Technical Abstract: The USDA Water Erosion Prediction Project (WEPP) watershed model is a distributed parameter, continuous model built as an extension of the WEPP hillslope model. The model was developed to predict erosion effects from agricultural management practices within small watersheds. The watershed model contains two primary components: channel and impoundment. The channel component can be further divided into the hydrology and erosion components. Channel infiltration is calculated by a Green-Ampt equation. A continuous water balance is maintained on the channels that includes calculation of evapotranspiration, percolation, canopy rainfall interception, and surface depressional storage. The peak runoff rate at the channel outlet is calculated using either the Rational equation or the equation used in the CREAMS model. Flow depth and hydraulic shear stress along the channel are computed by regression equations based on a numerical solution of the steady state spatially varied flow equation. Detachment, transport, and deposition within concentrated flow channels are calculated by a steady state solution to the sediment continuity equation. The impoundment component routes runoff and sediment through several types of impoundments including farm ponds, culverts, filter fences and check dams. The model extends the capability of the hillslope model to provide erosion prediction technology for agricultural watersheds based on fundamentals of erosion theory, soil and plant science, channel flow hydraulics, and rainfall-runoff relationships. This paper provides an overview of the watershed model conceptual framework and structure. In addition, mathematical representations of the processes simulated by the channel hydrology and erosion components are presented.