Application of the WEPP model to determine sources of run‐off and sediment in a forested watershed

This study investigates critical run‐off and sediment production sources in a forested Kasilian watershed located in northern Iran. The Water Erosion Prediction Project (WEPP) watershed model was set up to simulate the run‐off and sediment yields. WEPP was calibrated and validated against measured rainfall–run‐off–sediment data. Results showed that simulated run‐off and sediment yields of the watershed were in agreement with the measured data for the calibration and validation periods. While low and medium values of run‐off and sediment yields were adequately simulated by the WEPP model, high run‐off and sediment yield values were underestimated. Performance of the model was evaluated as very good and satisfactory during the calibration and validation stages, respectively. Total soil erosion and sediment load of the study watershed during the study period were determined to be 10 108 t yr^?1 and 8735 t yr^?1, respectively. The northern areas of the watershed with dry farming were identified as the critical erosion prone zones. To prioritize the subwatersheds based on their contribution to the run‐off and sediment production at the watershed's main outlet, unit response approach (URA) was applied. In this regard, subwatersheds close to the main outlet were found to have the highest contribution to sediment yield of the whole watershed. Results indicated that depending on the objective of land and water conservation practices, particularly, for controlling sediment yield at the main outlet, critical areas for implementing the best management practices may be identified through conjunctive application of WEPP and URA. Copyright © 2014 John Wiley & Sons, Ltd.     

Assessing the vulnerability to soil erosion of the Ukai Dam catchments using remote sensing and GIS

Abstract

The investigation of basins for planning soil conservation requires a selective approach to identify smaller hydrological units, which would be suitable for more efficient and targeted conservation management programmes. One criterion, generally used to determine the vulnerability of catchments to erosion, is the sediment yield of a basin. In India, sediment yield data are generally not collected for smaller sub-catchments and it becomes difficult to identify the most vulnerable areas for erosion that can be treated on a priority basis. An index-based approach, based on the surface factors mainly responsible for soil erosion, is suggested in this study. These factors include soil type, vegetation, slope and various catchment properties such as drainage density, form factor, etc. The method is illustrated with a case study of sub-catchments immediately upstream of the Ukai Reservoir located on the River Tapi in Gujarat State, India. The area is divided into 16 watersheds and different soil, vegetation, topography and morphology-related parameters are estimated separately for each watershed. Satellite data are used to evaluate the soil and vegetation indices, while a GIS system is used to evaluate the topography and morphology-related indices. The integrated effect of all the parameters is evaluated to find different areas vulnerable to soil erosion. Two watersheds were identified as being most susceptible to soil erosion. Based on the integrated index, a priority rating of the watersheds for soil conservation planning is recommended.     

Estimation of soil losses within plots as affected by different agricultural land management

Abstract

Proper agricultural land management strategies improve soil structural properties, thereby reducing soil loss by water erosion. This study was conducted to estimate soil losses from plots of different agricultural land management using the Water Erosion Prediction Project (WEPP) (95.7) model. The study took place in a semiarid region in Kenya. The mean annual rainfall was 694 mm. The WEPP (95.7) model was initially used to estimate total sediment loading from the catchment into a reservoir. The estimate was about 2871 t corresponding to an average sedimentation rate of 1063 t km^?2 year^?1, which was about 76% of the measured total sediment inflow into the reservoir. Soil losses were estimated within 10 plots on the catchment of different sizes and slopes with the following treatments: conventional tillage (hand hoeing) with maize and soybean intercropping (HOCOBE); conservation tillage (disc plough) with maize and soybean intercropping (COBEAN); conservation tillage with only maize cultivation (CNTCORN); and conservation tillage with only soybean cultivation (CNTBEAN). The soil loss reduction of COBEAN, CNTCORN and CNTBEAN relative to HOCOBE ranged between 27–47%, 16–29% and 12–25%, respectively, depending on the size and slope of the plot. In general, conservation tillage reduced soil loss relative to conventional tillage. However, with conservation tillage, the single cropping system resulted in greater soil loss than the intercropping system. In the case of single cropping with conservation tillage, the soil loss reduction for maize ranged between 4 and 9%, relative to soybean. Overall, the study showed that there would be a significant reduction of soil losses from plots if conservation tillage with an intercropping system (maize and soybean) were to be adopted on agricultural lands in semiarid regions.     

