Assessing water erosion in small watersheds using WEPP with GIS

Three different approaches using geographical information systems (GIs) and digital elevation models (DEMs) are described and evaluated for applying the Water Erosion Prediction Project (WEPP) model to assess water erosion in small watersheds. The first approach describes a typical application of the watershed version of WEPP using GIs only as an aidfor construction of required input files. The second approach presents an automated method for the application o f W P P through the extraction o f hillslopes and channels f iom DEMs. The third approach uses W P P model simulations on all possible flowpaths within a watershed. The three method were applied to s ix research watersheds: one f iom Trqnor, la., two f iom Watkinsville, Ga., and three fiom Holb Springs, Miss. A statistical analysis for all methods and watershed compared the predicted us. measured runoff and sediment yieM f iom watershed outlets on an eventby-event basis for runoff and sediment loss. The results indicate that the automatic hillslope method (the second approach) peforms as well as the manual technique (the first approach) for all watershed. A comparison of erosion from onb hillslopes for all three methods indicates that the f iwpa th method (the third approach) is statistically comparable to the other methods. Results of the analysis suggest that, given an accurate DEM and valid input a h a for a simple watershed, the automatic hillslope method can be used to facilitate the application of the watershed version o f WEPR and that predictions shouM be comparable to an expert user; application of WEPI? K q words: DEM, flowpaths, Geographic Information System, hydrology, modeling, run08 sediment. soil erosion, Water Erosion Prediction Project. atershed erosion is a cause of nonpoint source pollution that W can have an adverse effect on downstream and ecosystem water quality. Unfortunately, it is very expensive and impractical to monitor erosion in all watersheds of interest, hence, the need to predict erosion with modeling. The Water Erosion Prediction Project (WEPP) model is a process-based continuous simulation erosion model (Flanagan and Nearing 1995). O n e advantage WEPP has over existing models, such as the popular Universal Soil Loss Equation (USLE), is that deposition of sediment also can be predicted (Wischmeier and Smith 1978). In other words, soil loss on a complete continuous hillslope profile can be calculated.