Variable Width Filter Strips Designed with Terrain Analysis.

Filter strips are a widely-used practice for reducing the load of pollutants that leave agricultural fields in overland runoff. They are typically designed to intercept uniformly-distributed runoff with a constant width strip along a field margin. Non-uniform runoff flow, however, can reduce the effectiveness of a constant-width filter strip. Non-uniform flow is created by topographic undulations and swales in fields that concentrate runoff into certain locations and away from others. In these situations, filter strip effectiveness can be improved by placing more filter area where the runoff load is greater and less where the load is smaller. To do this, quantitative relationships were developed between the pollutant trapping efficiency and the ratio of filter strip area to upslope contributing area, i.e., buffer area ratio, using the Vegetative Filter Strip Model (VFSMOD. These relationships enable sizing each portion of a filter strip according to the size of the runoff load to that location. Terrain analysis and GIS are well suited for automating the variable-width design approach. The contributing area attribute can be easily calculated for each portion of the field margin and, when coupled with additional information on soil and slope, an appropriate filter area can be determined for that portion. This process is repeated for each segment along the entire field margin and produces a variable-width filter strip that matches the non-uniform pattern of runoff load to that margin. A map is created that shows the field and the associated variable-width design which facilitates field layout and the calculation of the total area of filter strip to be installed.