Long‐term effects of management practices on water‐driven soil erosion in an intense agricultural sub‐watershed: monitoring and modelling

The overarching objective of this research was to provide an improved understanding of the role of land use and associated management practices on long‐term water‐driven soil erosion in small agricultural watersheds by coupling the established, physically based, distributed parameter Water Erosion Prediction Project (WEPP) model with long‐term hydrologic, land use and soil data. A key step towards achieving this objective was the development of a detailed methodology for model calibration using physical ranges of key governing parameters such as effective hydraulic conductivity, critical hydraulic shear stress and rill/inter‐rill erodibilities. The physical ranges for these governing parameters were obtained based on in situ observations within the South Amana Sub‐Watershed (SASW) (～26 km²) of the Clear Creek, IA watershed where detailed documentation of the different land uses was available for a period of nearly 100 years. A quasi validation of the calibrated model was conducted through long‐term field estimates of water and sediment discharge at the outlet of SASW and also by comparing the results with data reported in the literature for other Iowa watersheds exhibiting similar biogeochemical properties. Once WEPP was verified, ‘thought experiments’ were conducted to test our hypothesis that land use and associated management practices may be the major control of long‐term erosion in small agricultural watersheds such as SASW. Those experiments were performed using the dominant 2‐year crop rotations in the SASW, namely, fall till corn–no till bean (FTC‐NTB), no till bean–spring till corn (NTB‐STC) and no till corn–fall till bean (NTC‐FTB), which comprised approximately 90% of the total acreage in SASW. Results of this study showed that for all crop rotations, a strong correspondence existed between soil erosion rates and high‐magnitude precipitation events during the period of mid‐April and late July, as expected. The magnitude of this correspondence, however, was strongly affected by the crop rotation characteristics, such as canopy/residue cover provided by the crop, and the type and associated timing of tillage. Tillage type (i.e. primary and secondary tillages) affected the roughness of the soil surface and resulted in increases of the rill/inter‐rill erodibilities up to 35% and 300%, respectively. Particularly, the NTC‐FTB crop rotation, being the most intense land use in terms of tillage operations, caused the highest average annual erosion rate within the SASW, yielding quadrupled erosion rates comparatively to NTB‐STC. The impacts of tillage operation were further exacerbated by the timing of the operations in relation to precipitation events. Timing of operations affected the ‘life‐time’ of residue cover and as a result, the degree of protection that residue cover offers against the water action on the soil surface. In the case of NTC‐FTB crop rotation, dense corn residue stayed on the ground for only 40 days, whereas for the other two rotations, corn residue provided a protective layer for nearly 7 months, lessening thus the degree of soil erosion. The cumulative effects of tillage type and timing in conjunction with canopy/residue cover led to the conclusion that land management practices can significantly amplify or deamplify the impact of precipitation on long‐term soil erosion in small agricultural watersheds. Copyright © 2009 John Wiley & Sons, Ltd.     

Variation in the sediment deposition behind check‐dams under different soil erosion conditions on the Loess Plateau,China

