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Representative Hillslope Methods for Applying the WEPP Model with DEMs and GIS

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In watershed modeling with WEPP, the process of manually identifying hillslopes and channels is very time consuming and can be subject to large variation between users. Furthermore, the representation of hillslope profiles is subjective and can differ between different modelers. To overcome this, modeling procedures called the Hillslope methods were developed that use geographical information systems (GIS) and digital elevation models (DEMs) to assess water erosion in small watersheds with the Water Erosion Prediction Project (WEPP) model. The Hillslope methods are automated procedures to develop hillslope and channel topographic characteristics from DEMs for use in the WEPP model. The objective of this study was therefore to determine which method of creating representative slope profile and representative hillslope profile lengths performs best. Three methods of creating a representative slope profile from DEMs were developed and tested: linear average, exponentially transformed average, and weighted average. Additionally, two methods to determine the representative hillslope profile length, called the Calcleng and Chanleng methods, were evaluated. The Calcleng method calculates a representative length of hillslope based on the weighted lengths of all flowpaths in a hillslope as identified through a DEM. The Chanleng method sets hillslope width equal to adjacent channel length and then computes a hillslope length from hillslope area divided by width. Actual DEMs from six research watersheds were used to test these methods. 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 different from each other. There were also no significant differences between the Calcleng and the Chanleng methods for sediment yields and runoff from the six watersheds. Theoretically, however, for more complex watersheds, the weighted average method for determination of representative slope profile gradient values and the Chanleng method to determine representative profile slope lengths are the preferred methods. These results help automate the application of WEPP to watersheds using GIS and DEMs.
Cochrane, T.A. , Flanagan, D.C.
Includes references
Transactions of the ASAE 2003 July-Aug, v. 46, no. 4
Journal Articles, USDA Authors, Peer-Reviewed
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