The Water Erosion Prediction Project (WEPP) watershed scale model is a continuous simulation tool thatextends the capability of the WEPP hillslope model to provide erosion prediction technology for small cropland andrangeland watersheds. The model is based on fundamentals of erosion theory, soil and plant science, channel flowhydraulics, and rainfall-runoff relationships, and contains hillslopes, channels, and impoundments as the primarycomponents. The hillslope and channel components can be further divided into hydrology and erosion components.Channel infiltration is calculated by a Green-Ampt Mein-Larson infiltration equation. A continuous channel waterbalance is maintained, including calculation of evapotranspiration, soil water percolation, canopy rainfall interception,and surface depressional storage. The channel peak runoff rate is calculated using either a modified Rational equation orthe equation used in the CREAMS model. Flow depth and hydraulic shear stress along the channel are computed byregression equations based on a numerical solution of the steady state spatially varied flow equations. Detachment,transport, and deposition within constructed channels or concentrated flow gullies are calculated by a steady statesolution to the sediment continuity equation. The impoundment component routes runoff and sediment through severaltypes of impoundment structures, including farm ponds, culverts, filter fences, and check dams. The purpose of this articleis to provide an overview of the model conceptual framework and structure. In addition, detailed mathematicalrepresentations of the processes simulated by the channel hydrology and erosion components are presented. The processessimulated by the impoundment component are not described in this article, but it does include impoundment effects onwatershed model channel peak discharge and time of concentration calculations.