To maintain a reasonable sediment regulation system in the middle reaches of the Yellow River, it is critical to determine the variation in sediment deposition behind check‐dams for different soil erosion conditions. Sediment samples were collected by using a drilling machine in the Fangta watershed of the loess hilly–gully region and the Manhonggou watershed of the weathered sandstone hilly–gully (pisha) region. On the basis of the check‐dam capacity curves, the soil bulk densities and the couplet thickness in these two small watersheds, the sediment yields were deduced at the watershed scale. The annual average sediment deposition rate in the Manhonggou watershed (702.0 mm/(km²·a)) from 1976 to 2009 was much higher than that in the Fangta watershed (171.6 mm/(km²·a)) from 1975 to 2013. The soil particle size distributions in these two small watersheds were generally centred on the silt and sand fractions, which were 42.4% and 50.7% in the Fangta watershed and 60.6% and 32.9% in the Manhonggou watershed, respectively. The annual sediment deposition yield exhibited a decreasing trend; the transition years were 1991 in the Fangta watershed and 1996 in the Manhonggou watershed (P < 0.05). In contrast, the annual average sediment deposition yield was much higher in the Manhonggou watershed (14011.1 t/(km²·a)) than in the Fangta watershed (3149.6 t/(km²·a)). In addition, the rainfalls that induced sediment deposition at the check‐dams were greater than 30 mm in the Fangta watershed and 20 mm in the Manhonggou watershed. The rainfall was not the main reason for the difference in the sediment yield between the two small watersheds. The conversion of farmland to forestland or grassland was the main reason for the decrease in the soil erosion in the Fangta watershed, while the weathered sandstone and bare land were the main factors driving the high sediment yield in the Manhonggou watershed. Knowledge of the sediment deposition process of check‐dams and the variation in the catchment sediment yield under different soil erosion conditions can serve as a basis for the implementation of improved soil erosion and sediment control strategies, particularly in semi‐arid hilly–gully regions. Copyright © 2018 John Wiley & Sons, Ltd.     

Spatial discretization of large watersheds and its influence on the estimation of hillslope sediment yield

The combined use of water erosion models and geographic information systems has facilitated soil loss estimation at the watershed scale. Tools such as the Geo‐spatial interface for the Water Erosion Prediction Project (GeoWEPP) model provide a convenient spatially distributed soil loss estimate but require discretization to identify hillslopes and channels. In GeoWEPP, the TOpographic PArameteriZation (TOPAZ) model is used as an automated procedure to extract a watershed boundary, hillslopes and channels from a digital elevation model (DEM). Previous studies in small watersheds have shown that the size of the hillslopes and the channel distribution affect the model estimates, but in large watersheds, the effects on the soil loss estimates have yet to be tested. Therefore, the objective of this study was to evaluate the effect of discretization on the hillslope sediment yield estimates using GeoWEPP in two large watersheds (>10 km²). The watersheds were selected and discretized varying the TOPAZ parameters [critical source area (CSA) and minimum source channel length (MSCL)] in a 30‐m resolution digital elevation model. The drainage networks built with TOPAZ were compared with each other using the drainage density index. The results showed that the discretization affected hillslope sediment yield estimates and their spatial distribution more than the total runoff. The drainage density index and the hillslope sediment yield were proportional but inversely related; thus, soil loss estimates were highly affected by the spatial discretization. As a result of this analysis, a method to choose the CSA and MSCL values that generates the greatest fraction of hillslopes having profile lengths less than 200 m was developed. This slope length condition is particularly crucial when using the WEPP and GeoWEPP models, in order for them to produce realistic estimates of sheet and rill erosion. Finally, and as a result of this analysis, a more reliable method was developed for selecting the TOPAZ channel network parameters (CSA and MSCL). Copyright © 2015 John Wiley & Sons, Ltd.     

Identification of soil erosion and fluvial sediment problems

A computer model has been used to estimate soil loss and sediment yield from irregular field-size units of small watersheds. Input to the model includes spring data (i.e. relating to February through May) for the independent variables of the Universal Soil Loss Equation, and for factors such as surface roughness, an index of overland runoff, and proximity to the stream. Output from the model includes maps of seasonal estimates of potential soil losses, field sediment delivery ratios, and expected sediment yields. On the basis of selected erosion and sediment yield tolerances, the output information has been analysed to identify watershed areas which (1) exhibit both erosion and sediment yield problems; (2) exhibit only erosion problems; (3) exhibit only sediment yield problems; and (4) exhibit neither erosion nor sediment yield problems. The percentage of the watershed area in each category and the percentage of the watershed soil loss and sediment loads contributed by each category are also identified. Application of the procedure for planning remedial control programs for five watersheds is discussed.     

