R2, Nash-Sutcliffe Efficiency) confirm the considerable agreements between predicted and observed TWS in the Amazon (R2=0.95), Orinoco (R2=0.94), Tocantins (R2=0.91), and Chobut (R2=0.88). However, GRACE-hydrological signals in some regions are somewhat complex given the relatively higher uncertainties in the multivariate models employed in this study.

The modeling of different scale hydrologic processes in aquatories

The results of mathematical and physical modeling of global and local hydrological processes in the water area are presented. By the method of mathematical modeling revealed variability of large-scale circulatory flow and mass transfer in a water-restricted area with complex obstacles.In laboratory conditions, by experimental methods, in the hydrodynamic tray, on the channel and the laboratory stand, detected of local hydrodynamic processes and mass transfer around local obstacles - three-row fuel burner rafts. Mechanisms of formation of erosion and transfer of the soil near and within the single and group structures of the grillages located on the laundering sandy bottom of the channel are revealed.

Future climate change impact assessment of watershed scale hydrologic processes in Peninsular Malaysia by a regional climate model coupled with a physically-based hydrology modelo

Impacts of climate change on the hydrologic processes under future climate change conditions were assessed over Muda and Dungun watersheds of Peninsular Malaysia by means of a coupled regional climate and physically-based hydrology model utilizing an ensemble of future climate change projections. An ensemble of 15 different future climate realizations from coarse resolution global climate models' (GCMs) projections for the 21st century was dynamically downscaled to 6km resolution over Peninsular Malaysia by a regional climate model, which was then coupled with the watershed hydrology model WEHY through the atmospheric boundary layer over Muda and Dungun watersheds. Hydrologic simulations were carried out at hourly increments and at hillslope-scale in order to assess the impacts of climate change on the water balances and flooding conditions in the 21st century. The coupled regional climate and hydrology model was simulated for a duration of 90years for each of the 15 realizations. It is demonstrated that the increase in mean monthly flows due to the impact of expected climate change during 2040–2100 is statistically significant from April to May and from July to October at Muda watershed. Also, the increase in mean monthly flows is shown to be significant in November during 2030–2070 and from November to December during 2070–2100 at Dungun watershed. In other words, the impact of the expected climate change will be significant during the northeast and southwest monsoon seasons at Muda watershed and during the northeast monsoon season at Dungun watershed. Furthermore, the flood frequency analyses for both watersheds indicated an overall increasing trend in the second half of the 21st century.

Habitat complexity influences fine scale hydrological processes and the incidence of stormwater runoff in managed urban ecosystems

Urban ecosystems have traditionally been considered to be pervious features of our cities. Their hydrological properties have largely been investigated at the landscape scale and in comparison with other urban land use types. However, hydrological properties can vary at smaller scales depending upon changes in soil, surface litter and vegetation components. Management practices can directly and indirectly affect each of these components and the overall habitat complexity, ultimately affecting hydrological processes. This study aims to investigate the influence that habitat components and habitat complexity have upon key hydrological processes and the implications for urban habitat management.Using a network of urban parks and remnant nature reserves in Melbourne, Australia, replicate plots representing three types of habitat complexity were established: low-complexity parks, high-complexity parks, and high-complexity remnants. Saturated soil hydraulic conductivity in low-complexity parks was an order of magnitude lower than that measured in the more complex habitat types, due to fewer soil macropores. Conversely, soil water holding capacity in low-complexity parks was significantly higher compared to the two more complex habitat types. Low-complexity parks would generate runoff during modest precipitation events, whereas high-complexity parks and remnants would be able to absorb the vast majority of rainfall events without generating runoff. Litter layers on the soil surface would absorb most of precipitation events in high-complexity parks and high-complexity remnants. To minimize the incidence of stormwater runoff from urban ecosystems, land managers could incrementally increase the complexity of habitat patches, by increasing canopy density and volume, preserving surface litter and maintaining soil macropore structure.

