Surface Runoff Generation Research Articles

Erosivity, surface runoff, and soil erosion estimation using GIS-coupled runoff–erosion model in the Mamuaba catchment, Brazil

This study evaluates erosivity, surface runoff generation, and soil erosion rates for Mamuaba catchment, sub-catchment of Gramame River basin (Brazil) by using the ArcView Soil and Water Assessment Tool (AvSWAT) model. Calibration and validation of the model was performed on monthly basis, and it could simulate surface runoff and soil erosion to a good level of accuracy. Daily rainfall data between 1969 and 1989 from six rain gauges were used, and the monthly rainfall erosivity of each station was computed for all the studied years. In order to evaluate the calibration and validation of the model, monthly runoff data between January 1978 and April 1982 from one runoff gauge were used as well. The estimated soil loss rates were also realistic when compared to what can be observed in the field and to results from previous studies around of catchment. The long-term average soil loss was estimated at 9.4 t ha(-1) year(-1); most of the area of the catchment (60%) was predicted to suffer from a low- to moderate-erosion risk (<6 t ha(-1) year(-1)) and, in 20% of the catchment, the soil erosion was estimated to exceed > 12 t ha(-1) year(-1). Expectedly, estimated soil loss was significantly correlated with measured rainfall and simulated surface runoff. Based on the estimated soil loss rates, the catchment was divided into four priority categories (low, moderate, high and very high) for conservation intervention. The study demonstrates that the AvSWAT model provides a useful tool for soil erosion assessment from catchments and facilitates the planning for a sustainable land management in northeastern Brazil.

Small-scale contact angle mapping on undisturbed soil surfaces

Abstract Research of the last years pointed out that most soils are neither completely hydrophilic nor hydrophobic, but exhibit a subcritical level of water repellency (i.e. contact angle, CA &gt; 0° and &lt; 90°). Soil water repellency (SWR) is mainly caused by organic compounds of different origin and structure, showing the relevance of biofilms and organic coatings present at many particle surfaces. Despite the importance of SWR for hydraulic processes like preferential flow phenomena, generation of heterogeneous moisture patterns, or surface run-off generation, detailed investigations on the spatial variability of SWR at various scales have rarely been carried out. We introduce a new and easy-to-apply operation for measuring the spatial distribution of SWR using a modified sessile drop method for direct optical assessment of CA at a small scale. The specific objectives of this paper are to apply a sampling and preparation technique that preserves the original spatial arrangement of soil particles and to characterize soil wettability in terms of CA at a high spatial resolution. Results revealed that the sampling and preparation technique allows determination of CA at the millimeter scale using droplets of 1 μL volume. Direct measurement on grain surfaces of the sand fraction is possible for grain sizes &gt; 300 μm using drop volumes down to 0.1 μL. Geostatistical evaluation showed that the measurement grid scale is below the range of spatial dependency for droplets of 1 μL volume, but not for measurements on single grains (pure nugget effect). Results show further that the small-scale differences in wettability, especially for CA &lt; 90°, cannot be detected by the conventional WDPT test. From these findings it can be concluded that the proposed technique allows the identification of small-scale variations in wettability that may promote the formation of heterogeneous flow fields and moisture patterns in soil under unsaturated conditions.

Consequências hidrológicas da mudança de uso da terra de floresta para pastagem na região da floresta tropical pluvial atlântica

The Atlantic rain forest is the most endangered ecosystem in Brazil. Its degradation has started since 1500 when the European settlers arrived. Despite of all land use changes that have occurred, hydrological studies carried out in this biome have been limited to hydrological functioning of rain forests only. In order to understand the hydrological consequences of land-use change from forest to pasture, we described the hydrological functioning of a pasture catchment that was previously covered by tropical rain forest. To reach this goal we measured the precipitation, soil matric potential, discharge, surface runoff and water table levels during one year. The results indicated that there is a decrease in surface soil saturated hydraulic conductivity. However, as low intensity rainfall prevails, the lower water conductivity does not necessarily leads to a substantially higher surface runoff generation. Regarding soil water matric potential, the pasture presented higher moisture levels than forest during the dry season. This increase in soil moisture implies in higher water table recharge that, in turn, explain the higher runoff ratio. This way, land-use change conversion from forest to pasture implies a higher annual streamflow in pasture catchments. Nonetheless, this increase in runoff due to forest conversion to pasture implies in losses of biological diversity as well as lower soil protection.

