Planning and assessment in land- and water-resource management are evolving from simple, local-scale problems toward complex, spatially explicit regional ones. Such problems have to be addressed with distributed models that can compute runoff and erosion at different spatial and temporal scales. The extensive data requirements and the difficult task of building input parameter files, however, have long represented an obstacle to the timely and cost-effective use of such complex models by resource managers. In addition, to evaluate management practices and their impacts on water quality, land and resource managers need to describe and simulate the impacts of land use and best management practices (BMPs) on watershed response. The United States Department of Agriculture (USDA)-Agricultural Research Service (ARS) and the Southwest Watershed Research Center (SWRC), in cooperation with the U.S. Environmental Protection Agency (EPA)-Office of Research and Development (ORD), and the University of Arizona (UA), have developed the Automated Geospatial Watershed Assessment (AGWA) Geographic Information System (GIS) tool to facilitate the distributed hydrological modeling process. The quality assurance and quality control measures used in the development of AGWA are described in PB2003-103047 (EPA/600/R-02/046).