Using a transient infiltrometric technique for intensively sampling field‐saturated hydraulic conductivity of a clay soil in two runoff plots

AbstractThe point measurement of soil properties allows to explain and simulate plot scale hydrological processes. An intensive sampling was carried out at the surface of an unsaturated clay soil to measure, on two adjacent plots of 4 × 11 m2 and two different dates (May 2007 and February–March 2008), dry soil bulk density, ρb, and antecedent soil water content, θi, at 88 points. Field‐saturated soil hydraulic conductivity, Kfs, was also measured at 176 points by the transient Simplified Falling Head technique to determine the soil water permeability characteristics at the beginning of a possible rainfall event yielding measurable runoff. The ρb values did not differ significantly between the two dates, but wetter soil conditions (by 31%) and lower conductivities (1.95 times) were detected on the second date as compared with the first one. Significantly higher (by a factor of 1.8) Kfs values were obtained with the 0.30‐m‐diameter ring compared with the 0.15‐m‐diameter ring. A high Kfs (> 100 mm h−1) was generally obtained for low θi values (< 0.3 m3m−3), whereas a high θi yielded an increased percentage of low Kfs data (1–100 mm h−1). The median of Kfs for each plot/sampling date combination was not lower than 600 mm h−1, and rainfall intensities rarely exceeded 100 mm h−1 at the site. The occurrence of runoff at the base of the plot needs a substantial reduction of the surface soil permeability characteristics during the event, probably promoted by a higher water content than the one of this investigation (saturation degree = 0.44–0.62) and some soil compaction due to rainfall impact. An intensive soil sampling reduces the risk of an erroneous interpretation of hydrological processes. In an unstable clay soil, changes in Kfs during the event seem to have a noticeable effect on runoff generation, and they should be considered for modeling hydrological processes. Copyright © 2012 John Wiley & Sons, Ltd.

Multi‐site evaluation of hydrology component of SWAT in the coastal plain of southwest Georgia

AbstractSimulation of watershed scale hydrologic and water quality processes is important for watershed assessments. Proper characterization of the accuracy of these simulations, particularly in cases with limited observed data, is critical. The Soil & Water Assessment Tool (SWAT) is frequently used for watershed scale simulation. The accuracy of the model was assessed by extrapolating calibration results from a well studied Coastal Plain watershed in Southwest Georgia, USA, to watersheds within the same geographic region without further calibration. SWAT was calibrated and validated on a 16.7‐km2 subwatershed within the Little River Experimental Watershed by varying six model parameters. The optimized parameter set was then applied to a watershed of similar land use and soils, a smaller watershed with different land use and soils and three larger watersheds within the same drainage system without further calibration. Simulation results with percent bias (PB) ±15% ≤ PB < ±25% and Nash–Sutcliffe efficiency (NSE) 0.50 < NSE ≤ 0.65 were considered to be satisfactory, whereas those with PB < ±10% and 0.75 < NSE ≤ 1.00 were considered very good. With these criteria, simulation results for the five non‐calibration watersheds were satisfactory to very good. Differences across watersheds were attributed to differences in soils, land use, and surficial aquifer characteristics. These results indicate that SWAT can be a useful tool for predicting streamflow for ungauged watersheds with similar physical characteristics to the calibration watershed studied here and provide an indication of the accuracy of hydrologic simulations for ungauged watersheds. Copyright © 2012 John Wiley & Sons, Ltd.

Linking Global Climate Models to an Integrated Hydrologic Model: Using an Individual Station Downscaling Approach

Abstract:This paper implements an individual station‐based downscaled approach based on a quantil–quantile bias correction mapping to further downscale regional gridded simulated output into individual station locations. We describe and propose a framework to optimize the usefulness of this downscaling approach over small watersheds in mountain regions, where downscale gridded data (∼10km) is still too coarse for use in sub‐regional hydrologic simulation studies. Two regional downscaled data sets were downscaled to individual station locations: the Desert Research Institute‐Regional Climate Model based on the Weather Research and Forecasting model (DRI‐RCM; 12 km resolution); and the US Bureau of Reclamation Bias‐Corrected Spatial Disaggregation (BCSD; 12 km resolution). We applied the station‐based downscaled data in the integrated hydrologic model, GSFLOW, as adapted to the Incline Creek watersheds in the northeastern Lake Tahoe basin. Hydrologic model results showed that the proposed downscaled approach provides reasonable downscaled climate data that can be used to evaluate sub‐regional scale hydrologic processes.