The Impact of Asynchronicity on Event‐Flow Estimation in Basin‐Scale Hydrologic Model Calibration1

Abstract: The calibration of basin‐scale hydrologic models consists of adjusting parameters such that simulated values closely match observed values. However, due to inevitable inaccuracies in models and model inputs, simulated response hydrographs for multiyear calibrations will not be perfectly synchronized with observed response hydrographs at the daily time step. An analytically derived formula suggests that when timing errors are significant, traditional calibration approaches may generally underestimate the total event‐flow volume. An event‐adaptive time series is developed and incorporated into the Nash‐Sutcliffe Efficiency objective function to diagnose the potential impact of event‐flow synchronization errors. Test sites are the 50 km2 Subwatershed I of the Little River Experimental Watershed (LREWswI) in southeastern Georgia, and the 610 km2 Little Washita River Experimental Watershed (LWREW) in southwestern Oklahoma, with the Soil and Water Assessment Tool used as the hydrologic model. Results suggest that simulated surface runoff generation is 55% less for LREWswI when the daily time series is used compared with when the event‐adaptive technique is used. Event‐flow generation may also be underestimated for LWREW, but to a lesser extent than it may be for LREWswI, due to a larger portion of the event flow being lateral flow.

Assessment of soil erosion risk using SWAT model

Soil erosion is one of the most serious land degradation problems and the primary environmental issue in Mediterranean regions. Estimation of soil erosion loss in these regions is often difficult due to the complex interplay of many factors such as climate, land uses, topography, and human activities. The purpose of this study is to apply the Soil and Water Assessment Tool (SWAT) model to predict surface runoff generation patterns and soil erosion hazard and to prioritize most degraded sub-catchment in order to adopt the appropriate management intervention. The study area is the Sarrath river catchment (1,491 km2), north of Tunisia. Based on the estimated soil loss rates, the catchment was divided into four priority categories for conservation intervention. Results showed that a larger part of the watershed (90 %) fell under low and moderate soil erosion risk and only 10 % of the watershed was vulnerable to soil erosion with an estimated sediment loss exceeding 10 t ha−1 year−1. Results indicated that spatial differences in erosion rates within the Sarrath catchment are mainly caused by differences in land cover type and gradient slope. Application of the SWAT model demonstrated that the model provides a useful tool to predict surface runoff and soil erosion hazard and can successfully be used for prioritization of vulnerable areas over semi-arid catchments.

Characteristics of runoff and sediment generation of forest vegetation on a hill slope by use of artificial rainfall apparatus

We studied the impact of forest vegetation on soil erosion, surface runoff, and sediment generation by using field simulated rainfall apparatus. We measured runoff and sediment generation of five 4.5 × 2.1 m runoff plots (a bare soil as a control; two Pinus tabulaeformis forest plots and two Platycladus orientalis forest with row spacing of 1 m × 1 m and 1.5 m × 1.5 m, respectively) in Beijing Jiu Feng National Forest Park under three rainfall intensities (0.42, 0.83, 1.26 mm per minute). Forest vegetation significantly reduced soil erosion and sediment yield. Mean total runoff volume in the four tree stand plots was 93% of that in the control plot, demonstrating the limited effectiveness of forest vegetation in runoff control. With increasing rainfall intensity, runoff reduction in forest plots declined from 28.32% to 2.1%. Similar trends in runoff coefficient and the relationship between runoff volume and rainfall duration was observed. Mean total sediment yield and mean sediment yield reduction rate under different treatments was 55.05% and 43.17% of those in the bare soil control plot, respectively. Rainfall intensity played an important role in runoff and sediment generation processes, and had a greater impact on runoff than on soil erosion and sediment generation. When considering several factors in runoff and sediment transport processes, the P. tabulaeform plot with row spacing at 1 × 1 m had a greater effect on soil and water conservation than did other forested plots.

