The AGNPS model was used to assess the impact of management practices on the water quantity and quality from Owl Run, a 1153-ha watershed in the Piedmont Region of Virginia. Prior to this assessment, the model was calibrated using 2 years of hydrologic and water-quality data from the same watershed. It was concluded that the model is applicable to nonpoint source (NPS) impact assessment for watersheds similar to Owl Run. Better agreement was found between simulated and observed runoff volumes than between simulated and observed peak rates, sediment or nutrient yields. An annualization procedure, based on frequency analyses of storms and rainfall erosivity factors, and the joint probabilities of occurrence at different crop stages, were used to estimate annual average NPS loadings. The results were found to be close to average observed values for the watershed. The model was also used to simulate the effects of the application of seven different best management practice (BMP) scenarios on the watershed. The reduction rates in simulated pollutant loadings and the costs for BMP implementation were used to identify appropriate BMPs for the watershed. (C) 1997 Elsevier Science Ltd.
