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 decemposition. The model uses a daily time step to calculate 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 plant communities along the hillslope.

The watershed version links hillslope elements of specified widths together with channel elements with given erodibility properties and impoundment elements.

A grid version will be developed that can allow the linkage of numerous hillslopes to model the erosion and sediment transport processes in large basins. WEPP will model basins that can have a single storm basinwide, and where upland, rather than channel, processes dominate sediment yields.

Input Files

The hillslope version requires 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 hill profile.

The soil file can contain up to 10 layers of soil describing the texture and other physical properties of the soil. The soil file also specifies the conductivity and the erodibility of the surface layer.

The management file contains descriptions of each of the plant communities and descriptions of each of the surface effects of management operations 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. 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.

Forest Research

Currently, the computer programs and database necessary to run a mountain climate weather sequence generator are being completed. Erodibility and conductivity values for undisturbed and disturbed harvest areas and forest roads have been measured, and from those results, methods to predict forest soil erodibility are being developed.

Typical watershed input files to describe road prisms within watersheds are being developed, along with an interface to simplify the file building process.

A forest growth model is being developed which will allow the prediction of plant regeneration following harvesting. The model will include competition between grass, shrubs, and trees for radiation and soil water.