Step changes in the flood frequency curve: Process controls

Empirical distribution functions of flood peaks in small catchments sometimes show discontinuities in the slope; that is, the largest flood peaks are significantly larger than the rest of the record. The aim of this paper is to understand whether these discontinuities, or step changes, can be a consistent effect of hydrological processes. We conducted field surveys in two Austrian alpine catchments 73 km2in size to map the spatial patterns of surface runoff generation and hydrogeologic storage. On the basis of this information, we selected the parameters of a distributed continuous runoff model, which is designed to simulate well the point when the storage capacity of the catchment is exhausted. Then we calibrated a stochastic rainfall model and performed Monte Carlo simulations of runoff to generate flood frequency curves for the two catchments. The curves exhibit a step change around a return period of 30 years. An analysis of the storage capacities suggests that this step change is due to a threshold of storage capacity being exceeded, which causes fast surface runoff in large parts of the catchments. The threshold occurs when the storage within the catchment is spatially rather uniform. To identify step changes, reliable estimates of the catchment storage capacity are needed on the basis of detailed hydrogeological information. The occurrence of a step change is of importance for estimating low‐probability floods since the flood estimates with the step change accounted for can be significantly different from those based on commonly used distribution functions. We therefore suggest that step changes in the flood frequency curve of small catchments can be real and their possible presence should be taken into account in design flood estimation.

讨赖河流域月降水时间序列中的混沌现象

降水是我国西北干旱内陆河流域出山径流的重要补给来源，很大程度上影响着流域水资源的形成转化、开发利用以及旱涝灾害等过程。本文以讨赖河流域为研究区，基于流域上、中、下游3个典型站点的月降水序列资料，采用混沌理论分析方法，以饱和关联维数和Kolmogorov熵为控制参量，计算并探讨该区月降水序列中的混沌特性。计算结果表明，讨赖河流域降水过程在月时间尺度上具有混沌特征，3个典型站的1/Kolmogorov值得出的流域降水的可预报长度为10个月左右。本研究有助于深入理解干旱内陆河流域复杂降水过程变化规律，对降水–径流过程的研究具有序列特征方面的指导意义。 As one of the important sources, rainfall in upper reaches of inland rivers in arid regions of northwestern Chinaplays key roles in not only the surface runoff generation, water transformation and utilization, but also regional drought and flooding to some certain extent. For a better understanding of characteristics of the rainfall, theTaolaiRiver basinis selected as the study area. The method of Chaos is used to determine the two key parameters of the Saturation Correlation Dimension and Kolmogorov Entropy based on the statistics of the observed monthly rainfall series from three representatively hydro-meteorological stations in the upper, middle and lower reaches of the river, respectively. Determination of the two key parameters resulted in relatively chaotic features for the time series of the monthly rainfall across theTaolaiRiver basin. Furthermore, reciprocal of the Kolmogorov entropy described a possible temporal forecasting scale of 10 months in this region. This study can help to understand the complex rainfall regime and analyze precipitation-runoff processes in arid regions.

Effects of land use, land cover and rainfall regimes on the surface runoff and soil loss on karst slopes in southwest China

Abstract Surface runoff and soil loss from 2007 to 2010 related to land use and rainfall regimes in karst hill slopes in Guizhou Province, southwest China, were analyzed. Using the hierarchical clustering method, sixty-one rainfall events under the subtropical monsoon climate condition were classified into 5 types of rainfall regimes according to the depth, maximum 30-min intensity, and duration of rainfall. In our study, we first demonstrated that the amounts of surface runoff and soil loss on the karst hill slopes were very small compared to the non-karst areas, because the dual hydrological structure in the karst region, including ground and underground drainage systems, could influence the processes of rainfall recharge and runoff generation. Most rainfall water was transported underground through limestone fissures and fractures, while little was in the form of surface runoff. Second, the runoff and soil loss were affected by land use management and vegetation cover. Soil loss was intensified in a descending order to five types of land uses: pastureland > burned area > cropland > combination vegetation land > young forestland. Third, the runoff and soil loss exhibited remarkable variances among different rainfall regimes. Large runoff and soil loss were mainly created by heavy rainfall storms with a rainfall depth of more than 40 mm and a maximum 30-min rainfall intensity of over 30 mm h− 1. In addition, rainfall storms with large antecedent precipitations could also produce large runoff and soil loss. These observations indicated that limestone fissures and fractures play important roles in surface runoff generation on karst limestone slopes due to their large storage capacity and high infiltration rate. Lastly, the soil erosion risk in the karst pure limestone slope is quite high and should be paid particular attention, especially in regards to over-grazing because the soil loss created by a single heavy rainstorm in pastureland was 5 times the annual soil loss tolerance.

