ABSTRACT. Structural best management practices (BMPsest management practices (BMPs) are routinely used to reduce nonpoint-source pollution resulting from agricultural activities and improve water quality. Studies have demonstrated how structural management practices can improve water quality, but the duration of their effectiveness and performance is largely unknown. Although a "design life" has been established by the USDA Natural Resources Conservation Service for most structural BMPs (e.g., USDA-NRCS, 2004), the ability of a BMP to function effectively throughout its assigned design life is uncertain, and its continued effectiveness past the design life is unknown. Quantifying the impact some 25 years after BMPs are installed would provide insight into water quality improvement taking place over time due to BMP implementation.Watershed modeling is one approach to analyzing the water quality impact, both short and long term, of BMP implementation. Watershed models have been used for decades to study nonpoint-source pollution and the impact of non-structural BMPs, but a limited number of studies have

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Federal conservation agencies are recognizing the need to account for the millions of dollars spent nationwide on conservation programs focused on implementing best management practices (BMPs), some of which have been in existence for decades. A cost-benefit analysis for many of these programs is difficult due to the limited water quality and cost data available, and because attempts to quantify the water quality benefits obtained from BMP implementation is problematic. A cost-benefit analysis was performed on a large watershed management project that installed hundreds of BMPs in the mid-1970s, the Black Creek Project. Water quality improvement for sediment and total phosphorus reduction due to BMP implementation was estimated in 2000 dollars using off-site benefit estimates, fertilizer nutrient costs and water quality trading values. The benefits received from the BMPs did not outweigh the costs for implementing and maintaining the BMPs. Benefits not captured in this economic analysis include lessons learned and used outside the watershed by the conservation community, gully erosion, erosion deposited within the watershed, nitrogen reduction, wildlife habitat improvement, human and aquatic ecosystem health, aesthetics, downstream impacts, intangible impacts and the needs of future generations. This study shows that the tools needed to compute an accurate comparison of benefits and costs concerning water quality are lacking. Economic analysis of conservation planning should continue, but should not be the sole determining factor when deciding if a conservation project is worthwhile.

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