Evaluating Potential Phosphorus Management Impacts in the Lake Eucha Basin Using SWAT

Lake Eucha is a nexus of water quality conflicts between agribusiness and environmentalists, urban and rural stakeholders, and municipalities and state governments. The decline in water quality in Lake Eucha, a drinking water supply reservoir for the city of Tulsa, has been attributed to both municipal wastewater treatment plant effluent and the application of poultry litter as a fertilizer within the basin. Litigation has forced policy changes in the basin with little knowledge of their eventual impact on water quality. The Soil and Water Assessment Tool (SWAT) model was used to evaluate the effect of various nutrient management scenarios, including those forced by the litigation, on nutrient loads to Lake Eucha. The model was calibrated for streamflow and total phosphorus using a complex shuffled evolution approach on a 75 GHz computer cluster. These predictions were used to identify management strategies predicted to meet established in-lake water quality standards. SWAT predicted that the litigation-imposed changes in the basin have reduced potential phosphorus load to Lake Eucha by 39%. The model also predicted that additional changes will be needed to meet the current water quality standard. This study illustrates how a watershed model can be used to assess potential management scenarios and identify strategies to meet a particular water quality standard. This information allows policy makers to make more informed water quality management decisions.