Watershed Loading Functions for Nonpoint Sources

A modeling procedure has been developed to estimate nonpoint source pollution from agricultural runoff in large watersheds. The procedure is based on simple equations for runoff and soil loss prediction. It is designed for use in situations where resources are not available for extensive water quality sampling or more sophisticated modeling efforts. The simple models or loading functions were tested by comparing predicted and observed nitrogen and phosphorus mass fluxes in streamflow from a 391-km² Pennsylvania watershed for a 16-month period. The comparisons indicated that loading functions provided reasonable estimates of dissolved nitrogen, solid-phase nitrogen and total phosphorus mass fluxes in runoff from the watershed. Rather, these simple methods are most suitable as screening devices to: (1)Identify the approximate magnitudes of nonpoint source loading of pollutants in agricultural runoff; and (2)evaluate the likely changes in loadings associated with alternative agricultural management practices.