Nonpoint source pollution (NPSP) is the export to receiving waters of nutrients originating from diffuse sources. This research documents a methodology for confirming reductions in NPSP resulting from implementation of agricultural Best Management Practices (BMPs). It employs that methodology to confirm the success of BMPs implemented in Graywood Gully, a study sub-watershed that drains into Conesus Lake, NY. Evaluating the effects of BMPs in agricultural watersheds is often complicated by significant temporal variability in weather and hydrologic conditions. In many cases NPSP demonstrates much greater variability in response to antecedent hydrologic/ meteorologic conditions than to commonly implemented BMPs. In essence, weather variability can mask the beneficial effects of the BMPs. By using the Thornthwaite-Mather procedure to model soil moisture status in addition to event rainfall total, it is possible to remove the major sources of weather/hydrologic-related variability, essentially reducing the number of experimental variables to the BMP itself. Application of this method to the Graywood sub-watershed reveals that BMPs can greatly reduce export of NPSP generated pollutants to receiving waters. Estimates of NPSP reductions range from 53% for soluble reactive phosphorus to 89% for nitrate.