Monthly spatial distributed water resources assessment: a case study

Water resource conservation is of utmost importance, especially for agriculture in developing countries. Frequent occurrences of water shortage have driven more social efforts in researching on water resources spatial distribution, as the land cover changes recently have shown positive influences. For the purpose of efficient water resources management, hydrological processes under different types of land covers and soil textures are supposed to be accurately analyzed and evaluated. Recently developed distributed hydrological mode (DHM) has been a strong hydro-cycle simulation tool for inferring variability and heterogeneity of water resources distribution. In this paper, a spatially distributed Water and Energy Transfer between Soil, Plants and Atmosphere under quasi Steady State (WetSpass) model was introduced in the distributed hydro-cycle simulation on upstream Han river basin. The simulation time-step of WetSpass model was modified from originally one season to currently one month. In addition, an experiential non-linear routing algorithm was integrated into WetSpass for discharge confluence. The study area was delineated into 12 upstream to downstream routing related catchments whose land covers and soil textures were investigated and illustrated. Model verification was completed through the calibration of simulated hydrograph against observation using eleven years of continuous precipitation and meteorological data. Moreover, four criteria were used to evaluate the model performance and the calibrated results of routing parameters were discussed. Furthermore, the distribution of surface runoff generation, evapotranspiration and groundwater recharge were illustrated and analyzed considering the spatial heterogeneity of land cover and soil texture. Results showed that water resource spatial distribution and hydrological processes were closely related to land cover and soil texture and the model had achieved a success in hydro-cycle modeling of upstream Han river basin.

Simulating hydrologic response of a pasture hillslope in North Alabama using the Hortonian Infiltration and Runoff/On model

A physically based, fully distributed Hortonian Infiltration and Runoff/On hydrologic model was used to model infiltration excess as the dominant runoff generation mechanism on a pasture hillslope. The model was evaluated for its applicability to simulate spatial and temporal variability of runoff generation areas observed on a pasture hillslope in the Sand Mountain region of North Alabama, United States. Three rainfall events of varying intensity and duration were simulated for a highly instrumented pasture hillslope to study the dynamics of runoff generation and runon areas. Calibration and cross validation were performed on all three rainfall events. Performance of the Hortonian Infiltration and Runoff/On–simulated hydrographs was evaluated using root mean squared error, coefficient of determination and Nash-Sutcliffe coefficient of efficiency. The calibrated model for the first event resulted in a root mean squared error of 1.18 m³ (41.7 ft³) for runoff volume; the next two events resulted in root mean squared errors of less than 1 m³ (35.3 ft³). Similarly, the coefficient of determination and Nash-Sutcliffe coefficient of efficiency values for all three events were greater than 0.70 for the calibrated model. Results from cross validation showed that the Hortonian Infiltration and Runoff/On model-simulated runoffs were in agreement with the observed data. In addition, the model simulated spatial and temporal variations in runoff generation, and runon areas were in agreement with observed variations. Model results helped explain the interactions among hydrologic and climatic characteristics, such as topography, soil parameters, and rainfall variations, and their connections to surface runoff–generation processes. Although the model does not simulate subsurface lateral flow, it shows promise for identifying runoff generation and runon areas for controlling nonpoint source pollution from pasture hillslopes in this and similar regions.

