Using SWAT for sub-field identification of phosphorus critical source areas in a saturation excess runoff region

The identification of phosphorus critical source areas (CSAs) is an important step in reducing nonpoint-source phosphorus contributions to lakes and streams, providing the basis for targeted mitigation and efficient utilization of limited resources. A high-resolution SWAT model, consisting of sub-field level hydrologic response units, was applied to Lake Champlain's Missisquoi Bay Watershed, located along the United States-Canadian border. Because this region of the northeastern United States is dominated by saturation excess runoff, a novel approach for adjusting SCS (Soil Conservation Service) curve numbers based on the local compound topographic index was developed. After calibration and validation, the SWAT model was run for a 30-year period to identify the phosphorus CSAs throughout the watershed, determining that 20% of the watershed produces 74% of the total phosphorus load. The resulting sub-field level CSA map is currently being hosted online, providing a valuable tool for natural resources planners, land owners, and scientists. [GRAPHICS] Editor Z.W. Kundzewicz; Guest editor V. Krysanova