Combined Monitoring and Modeling Indicate the Most Eff ective Agricultural Best Management Practices Zachary

1798 Although water quality problems associated with agricultural nonpoint source (NPS) pollution have prompted the rapid and widespread adoption of a variety of so called “best management practices” (BMPs), there have been few realistic eff orts to assess their combined eff ectiveness in reducing NPS pollution. Th is study used the Variable Source Loading Function (VSLF) model, a distributed watershed model, to simulate phosphorus (P) loading from an upstate New York dairy farm before and after the implementation of a suite of BMPs. With minimal calibration, the model calculates the dissolved P (DP) losses from impervious surfaces (e.g., barnyards), the plant/soil complex, fi eld-applied manure, and loads associated with basefl ow conditions. Th e simulated DP loads agreed well with measured loads for both the pre-BMP and post-BMP periods. More importantly, results showed that BMPs reduced DP loads by 35%, which is over half of the expected reduction if all manure was removed from the watershed, i.e., ~50% reduction. Th e model results indicate that had no BMPs been installed DP loads would be ~37% greater than observed at the watershed outlet. Th e most eff ective BMPs were those that disassociated pollutant loading areas from areas prone to generating runoff , i.e., hydrologically sensitive areas. By contrast, attempts to reduce P content in manure were somewhat less eff ective. Th is study demonstrates that a combination of distributed, mechanistic modeling and long-term monitoring provides better insights into the eff ectiveness of water quality protection eff orts than either individually. Combined Monitoring and Modeling Indicate the Most Eff ective Agricultural Best Management Practices