Concentrated runoff flow : implications for buffer design and water quality benefits
Non-uniform or concentrated flow of surface runoff from agricultural fields can reduce the pollutant trapping effectiveness of buffer strips having constant width along a riparian zone or field margin. Effectiveness and cost-effectiveness might be improved by reconfiguring the buffer to be larger where more runoff flows and smaller where runoff is less. A GIS tool has been developed that accounts for non-uniform patterns of runoff flow which can be used for assessing performance of buffers and for designing them. This tool was used to assess the effect of nonuniform runoff on sediment trapping efficiency of constant-width buffer designs and to compare performance of constant-width and variable-size configurations. The tool is an ArcGIS extension based on the design model of Dosskey et al. (2011). It employs a digital elevation model (DEM) to divide the riparian area or field margin into many segments, determine contributing area and slope to each one and, then, design for a buffer area ratio that provides a specified level of trapping efficiency. The assessment procedure employs these same algorithms, but in a different order; first, determining the existing buffer area ratio and, then, calculating it's trapping efficiency. Results using this tool on a sample of fields in the Midwestern U.S. suggest that variable-size designs can be more than twice as effective per unit buffer area as conventional constant-width designs. Producing cost-effective designs and accurate performance assessments of buffers requires accounting for detailed spatial patterns of runoff flow from agricultural fields.