Modeling Long-Term Water Quality Impact of Structural BMPS

Structural best management practices (BMPs) that reduce soil erosion and nutrient losses have been recommendedand installed on agricultural land for years. A structural BMP is expected to be fully functional only for a limited period afterinstallation, after which degradation of the BMP is likely to lead to a reduction in the water quality improvement providedby the BMP. Assessing the impact of BMPs on water quality is of widespread interest, but no standard methods exist todetermine the water quality impact of structural BMPs, particularly as the impact changes through time. The objective of thisstudy was to determine the long-term (~20 year) impact of structural BMPs in two subwatersheds of Black Creek on sedimentand phosphorus loads using the Soil and Water Assessment Tool (SWAT) model. The BMPs were represented by modifyingSWAT parameters to reflect the impact the practice has on the processes simulated within SWAT, both when practices are fullyfunctional and as their condition deteriorates. The current condition of the BMPs was determined using field evaluationresults from a previously developed BMP condition evaluation tool. Based on simulations in the two subwatersheds, BMPsin good condition reduced the average annual sediment yield by 16% to 32% and the average annual phosphorus yield by10% to 24%. BMPs in their current condition reduced sediment yield by only 7% to 10% and phosphorus yield by 7% to 17%.