Distributed discharge and sediment concentration predictions in the sub‐humid Ethiopian highlands: the Debre Mawi watershed

Experimental research in the Ethiopian highlands found that saturation excess induced runoff and erosion are common in the sub‐humid conditions. Because most erosion simulation models applied in the highlands are based on infiltration excess, we, as an alternative, developed the Parameter Efficient Distributed (PED) model, which can simulate water and sediment fluxes in landscapes with saturation excess runoff. The PED model has previously only been tested at the outlet of a watershed and not for distributed runoff and sediment concentration within the watershed. In this study, we compare the distributed storm runoff and sediment concentration of the PED model against collected data in the 95‐ha Debre Mawi watershed and three of its nested sub‐watersheds for the 2010 and 2011 rainy seasons. In the PED model framework, the hydrology of the watershed is divided between infiltrating and runoff zones, with erosion only taking place from two surface runoff zones. Daily storm runoff and sediment concentration values, ranging from 0.5 to over 30 mm and from 0.1 to 35 g l^?1, respectively, were well simulated. The Nash Sutcliffe efficiency values for the daily storm runoff for outlet and sub‐watersheds ranged from 0.66 to 0.82, and the Nash–Sutcliffe efficiency for daily sediment concentrations were greater than 0.78. Furthermore, the model uses realistic fractional areas for surface and subsurface flow contributions, for example between saturated areas (15%), degraded areas (30%) and permeable areas (55%) at the main outlet, while close similarity was found for the remaining hydrology and erosion parameter values. One exception occurred for the distinctly greater transport limited parameter at the actively gullying lower part of the watershed. The results suggest that the model based on saturation excess provides a good representation of the observed spatially distributed runoff and sediment concentrations within a watershed by modelling the bottom lands (as opposed to the uplands) as the dominant contributor of the runoff and sediment load. Copyright © 2014 John Wiley & Sons, Ltd.     

Evaluation of a dynamic multi‐class sediment transport model in a catchment under soil‐conservation agriculture

Soil erosion models are essential tools for the successful implementation of effective and adapted soil conservation measures on agricultural land. Therefore, models are needed that predict sediment delivery and quality, give a good spatial representation of erosion and deposition and allow us to account for various soil conservation measures. Here, we evaluate how well a modified version of the spatially distributed multi‐class sediment transport model (MCST) simulates the effectiveness of control measures for different event sizes. We use 8 year runoff and sediment delivery data from two small agricultural watersheds (0·7 and 3·7 ha) under optimized soil conservation. The modified MCST model successfully simulates surface runoff and sediment delivery from both watersheds; one of which was dominated by sheet and the other was partly affected by rill erosion. Moreover, first results of modelling enrichment of clay in sediment delivery are promising, showing the potential of MCST to model sediment enrichment and nutrient transport. In general, our results and those of an earlier modelling exercise in the Belgian Loess Belt indicate the potential of the MCST model to evaluate soil erosion and deposition under different agricultural land uses. As the model explicitly takes into account the dominant effects of soil‐conservation agriculture, it should be successfully applicable for soil‐conservation planning/evaluation in other environments. Copyright © 2008 John Wiley & Sons, Ltd.     

Piecewise prediction model for watershed‐scale erosion and sediment yield of individual rainfall events on the Loess Plateau,China

Establishing a universal watershed‐scale erosion and sediment yield prediction model represents a frontier field in erosion and soil/water conservation. The research presented here was conducted on the Chabagou watershed, which is located in the first sub‐region of the hill‐gully area of the Loess Plateau, China. A back‐propagation artificial neural model for watershed‐scale erosion and sediment yield was established, with the accuracy of the model, then compared with that of multiple linear regression. The sensitivity degree of various factors to erosion and sediment yield was quantitatively analysed using the default factor test. On the basis of the sensitive factors and the fractal information dimension, the piecewise prediction model for erosion and sediment yield of individual rainfall events was established and further verified. The results revealed the back‐propagation artificial neural network model to perform better than the multiple linear regression model in terms of predicting the erosion modulus, with the former able to effectively characterize dynamic changes in sediment yield under comprehensive factor conditions. The sensitivity of runoff erosion power and runoff depth to the erosion and sediment yield associated with individual rainfall events was found to be related to the complexity of surface topography. The characteristics of such a hydrological response are thus closely related to topography. When the fractal information dimension is greater than the topographic threshold, the accuracy of prediction using runoff erosion power is higher than that of using runoff depth. In contrast, when the fractal information dimension is smaller than the topographic threshold, the accuracy of prediction using runoff depth is higher than that of using runoff erosion power. The developed piecewise prediction model for watershed‐scale erosion and sediment yield of individual rainfall events, which introduces runoff erosion power and runoff depth using the fractal information dimension as a boundary, can be considered feasible and reliable and has a high prediction accuracy. Copyright © 2013 John Wiley & Sons, Ltd.     