A compilation and meta-analysis of rainfall simulation data on arable soils

► We compiled and analyzed a data set of 726 rainfall simulations on 209 plots. ► This allowed to overcome deficiencies inherent in single-source data sets. ► The compilation gives information on infiltration and runoff not obtainable otherwise. ► The compilation allowed defining basic standards for rainfall simulations studies. Rainfall simulations are a useful and important tool in studying infiltration, surface runoff generation, soil erosion and nutrient as well as agro-chemical transport from arable land. Such simulations are time-consuming and costly and hence are usually only carried out under a limited variation of settings necessary to answer specific research questions. Therefore, it is difficult to use rainfall simulation data for hypothesis testing in a more general sense or to parameterize hydrological or erosion models applicable under a wider range of environmental conditions. To overcome these restrictions and to set-up a broader basis for following up studies, we analyzed, harmonized and filled gaps of a large set of existing rainfall simulations carried out by five different research groups in Germany. This covered 726 rainfall simulations (24,384 runoff measurements) carried out on 209 plots under a wide range of conditions for which 4 rain properties, 5 plot properties, 20 soil properties, 5 land use properties and 2 runoff properties were compiled. These data were quality controlled and made available for public use ( Seibert et al., 2011 ). The most important deficiencies were smoothed runoff measurements, missing time to ponding data, different soil descriptions including frequent gaps in stone content, inconsistent moisture measurements and sometimes rather rough measurements of surface cover. The calculation of the geometric mean particle diameter, time since tillage and the application of different site specific procedures supported harmonization and helped to overcome several of these deficiencies. A satisfying gap filling procedure was developed for time to ponding. The most important inconsistencies that could not be removed were different depths of moisture measurement. Hence, there is a need to define a set of basic variables that always should be measured and documented with defined standards to enable comparison between different studies, to assess the boundary conditions of validity and possibly to make wider use of individual data sets by combining several of them.

Spatial distribution of top soil water content in an experimental catchment of Southeast Brazil

Soil water content is essential to understand the hydrological cycle. It controls the surface runoff generation, water infiltration, soil evaporation and plant transpiration. This work aims to analyze the spatial distribution of top soil water content and to characterize the spatial mean and standard deviation of top soil water content over time in an experimental catchment located in the Mantiqueira Range region, state of Minas Gerais, Brazil. Measurements of top soil water content were carried out every 15 days, between May/2007 and May/2008. Using time-domain reflectometry (TDR) equipment, 69 points were sampled in the top 0.2 m of the soil profile. Geostatistical procedures were applied in all steps of the study. First, the spatial continuity was evaluated, and the experimental semi-variogram was modeled. For the development of top soil water content maps over time a co-kriging procedure was used having the slope as a secondary variable. Rainfall regime controlled the top soil water content during the wet season. Land use was also another fundamental local factor. The spatial standard deviation had low values under dry conditions, and high values under wet conditions. Thus, more variability occurs under wet conditions.

Preferential Flow Effects on Infiltration and Runoff in Grassland and Forest Soils

Due to the heterogeneity of soil hydraulic properties, there are many ways in which natural soils respond to rainfall, making upscaling of flow processes from plot to catchment scale difficult. The objectives of this study were to qualitatively characterize the flow pathways on forest and grassland hillslopes and to quantitatively define the relevant parameters controlling surface runoff generation by using infiltration and dye tracer experiments supplemented with measurements of the saturated hydraulic conductivity Ksat, the structural porosity nSP, and the pore‐size distribution PSD. While infiltration excess overland flow dominates in grassland, forest soil structure characterized by relatively high values of Ksat and nSP enhances the infiltrability of the soil and consequently prevents or at least reduces surface runoff. The dye patterns suggest that macropores are more efficient in forest than in grassland soil. The low efficiency of grassland soil macropores in transporting all water vertically downward can be explained by (i) the fine and dense few topsoil layers caused by the land use that limit water flux into the underlying macropores and (ii) their restricted number, their tortuosity, and the restricted interaction between macropores and the matrix below the topsoil layer. The larger root water uptake of forest soil as compared to grassland soil can be viewed as an additional factor enhancing its storage capacity and, consequently, may reduce the generation of surface runoff. It remains unclear, however, what effect the low interaction between macropores and soil matrix in the upper part of the subsoil has on surface runoff in grassland soil; this should be investigated in future studies.

