WATER EROSION PREDICTION PROJECT

August, 1998

Description The Water Erosion Prediction Project (WEPP) soil erosion model is being developed by an interagency group of scientists including the USDA's Forest Service, Agricultural Research Service, and Soil Conservation Service, and the Dept. of Interior's Bureau of Land Management and US Geological Survey. Scientists from these agencies throughout the United States have been working since 1985 to develop an erosion prediction model to replace the Universal Soil Loss Equation (USLE). The WEPP model is a complex computer program that describes the processes that lead to erosion. These processes include infiltration and runoff; soil detachment, transport, and deposition; and plant growth, senescence, and residue decomposition. The model daily calculates the soil water content in multiple layers and plant growth/decomposition. The effects of tillage processes and soil consolidation are also modeled. Topographic Versions The WEPP model will be available in three versions. The base model is a hillslope version that predicts soil erosion from a single hillslope profile of any length. The hillslope can have a complex shape, and can include numerous soils and crops along the hillslope. The watershed version links hillslope elements of specified widths together with channel and impound-ment elements. A grid or GIS version is envisioned that will link numerous hillslopes to model the erosion and sediment transport processes in large basins. WEPP will model basins that can have a single storm basin-wide, and where upland, rather than channel, processes dominate sediment yields. Input Files The hillslope version contains four input files. A daily climate file includes the description of daily precipitation, temperatures, radiation and wind. A climate generator is available to generate typical weather sequences for non-mountainous terrain from a 60-mile grid of weather station records. The slope file contains two or more sets of points describing the slope at intervals along the profile.

The soil file can contain up to 10 layers of soil describing the texture and other physical properties of the soil. The most critical inputs are the erodibility and hydraulic conductivity of the surface layer. The management file contains descriptions of each of the plant communities and descriptions of each of the tillage systems employed. The file also contains the surface condition at the start of the simulation. A structure file links hillslope elements and channel elements together in the watershed version. File builders assist with the development of all of the files. Forest Applications In forests, the majority of sediment comes from forest roads, and in some conditions, from skid trails or burned areas. It is, however, necessary to model the entire watershed to estimate amounts of runoff contributing to stream flows to determine how much of the eroded sediment is transported down-stream. It will be necessary to use the watershed version for forest conditions to be able to determine the interaction between roads and undisturbed areas. A set of typical templates for forested areas and roads are available for the hillslope version. Forest Research A weather sequence generator is being developed for the mountains in the western USA which will generate a stochastic sequence of climate from any set of weather data, such as a coop station or a snotel station. Currently, the computer programs and support data-base necessary to run the mountain climate weather sequence generator are being developed. Erodibility and conductivity values for undisturbed and disturbed forests and forest roads have been measured, and from those results, methods to predict forest soil erodibility are now available. Current research is extending erodibility measurement to road aggregate materials, spatially distributed fire effects, and upland forest watershed processes. Typical watershed input files to describe road prisms and simple watersheds are being developed, along with improved interfaces for all applications. For more information contact W. Elliot, Project Leader, Moscow Forestry Sciences Lab. (208) 883 2338; email: welliot/rmrs_moscow@fs.fed.us