A design aid for sizing filter strips using buffer area ratio
Nonuniform field runoff can reduce the effectiveness of filter strips that are a uniform size along a field margin. Effectiveness can be improved by placing more filter strip where the runoff load is greater and less where the load is smaller. A modeling analysis was conducted of the relationship between pollutant trapping efficiency and the ratio of filter strip area to upslope contributing area, i.e., buffer area ratio. The results were used to produce an aid for designing filter strips having consistent effectiveness along field margins where runoff load is nonuniform. Simulations using the process-based Vegetative Filter Strip Model show that sediment and water trapping efficiencies of a filter strip increase nonlinearly as the buffer area ratio gets larger. Site characteristics, including slope, soil texture, and upslope soil cover management practices, help to define this relationship more accurately. Using the Vegetative Filter Strip Model simulation results, a graphical design aid was developed for estimating the buffer area ratio required to achieve specific trapping efficiencies for different pollutants under a broad range of agricultural site conditions. A single graph was produced showing simulation results for seven scenarios as a family of lines that divide the full range of possible relationships between trapping efficiency and buffer area ratio and into fairly even increments. Simple rules guide the selection of one line that best describes a given field situation by considering slope, soil texture, and field cover management practices. Relationships for sediment-bound and dissolved pollutants are interpreted from the Vegetative Filter Strip Model results for sediment and water. The design aid is easy to use, accounts for several major variables that determine filter strip performance, and is based on a validated, process-based, mathematical model. The use of this design aid will enable a more precise fit between filter size and runoff load where runoff from agricultural fields is nonuniform.