The Daily Erosion Project – daily estimates of water runoff,soil detachment,and erosion

Water runoff and sediment transport from agricultural uplands are substantial threats to water quality and sustained crop production. To improve soil and water resources, farmers, conservationists, and policy‐makers must understand how landforms, soil types, farming practices, and rainfall interact with water runoff and soil erosion processes. To that end, the Iowa Daily Erosion Project (IDEP) was designed and implemented in 2003 to inventory these factors across Iowa in the United States. IDEP utilized the Water Erosion Prediction Project (WEPP) soil erosion model along with radar‐derived precipitation data and government‐provided slope, soil, and management information to produce daily estimates of soil erosion and runoff at the township scale (93 km² [36 mi²]). Improved national databases and evolving remote sensing technology now permit the derivation of slope, soil, and field‐level management inputs for WEPP. These remotely sensed parameters, along with more detailed meteorological data, now drive daily WEPP hillslope soil erosion and water runoff estimates at the small watershed scale, approximately 90 km² (35 mi²), across sections of multiple Midwest states. The revisions constitute a substantial improvement as more realistic field conditions are reflected, more detailed weather data are utilized, hill slope sampling density is an order of magnitude greater, and results are aggregated based on surface hydrology enabling further watershed research and analysis. Considering these improvements and the expansion of the project beyond Iowa it was renamed the Daily Erosion Project (DEP). Statistical and comparative evaluations of soil erosion simulations indicate that the sampling density is adequate and the results are defendable. The modeling framework developed is readily adaptable to other regions given suitable inputs. © 2017 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.     

Lumped surface and sub‐surface runoff for erosion modeling within a small hilly watershed in northern Vietnam

Developing models to predict on‐site soil erosion and off‐site sediment transport at the agricultural watershed scale represent an on‐going challenge in research today. This study attempts to simulate the daily discharge and sediment loss using a distributed model that combines surface and sub‐surface runoffs in a small hilly watershed (< 1 km²). The semi‐quantitative model, Predict and Localize Erosion and Runoff (PLER), integrates the Manning–Strickler equation to simulate runoff and the Griffith University Erosion System Template equation to simulate soil detachment, sediment storage and soil loss based on a map resolution of 30 m × 30 m and over a daily time interval. By using a basic input data set and only two calibration coefficients based, respectively, on water velocity and soil detachment, the PLER model is easily applicable to different agricultural scenarios. The results indicate appropriate model performance and a high correlation between measured and predicted data with both Nash–Sutcliffe efficiency (Ef) and correlation coefficient (r²) having values > 0.9. With the simple input data needs, PLER model is a useful tool for daily runoff and soil erosion modeling in small hilly watersheds in humid tropical areas. Copyright © 2013 John Wiley & Sons, Ltd.     