Variable self‐similar sinuosity properties within simulated river networks

ABSTRACTRiver networks have been shown to obey power scaling laws and to follow self‐organization principles. Their self‐similar (fractal) properties open a path to relate small scale and large scale hydrological processes, such as erosion, deposition or geological movements. However, the existence of a self‐similar dimension has only been checked using either the whole channel network or, on the contrary, a single channel link. No study has explicitly addressed the possible spatial variation of the self‐similar properties between these two extreme geomorphologic objects. Here, a new method based on self‐similarity maps (SSM) is proposed to spatially explore the stream length self‐similar dimension Dl within a river network. The mapping principle consists in computing local self‐similar dimensions deduced from a fit of stream length estimations using increasing divider sizes. A local uncertainty related to the fit quality is also computed and localized on every stream. To assess the efficiency of the approach, contrasted river networks are simulated using optimal channel networks (OCN), where each network is characterized by an exponent γ conditioning its overall topology. By building SSM of these networks, it is shown that deviations from uniform self‐similarity across space occur. Depending on the type of network (γ parameter), these deviations are or are not related to Strahler's order structure. Finally, it is found numerically that the structural averaged stream length self‐similar dimension Dl is closely related to the more functional γ parameter. Results form a bridge between the studies on river sinuosity (single channel) and growth of channel networks (watershed). As for every method providing spatial information where they were lacking before, the SSM may soon help to accurately interpret natural networks and help to simulate more realistic channel networks. Copyright © 2011 John Wiley & Sons, Ltd.

Development of a coupled soil erosion and large‐scale hydrology modeling system

Soil erosion models are usually limited in their application to the field scale; however, the management of land resources requires information at the regional scale. Large‐scale physically based land surface schemes (LSS) provide estimates of regional scale hydrologic processes that contribute to erosion. If scaling issues are adequately addressed, coupling an LSS to a physically based erosion model can provide a tool to study the regional impact of soil erosion. A coupling scheme was developed using the Variable Infiltration Capacity (VIC) model to produce hydrologic inputs for the stand‐alone Water Erosion Prediction Project‐Hillslope Erosion (WEPP‐HE) program, accounting for both temporal and spatial scaling issues. Precipitation events were disaggregated from daily to hourly and used with the VIC model to generate hydrologic fluxes. Slope profiles were downscaled from 30 arc second to 30 m hillslopes. Additionally, soil texture and erodibility were adjusted with simplified assumptions based on the full WEPP model. Soil erosion at the large scale was represented on a VIC model grid cell basis by applying WEPP‐HE to subsamples of 30 m hillslopes. On an average annual basis, results showed that the coupled model was comparable with full WEPP model predictions. On an event basis, the coupled model system captured more small erosion events, with erodibility adjustments of the same magnitude as from the full WEPP model simulations. Differences in results can be attributed to discrepancies in hydrologic data calculations and simplified assumptions in vegetation and soil erodibility. Overall, the coupled model demonstrated the feasibility of erosion prediction for large river basins.

A methodological framework for integrating computational fluid dynamics and ecological models applied to juvenile freshwater mussel dispersal in the Upper Mississippi River

Interdisciplinary research in hydraulics and ecology for river management and restoration must integrate processes that occur over a wide range of spatial and temporal scales, which presents a challenge to ecohydraulics modelers. Computational fluid dynamics (CFD) models are being more widely used to determine flow fields for ecohydraulics applications. In the Upper Mississippi River (UMR), the mussel dynamics model was developed as a tool for management and conservation of freshwater mussels (Unionidae), which are benthic organisms, imperiled in North America, that are inextricably linked with the hydraulics of river flow. We updated the juvenile dispersal component of the mussel dynamics model by using stochastic Lagrangian particle tracking in a three dimensional flow field output from CFD models of reaches in the UMR. We developed a methodological framework to integrate hydrodynamic data with the mussel dynamics model, and we demonstrate the use of the juvenile dispersal model employed within the methodological framework in two reaches of the UMR. The method was used to test the hypothesis that impoundment affects the relationship of some hydraulic parameters with juvenile settling distribution. Simulation results were consistent with this hypothesis, and the relationships of bed shear stress and Froude number with juvenile settling were altered by impoundment most likely through effects on local hydraulics. The methodological framework is robust, integrates Eulerian and Lagrangian reference frameworks, and incorporates processes over a wide range of temporal and spatial scales, from watershed scale hydrologic processes (decades), to reach scale (km) processes that occur over hours or days, and turbulent processes on spatial scales of meter to millimeter and times scales of seconds. The methods are presently being used to assess the impacts of pre- and early post-settlement processes on mussel distributions, including the effects of bed shear stress, and the sensitivity of the location of the host fish when juveniles excyst, on juvenile settling distribution.

