Technical Abstract: One of the factors driving change in grassland ecosystems is concern about environmental quality. The Conservation Effects Assessment Project (CEAP), led by USDA NRCS and ARS, is designed to quantify environmental effects of conservation programs in national benchmark watersheds, one of which is the Fort Cobb Reservoir Watershed (FCRW) in Caddo County, OK. One conservation practice under investigation in the FCRW by USDA-ARS Grazinglands Research Laboratory scientists is the conversion of cropland to perennial grasses such as Bermuda grass. Because it is not logistically and economically feasible to carry out field experiments alone to determine the benefits of conservation programs on the environment at the watershed scale, a modeling approach is also being used for the CEAP. The Soil and Water Assessment Tool (SWAT) model was used in this study to quantify the impact of converting cropland into perennial grasses on top soil loss and surface water resources. Because conversion of cropland to perennial grasses decreases exposed soil surfaces and significantly reduces the amount of nutrients applied, it is anticipated that model results will show a reduction in sediment and associated contaminant transport in surface water and a reduction of nutrients in ground water, which will lead to improved water quality. Therefore, the goal of this study will be to quantify this anticipated water quality improvement resulting from the conversion of cropland to perennial grasses. The outcome will be useful in the selection of land areas and agricultural enterprises that may provide the most benefit resulting from farm cropland to grassland conversion.