Water Erosion Prediction Project (WEPP)
The erosion model of the Water Erosion Prediction Project (WEPP) is a continuous simulation computer program which predicts soil loss and sediment deposition from overland flow on hillslopes, soil loss and sediment deposition from concentrated flow in small channels, and sediment deposition in impoundments. In addition to the erosion components, it also includes a climate component which uses a stochastic weather generator to provide daily weather information, a hydrology component which is based on a modified Green-Ampt infiltration equation and solutions of the kinematic wave equations, a daily water balance component, a plant growth and residue decomposition component, and an irrigation component.
The WEPP model computes spatial and temporal distributions of soil loss and deposition, and provides explicit estimates of when and where in a watershed or on a hillslope that erosion is occurring so that conservation measures can be selected to most effectively control soil loss and sediment yield.
The WEPP model includes a number of conceptual components which are used to predict and calculate these estimates of soil detachment and deposition: (1) climate (rainfall, temperature, solar radiation, wind), (2) winter conditions (freeze-thaw cycles, snow accumulation and snow melt), (3) irrigation (stationary, sprinkler, furrow), (4) hydrological variables (infiltration, depressional storage, runoff), (5) water balance (evapotranspiration, percolation, drainage), 6) soil types and properties, (7) plant growth (cropland, rangeland, forests), (8) residue management and decomposition, (9) tillage impacts on infiltration and erodibility, (10) erosion (interrill, rill, channel), (11) deposition (rills, channels and impoundments), and (12) sediment delivery, particle sorting and enrichment.
Given the variety and detail of data needed for the model to operate reliably, WEPP can be used only at field level, with the maximum size of a ‘field’ 256 hectare. The actual size limit at which WEPP can be applied depends on the degree of complexity within a field. The procedure can be applied if the field contains rills and ephemeral gullies that can be farmed over, but not in agricultural fields or watersheds having incised, permanent channels such as classical gullies and stream channels. The procedure can be applied to constructed waterways like terrace channels and grassed waterways. In rangeland and forest applications, "fields" can include gullies about 1 to 2 m in width by about 1 m deep, but the procedure is not expected to apply to headcut erosion, sloughing of gully sidewalls, or the effects of seepage on erosion in concentrated flow channels.
The current (2012) version of WEPP has a Windows interface and contains the CLIGEN stochastic weather generator. It can be downloaded from https://www.ars.usda.gov/midwest-area/west-lafayette-in/national-soil-erosion-research/docs/wepp/wepp-downloads/.
Given the very detailed equations built into WEPP and the excellent documentation, WEPP is a research tool and of particular interest to scientists who want to understand the physics and interactions of the components underpinning erosion modeling.