AGNPS-based assessment of the impact of BMPs on nitrate-nitrogen discharging into an Illinois water supply lake

The Agricultural Non-Point-Source Pollution (AGNPS) model was used to simulate storm-generated rainfall-runoff and distribution of nitrate-nitrogen (nitrate-N) concentration in the Upper Sangamon River basin draining into Lake Decatur in Decatur, Illinois. This water supply reservoir for the city of Decatur receives water from the 2,400-square-kilometer Lake Decatur agricultural watershed and has high nitrate-N concentrations periodically exceeding the 10 milligram per liter drinking water standard of the U.S. and Illinois Environmental Protection Agencies (USEPA and IEPA). Using monitored and estimated data, the model was calibrated and validated to predict closely-observed runoff volumes and nitrate-N concentrations. Sensitive model parameters were identified. The calibrated model was used to evaluate the effects of different agricultural and watershed management practices (BMPs) for reducing nitrate-N discharge into the lake. Model parameters were identified to represent the BMPs. Model results showed variations of nitrate-N discharge into the lake with respect to different BMPs and BMP applications at different areas of the watershed. The model was found useful in the preliminary evaluations of the BMPs in the Lake Decatur watershed for reducing nitrate-N discharge into the lake. The BMP evaluation approach and the results may be useful to users of the AGNPS model and other non-point-source pollution models. Model limitations must be kept in mind in interpreting the results from this empirically-based model and in using them to solve practical problems.