Treating an artificial catchment as ungauged: Increasing the plausibility of an uncalibrated, process-based SVAT scheme by using additional soft and hard data

The 1-D process-based model SIMULAT was applied to the 6 ha large artificial catchment “Chicken Creek” in Lausatia, Germany. Within the framework of a model intercomparison study, data availability was improved step by step, starting from sparse data conditions. Initially, the model was parameterised based on transfer functions (e.g., soil hydraulic properties were estimated from pedotransfer functions) and literature (e.g., plant parameters, boundary conditions), only. Then parameterisation was revised based on field inspection and additional quantitative data (e.g., from point measurements). Finally, soil moisture data were used for validation and calibration purposes. During this parameterisation process, model results became increasingly plausible although calibration and validation against observed discharge were not feasible because discharge data were not available to the modellers. Simulated discharge dynamics changed from an initially base flow-dominated and continuous flow regime to a system in which different flow components contribute similarly to the event-based total discharge, better conforming to the hydrological process understanding with respect to the development of a gully network. Qualitative information (=soft data) gained from a field visit particularly contributed to this improvement in process understanding towards a flow regime dominated by surface runoff, while additional quantitative information on system characteristics rather served the purpose of verifying (or revising) of model parameterisation and defining appropriate initial conditions. An evaluation of simulated surface runoff rates based on event-based discharge information for a subcatchment revealed that the model overestimated the surface runoff generation for all advanced modelling steps. A final validation of model results is not yet feasible as continuous discharge data at the catchment outlet are not available so far. However, the model application indicated that integration of soft and quantitative information can considerably increase the plausibility of model results in the case of poorly gauged basins while applying transfer functions developed under natural conditions might fail in artificial catchments.

Models as multiple working hypotheses: hydrological simulation of tropical alpine wetlands

Tropical alpine grasslands, locally known as paramos, are the water towers of the northern Andes. They are an essential water source for drinking water, irrigation schemes and hydropower plants. But despite their high socio-economic relevance, their hydrological processes are very poorly understood. Since environmental change, ranging from small scale land-use changes to global climate change, is expected to have a strong impact on the hydrological behaviour, a better understanding and hydrological prediction are urgently needed. In this paper, we apply a set of nine hydrological models of different complexity to a small, well monitored upland catchment in the Ecuadorian Andes. The models represent different hypotheses on the hydrological functioning of the paramo ecosystem at catchment scale. Interpretation of the results of the model prediction and uncertainty analysis of the model parameters reveals important insights in the evapotranspiration, surface runoff generation and base flow in the paramo. However, problems with boundary conditions, particularly spatial variability of precipitation, pose serious constraints on the differentiation between model representations.

Observation of hydrological processes and structures in the artificial Chicken Creek catchment

Hydrological catchments are the essential observation spaces for hydrological and ecological studies on a landscape scale. Generally, natural catchments are characterised by high complexity and heterogeneity with uncertainties in boundaries, interior structures and properties. Artificially created, spatially and structurally well-defined hydrological catchments might be a suitable tool for enabling progress in hydrological research. “Chicken Creek”, one of the largest artificial catchments, with clearly defined boundary conditions and extended monitoring facilities, was set up on a dump site associated with opencast mining activities in East Germany. This catchment, left to undisturbed succession, has enabled the observation of ecosystem development from its very beginnings. A sophisticated measurement concept covering hydrological, meteorological and soil physical parameters was designed. It aimed at the detection and description of the hydrological processes that form structures and are controlled by structures at the catchment scale. In mature natural catchments that have already achieved a stage of dynamic equilibrium with evolved runoff structures and land cover, hot spots of hydrological interest are normally known or easily identifiable prior to the establishment of any monitoring concept. For a newly created artificial catchment, there is no recorded experience regarding hydrological development and behaviour. Under the premise of looking at a defined, initially unstructured, homogenous, isotropic, “virgin” area, a primarily grid-based observation concept was chosen as the initial approach. As components of the progressive development of the ecosystem, new soil and runoff structures have been formed, and vegetation has spread on the initially uncovered surface. As a function of the changed hydrological properties and the improved system knowledge, the initial observation programme was iteratively extended and adjusted by the addition of more structure- and process-related installations. In this study, the design of the experimental observation set-up, the gathered experiences and the selected results of a large number of the most various hydrological data are presented. The combined analysis of measurement data and qualitative information worked to determine that the precipitation-induced surface runoff generation and the filling of the catchment storage were the main relevant hydrological processes in the initial development phase of the Chicken Creek catchment. Formed by the dominating surface runoff, erosion gullies are the most easily distinguishable structures influencing the runoff process. Despite the well-known catchment boundaries and the extensive monitoring, the catchment water balance has not been adequately closed so far. Due to the catchment’s properties (porosity, morphology, land cover), which are less homogeneous than initially assumed, the determination of the catchment’s evapotranspiration and changes in storage remains the challenge for understanding the Chicken Creek catchment.

