This report describes an initial attempt to combine the detailed simulation of agricultural runoff and soil processes, which calculates surface and subsurface pollutant transport to receiving waters, with subsequent simulation of instream transport and transformations. The result is a comprehensive simulation of watershed hydrology and water quality. Comparison of water quality conditions under conventional agronomic practices and Best Management Practices (BMPs) provides the basis for determining the net effects and associated benefits of BMP implementation. Using simulated concentrations of pesticides and other toxic pollutants in conjunction with lethality-duration information, the frequency of acute and chronic toxic conditions can be determined to assess the aquatic impact of proposed practices.