Modeling Phosphorus in the Appoquinimink Watershed with AGNPS

Portions of the Appoquinimink River in New Castle County, Delaware have been declared in danger of eutrophication by the Delaware Department of Natural Resources and Environmental Control due to high phosphorus levels. The Appoquinimink watershed is approximately 12,400 hectares and is comprised primarily of agricultural lands interspersed with woodlands and rural communities and development. Previous research efforts resulted in the successful implementation of many Best Management Practices (BMPs); however, reduction of soluble P loads into surface waters remains a problem. The AGNPS model was used to model additional BMPs specifically targeting soluble P load reductions. BMPs included land use changes, adjustments of fertilization levels and application methods. Following a sensitivity analysis and calibration of the model, BMPs were tested using both growing and dormant season data sets for 1 year, 2 year and 10 year storm events. The most successful BMP was reduction of fertilization levels. Other BMPs were not found to have as great of significant effects.