Spatial statistical properties and scale transform analyses on the topographic index derived from DEMs in China

The topographic index (TI), frequently used in approximately characterizing the spatial distribution of variable source areas within a watershed, has been widely applied in topography-based land-surface process schemes coupled in regional or global climatic models. The TI concept, however, was originally developed for studying hill-slope scale hydrological processes and was most commonly used in simulations from small- to medium-sized watersheds. It is still questionable whether the TI computed from coarse-resolution digital elevation models (DEMs) for large-scale hydrology and climate studies can effectively reflect the spatial distribution of soil moisture, surface saturation, and runoff generation processes in most areas. In this study, we first proposed an improved multiple flow direction algorithm (IMFD) for accurately computing the TI distribution. We then evaluated the IMFD algorithm quantitatively by using four types of artificial mathematical surfaces. Subsequently, we conducted statistical analyses on the TI distributions computed with IMFD from 90×90 m 2 and 1000×1000 m 2 resolution DEM blocks sampled from across the whole of China. We found there are linear relationships between the mean TI values computed from the two different resolution DEMs in three sampled blocks of different sizes, i.e., 0.1°×0.1°, 0.5°×0.5° and 1°×1°. Systematic analyses further suggested that the forms of these linear relationships are evidently affected by the algorithm used for the TI computation, while the size, location, and number of the selected TI samples have minor effects on them. Finally, we investigated the influence of DEM resolution on the spatial statistical properties of TI. From the viewpoint of terrain discretization and smoothing effects, we also discussed the mechanism and the reasons causing the similarity on TI at different spatial resolutions.

HYDROLOGIC CYCLE AND HEAT TRANSPORT MODELING AND ITS APPLICATION TO AN URBANIZED WATERSHED

A physically distributed hydrological model (WEP model) was coupled with a newly developed physically distributed model of heat transport in river water to simulate watershed scale hydrological and heat transport processes and resulting effects on stream environment. The model is characterized by the capability of simulating runoff from a highly urbanized watershed with a dual drainage system of river channels and a sewer network, which enables appropriate prediction of inflows to wastewater treatment plants and river floods. Also, this model is designed to investigate factors affecting spatial and temporal variations of stream temperature by incorporating anthropogenic heat impacts and urban canopy processes. Applying the model to the Kanda River Watershed, stream flow and temperature as well as inflows to a WWTP are well simulated. The anthropogenic heat impact on stream temperature was also simulated.

Influence of soil texture on hydraulic properties and water relations of a dominant warm-desert phreatophyte

We investigated hydraulic constraints on water uptake by velvet mesquite (Prosopis velutina Woot.) at a site with sandy-loam soil and at a site with loamy-clay soil in southeastern Arizona, USA. We predicted that trees on sandy-loam soil have less negative xylem and soil water potentials during drought and a lower resistance to xylem cavitation, and reach E(crit) (the maximum steady-state transpiration rate without hydraulic failure) at higher soil water potentials than trees on loamy-clay soil. However, minimum predawn leaf xylem water potentials measured during the height of summer drought were significantly lower at the sandy-loam site (-3.5 +/- 0.1 MPa; all errors are 95% confidence limits) than at the loamy-clay site (-2.9 +/- 0.1 MPa). Minimum midday xylem water potentials also were lower at the sandy-loam site (-4.5 +/- 0.1 MPa) than at the loamy-clay site (-4.0 +/- 0.1 MPa). Despite the differences in leaf water potentials, there were no significant differences in either root or stem xylem embolism, mean cavitation pressure or Psi(95) (xylem water potential causing 95% cavitation) between trees at the two sites. A soil-plant hydraulic model parameterized with the field data predicted that E(crit) approaches zero at a substantially higher bulk soil water potential (Psi(s)) on sandy-loam soil than on loamy-clay soil, because of limiting rhizosphere conductance. The model predicted that transpiration at the sandy-loam site is limited by E(crit) and is tightly coupled to Psi(s) over much of the growing season, suggesting that seasonal transpiration fluxes at the sandy-loam site are strongly linked to intra-annual precipitation pulses. Conversely, the model predicted that trees on loamy-clay soil operate below E(crit) throughout the growing season, suggesting that fluxes on fine-textured soils are closely coupled to inter-annual changes in precipitation. Information on the combined importance of xylem and rhizosphere constraints to leaf water supply across soil texture gradients provides insight into processes controlling plant water balance and larger scale hydrologic processes.