Effect of watershed subdivision on simulation of water balance components

The present effect of watershed subdivision on simulated water balance components using the thoroughly tested Soil and Water Assessment Tool (SWAT) model has been evaluated for the Nagwan watershed in eastern India. Observed meteorological and hydrological data (daily rainfall, temperature, relative humidity and runoff) for the years 1995 to 1998 were collected and used. The watershed and sub‐watershed boundaries, slope and soil texture maps were generated using a geographical information system. A supervised classification method was used for land‐use/cover classification from satellite imagery of 1996. In order to study the effect of watershed subdivision, the watershed was spatially defined into three decomposition schemes, namely a single watershed, and 12 and 22 sub‐watersheds. The simulation using the SWAT model was done for a period of 4 years (1995 to 1998). Results of the study showed a perfect water balance for the Nagwan watershed under all of the decomposition schemes. Results also revealed that the number and size of sub‐watersheds do not appreciably affect surface runoff. Except for runoff, there was a marked variation in the individual components of the water balance under the three decomposition schemes. Though the runoff component of the water balance showed negligible variation among the three cases, variations were noticed in the other components: evapotranspiration (5 to 48%), percolation (2 to 26%) and soil water content (0·30 to 22%). Thus, based on this study, it is concluded that watershed subdivision has a significant effect on the water balance components. Copyright © 2005 John Wiley & Sons, Ltd.     

A new approach for linking event‐based upland sediment sources to downstream suspended sediment transport

In this study, we proposed a new approach for linking event sediment sources to downstream sediment transport in a watershed in central New York. This approach is based on a new concept of spatial scale, sub‐watershed area (SWA), defined as a sub‐watershed within which all eroded soils are transported out without deposition during a hydrological event. Using (rainfall) event data collected between July and November, 2007 from several SWAs of the studied watershed, we developed an empirical equation that has one independent variable, mean SWA slope. This equation was then used to determine event‐averaged unit soil erosion rate, Q_S/A, (in kg/km²/hr) for all SWAs in the studied watershed and calculate event‐averaged gross erosion E_ea (in kg/hr). The event gross erosion E_t (in kilograms) was subsequently computed as the product of E_ea and the mean event duration, T (in hours) determined using event hydrographs at the outlet of the studied watershed. Next, we developed two linear sediment rating curves (SRCs) for small and big events based on the event data obtained at the watershed outlet. These SRCs, together with T, allowed us to determine event sediment yield SY_e (in kilograms) for all events during the study period. By comparing E_t with SY_e, developing empirical equations (i) between E_t and SY_e and (ii) for event sediment delivery ratio, respectively, we revealed the event dynamic processes connecting sediment sources and downstream sediment transport. During small events, sediment transport in streams was at capacity and dominated by the deposition process, whereas during big events, it was below capacity and controlled by the erosion process. The key of applying this approach to other watersheds is establishing their empirical equations for Q_S/A and appropriately determining their numbers of SWAs. Copyright © 2011 John Wiley & Sons, Ltd.     

Using GIS and a digital elevation model to assess the effectiveness of variable grade flow diversion terraces in reducing soil erosion in northwestern New Brunswick,Canada

Flow diversion terraces (FDT) are commonly used beneficial management practice (BMP) for soil conservation on sloped terrain susceptible to water erosion. A simple GIS‐based soil erosion model was designed to assess the effectiveness of the FDT system under different climatic, topographic, and soil conditions at a sub‐basin level. The model was used to estimate the soil conservation support practice factor (P‐factor), which inherently considered two major outcomes with its implementation, namely (1) reduced slope length, and (2) sediment deposition in terraced channels. A benchmark site, the agriculture‐dominated watershed in northwestern New Brunswick (NB), was selected to test the performance of the model and estimated P‐factors. The estimated P‐factors ranged from 0·38–1·0 for soil conservation planning objectives and ranged from 0·001 to 0·45 in sediment yield calculations for water‐quality assessment. The model estimated that the average annual sediment yield was 773 kg ha^?1 yr ^?1 compared with a measured value of 641 kg ha^?1 yr^?1. The P‐factors estimated in this study were comparable with predicted values obtained with the revised universal soil loss equation (RUSLE2). The P‐factors from this study have the potential to be directly used as input in hydrological models, such as the soil and water assessment tool (SWAT), or in soil conservation planning where only conventional digital elevation models (DEMs) are available. Copyright © 2009 John Wiley & Sons, Ltd.     

