Precision agriculture techniques can be applied at field margins to improve performance of water quality protection practices. Effectiveness of vegetative buffers, conventionally designed to have uniform width along field margins, is limited by spatially non-uniform runoff from fields. Effectiveness can be improved by placing relatively wider buffer at locations where loads are greater. A GIS tool was developed that accounts for non-uniform flow and produces more-effective, variable-width, designs.

 

The design model was developed by simulation modeling using the Vegetative Filter Strip Model (VFSMOD-W) to produce relationships between pollutant trapping efficiency and buffer area ratio. These relationships were summarized by a family of equations which divide the full range of possible relationships into fairly even increments. To apply the model, one equation is selected that best describes a given field situation based on slope, soil texture, field cover management, and pollutant type. This equation is used to determine the buffer area ratio that would produce a desired level of trapping efficiency and it would be applied to the contributing area to each segment of field margin. This equation also can be used in reverse to estimate the performance of existing or hypothetical buffers.

 

The design model was, then, adapted for use in a GIS. The GIS tool employs an aerial orthophoto to define the field margin, a DEM grid to segment the field margin and determine contributing areas and slopes to each one, and a digital soil survey (SSURGO) for corresponding soil texture. The photo is used again to map the resulting buffer on the ground.

 

Results using the design model, both manually and the automated GIS tool, were compared to measured performance of filter strips in central Iowa. Measured performance was in close agreement with estimates provided by both the manual model and its GIS adaptation. Through terrain analysis, the GIS tool produces designs that can substantially improve the water quality performance of vegetative buffers.