Small Watershed Modeling with Wepp Using Grid-based Dems and Gis

Two different methods utilizing 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 and runoff in small watersheds. The first approach, called the Hillslope method, presents an automated method for the application of WEPP through the extraction of hillslopes and channels from DEMs. The second approach, called the Flowpath method, uses WEPP model simulations on all possible flowpaths within a watershed. The two methods were applied to five field size (0.59 to 2.7 ha) research watersheds, two from Watkinsville, Georgia, three from Holly Springs, Mississippi, and one small watershed (29 ha) in Treynor, Iowa. Both methods were evaluated for assessing watershed erosion using different DEM grid resolutions. Simulations were conducted using 1, 3, 5, and 10-meter resolution DEMs for the field sized watersheds and 5, 10, 15, and 20-meter resolutions for the Treynor watershed. Statistical analysis for all methods and watersheds, were performed by comparing predicted vs. measured runoff and sediment yield from the watershed outlets for runoff and sediment loss. A comparison of erosion from only hillslopes for all three methods indicates that the Flowpath method is statistically comparable to the other methods. Comparisons of both simulation methods to a manual application of WEPP by an expert user for all watersheds were also conducted at each DEM resolution level. The results indicate that the automatic Hillslope method performs as well as a manual application of WEPP by an expert user for all watersheds.