Assessing impacts of riparian buffer zones on sediment and nutrient loadings into streams at watershed scale using an integrated REMM‐SWAT model

Riparian buffer zones in agriculture dominated watersheds play important roles in reducing nonpoint source pollution into aquatic ecosystems and are widely used as a Best Management Practice. Assessment of the effectiveness of riparian buffer zones by modeling method is widely used for watershed management as field measurement‐based assessment is difficult and expensive. The integration of Riparian Ecosystem Management Model (REMM) and Soil and Water Assessment Tool (SWAT) has been developed to simulate the effect of nonpoint source pollution reduction by riparian buffer zones at subbasin scale. However, there are problems in using the integrated model at subbasin scale, as the size of subbasin partition could affect the pollutant reduction rate by riparian buffers. In this study, we partitioned a large watershed with size of 1331 ha into sub‐watersheds with sizes of 666, 333, 166, 83, 51, and 29 ha, and then compared the different simulation results. We found that the modeling could yield more convergent results when the sub‐watersheds were partitioned into suitable size. In the studied area, the suitable sub‐watershed size was less than about 166 ha for runoff and nitrogen and 83 ha for sediment and phosphorus. Among the eight sub‐watersheds (partitioned based on the size of 166 ha), results showed that the effects of riparian buffers on runoff and nutrient loading varied drastically. The reduction rate varied from 0.26% to 30.13% for runoff, 29.4% to 74.07% for sediment, 9.61% to 57.85% for nitrogen, and 18.61% to 68.12% for phosphorus, respectively.     

People’s perception and socioeconomic determinants of soil erosion:A case study of Samanalawewa watershed,Sri Lanka

Though soil erosion is an important concern in Sri Lanka, there is a dearth of baseline information on soil erosion in many ofits watersheds, which obstructs monitoring of soil erosion and mitigating its effects. In order to assess soil erosion in a critical watershed and to identify its determinants, the Samanalawewa watershed, which contains one of the main hydropower generating reservoirs in Sri Lanka, was selected for this study. Remote-sensing (RS) and geographic information system (GIS) based modeling...     

First‐year post‐fire erosion rates in Bitterroot National Forest,Montana

Accelerated runoff and erosion commonly occur following forest fires due to combustion of protective forest floor material, which results in bare soil being exposed to overland flow and raindrop impact, as well as water repellent soil conditions. After the 2000 Valley Complex Fires in the Bitterroot National Forest of west‐central Montana, four sets of six hillslope plots were established to measure first‐year post‐wildfire erosion rates on steep slopes (greater than 50%) that had burned with high severity. Silt fences were installed at the base of each plot to trap eroded sediment from a contributing area of 100 m². Rain gauges were installed to correlate rain event characteristics to the event sediment yield. After each sediment‐producing rain event, the collected sediment was removed from the silt fence and weighed on site, and a sub‐sample taken to determine dry weight, particle size distribution, organic matter content, and nutrient content of the eroded material. Rainfall intensity was the only significant factor in determining post‐fire erosion rates from individual storm events. Short duration, high intensity thunderstorms with a maximum 10‐min rainfall intensity of 75 mm h^?1 caused the highest erosion rates (greater than 20 t ha^?1). Long duration, low intensity rains produced little erosion (less than 0·01 t ha^?1). Total C and N in the collected sediment varied directly with the organic matter; because the collected sediment was mostly mineral soil, the C and N content was small. Minimal amounts of Mg, Ca, and K were detected in the eroded sediments. The mean annual erosion rate predicted by Disturbed WEPP (Water Erosion Prediction Project) was 15% less than the mean annual erosion rate measured, which is within the accuracy range of the model. Published in 2007 by John Wiley & Sons, Ltd.     

WATERSHED SEDIMENT YIELD AND RANGELAND HEALTH

lINTRoDUCTIoNDifferencesintheprevailinglanduseandmanagementofaridandsemiaridareasaredeterminedinlargepartbyclimate.AridareasgenerallyreceivetoolittleprecipitationtosupportdrylandagricultureordomesticlivestockgrazingalthoughtheyaregrazedbywildIife,andattimes,bydomesticlivestock.Incontrast,insemiaridareasadequatemoistureisusuallyavaiIableatsometimeduringtheyeartoproduceforageforlivestockandwildlife,andtherearesomeyearswhendrylandcropproductionissuccessful.However,bothclimatesarecharacterize…