Surface runoff and soil erosion estimation using the SWAT model in the Keleta Watershed, Ethiopia

AbstractThis study evaluates surface runoff generation and soil erosion rates for a small watershed (the Keleta Watershed) in the Awash River basin of Ethiopia by using the Soil and Water Assessment Tool (SWAT) model. Calibration and validation of the model was performed on monthly basis, and it could simulate surface runoff and soil erosion to a good level of accuracy. The simulated surface runoff closely matched with observed data (derived by hydrograph separation). Surface runoff generation was generally high in parts of the watershed characterized by heavy clay soils with low infiltration capacity, agricultural land use and slope gradients of over 25 per cent. The estimated soil loss rates were also realistic compared to what can be observed in the field and results from previous studies. The long‐term average soil loss was estimated at 4·3 t ha−1 y−1; most of the area of the watershed (∼80 per cent) was predicted to suffer from a low or moderate erosion risk (&lt;8 t ha−1 y−1), and only in ∼1·2 per cent of the watershed was soil erosion estimated to exceed 12 t ha−1 y−1. Expectedly, estimated soil loss was significantly correlated with measured rainfall and simulated surface runoff. Based on the estimated soil loss rates, the watershed was divided into four priority categories for conservation intervention. The study demonstrates that the SWAT model provides a useful tool for soil erosion assessment from watersheds and facilitates planning for a sustainable land management in Ethiopia. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Comparative diagnostic analysis of runoff generation processes in Oklahoma DMIP2 basins: The Blue River and the Illinois River

Summary This paper presents the results of a comparative diagnostic study of runoff generation processes in two test basins in Oklahoma: The Blue River at Blue and the Illinois River near Tahlequah. This study involves analysis of signatures of spatio-temporal runoff variability, extracted from both observed rainfall–runoff data and from predictions of a distributed, physically based rainfall–runoff model. Analysis of observed data in both basins indicates that event runoff coefficients are systematically higher in the wet season than in the dry season. Model predictions indicate that the transition from high to low runoff coefficients in the Blue River basin is linked to variations of water table depth and surface soil moisture, contributing to a seasonal switching of surface runoff generation mechanisms, from saturation excess to infiltration excess. In the Illinois River basin, however, due to more permeable soils, infiltration excess runoff occurs rarely. The differences in intra-annual patterns of runoff coefficients and runoff generation mechanisms can be partly explained by the seasonality of climate forcing and water table position. Despite the significant differences of runoff generation mechanisms between the two basins, spatial analysis of the model results reveals that in both watersheds, but especially so in the more humid Illinois River basin, saturation excess runoff and subsurface stormflow coexist in competition throughout the year. This competition is quantitatively shown to be controlled by the relative magnitudes of the saturated hydraulic conductivity of the soils and the topographic slope. In addition, the spatial variabilities of runoff generation processes also impact the spatial scaling behavior of runoff ratios, indicating the existence of a threshold watershed size beyond which the variability is averaged out.

The effect of the conditions of a landscape on its retention capacity

The effect of the conditions of a landscape on its retention capacityQuestions related to the occurrence, frequency, intensity, duration, characteristics and causes of floods have been discussed more in recent years. Two basic approaches to flood control often conflict. The first is based on the assumption of the considerable effect of a landscape's retention capacity, which can in fact prevent surface runoff generation and flood formation and can significantly transform flood wave. The second approach asserts that the retention capacity of a landscape is nearly negligible and that the only reliable flood protection can be provided by extending the technical structures of flood control measures mainly and directly on water courses. Two different approaches were applied to assess the effect of landscape conditions and revitalization measures on surface runoff and flood formation within a catchment and floodplain. The conclusion shows that the effect of landscape revitalization is very important, but mainly for low return periods of flood events, while for extreme events, the effect on landscapes and floodplains becomes less important and even negligible.