Representative Hillslope Methods for Applying the Wepp Model With Dems and Gis

In watershed modeling with WEPP, the process of manually identifying hillslopes and channels is very timeconsuming and can be subject to large variation between users. Furthermore, the representation of hillslope profiles issubjective and can differ between different modelers. To overcome this, modeling procedures called the Hillslope methodswere developed that use geographical information systems (GIS) and digital elevation models (DEMs) to assess water erosionin small watersheds with the Water Erosion Prediction Project (WEPP) model. The Hillslope methods are automatedprocedures to develop hillslope and channel topographic characteristics from DEMs for use in the WEPP model. Theobjective of this study was therefore to determine which method of creating representative slope profile and representativehillslope profile lengths performs best. Three methods of creating a representative slope profile from DEMs were developedand tested: linear average, exponentially transformed average, and weighted average. Additionally, two methods todetermine the representative hillslope profile length, called the Calcleng and Chanleng methods, were evaluated. TheCalcleng method calculates a representative length of hillslope based on the weighted lengths of all flowpaths in a hillslopeas identified through a DEM. The Chanleng method sets hillslope width equal to adjacent channel length and then computesa hillslope length from hillslope area divided by width. Actual DEMs from six research watersheds were used to test thesemethods. The results from the application of these methods were compared to each other and to measured sediment data.Results showed that the three methods for determining the representative slopes of the profiles were not significantly differentfrom each other. There were also no significant differences between the Calcleng and the Chanleng methods for sedimentyields and runoff from the six watersheds. Theoretically, however, for more complex watersheds, the weighted average methodfor determination of representative slope profile gradient values and the Chanleng method to determine representative profileslope lengths are the preferred methods. These results help automate the application of WEPP to watersheds using GIS andDEMs.