An evaluation of structural best management practices 20 years after installation.

摘要

Evaluating the impact of best management practices (BMPs) is a necessary component to ensure that BMPs are performing as intended. Studying the condition, effectiveness and water quality impact of BMPs is in its infancy. A sample of grassed waterways, grade stabilization structures, field borders, and parallel terraces, implemented during the Black Creek Project (one of the first large watershed demonstration projects to study the relationship between agricultural activities and the use of BMPs to improve water quality), were evaluated via physical inspection, watershed hydrology computer modeling and economic analysis.; Evaluation techniques that estimate the current condition of a BMP based on visual inspection and comparisons to selected original design dimensions were developed. The evaluation tools were tested for validity using an expert panel approach and for reliability based on inter-rater correlation, and applied to a subset of structural BMPs implemented during the Black Creek Project. One-third of the BMPs no longer remain, while those that remain are in fair condition and are partially functional.; The BMPs were represented in a watershed hydrology model to predict their long-term impact for reducing non-gully erosion and phosphorus losses. BMPs in varying conditions---good, fair or poor---were represented in the Soil and Water Assessment Tool (SWAT) by modifying parameters that reflect the impact the BMP has on the hydrologic processes occurring in SWAT. BMPs represented in good condition reduced the sediment and total P yields from 1--31% more than BMPs in varying condition. The simulations incorporated land use change from 1975 to 2003, during which pasture increased and row crops decreased over time. Applying 2003 land use resulted in a 35--59% sediment reduction and a 25--29% total P reduction when no BMPs were implemented compared to the 1975 land use.; An economic analysis was performed and identified some of the limitations encountered when performing a benefit-cost analysis for a conservation project. Soil erosion and phosphorus reductions predicted from SWAT were assigned a monetary value 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. A larger benefit-cost ratio was computed when accounting for cropping system changes, but was still less than one. A myriad of benefits pertaining to water quality, i.e. gully erosion, nitrogen reduction, wildlife habitat improvement, human and aquatic ecosystem health, downstream impacts, and intangible impacts, were not captured with the economic analysis because of an inability to assign a market value to a non-market good, water quality.