It can predict the effect of management decisions on water, sediment, and pesticide yield with reasonable accuracy for ungaged rural basins throughout the United States. Pesticide, Soil and Rainfall Coverage is for the entire U.S., all other site-specific parameters are user supplied. Processes considered include surface runoff, return flow, percolation, evapotranspiration, transmission losses, pond and reservoir storage, sedimentation, and crop growth. A weather generator allows precipitation, temperature, and solar radiation to be simulated when measured data is unavailable. The precipitation model is a first-order Markov chain model, while air temperature and solar radiation are generated from the normal distribution.
Return flow is calculated as a function of soil water content and return flow time.
Return flow travel times can be calculated from soil hydraulic properties or user-inputs. The percolation component uses a storage routing model combined with a crack-flow model to predict flow through the root zone. Evapotranspiration is estimated using Ritchie's ET model. Transmission losses in the stream channel are calculated as a function of channel dimensions, flow duration, and effective hydraulic conductivity of the channel bed. Pond storage is based on a water balance equation that accounts for inflow, outflow, evaporation, and seepage.