FREQUENCY OF CONCENTRATED FLOW THROUGH AGRICULTURAL RIPARIAN BUFFERS IN THE VIRGINIA COASTAL PLAIN AND PIEDMONT

Forested riparian buffers are intended to reduce sediment and nutrient pollution in runoff from agricultural fields. Numerous studies suggest forested buffers can be an effective practice when runoff is widely disseminated in buffers, achieving substantial reduction in sediment, particle-bound phosphorus, and nitrogen loads. However, effectiveness is reduced considerably when field topography concentrates runoff in topographic lows, which can lead to channelization, allowing rapid water movement through the buffer. Here we investigate how field runoff moves through points of concentrated flow in an effort to determine the frequency of buffer bypassing in the Virginia Coastal Plain and Piedmont (parts of the Chesapeake Bay watershed). We used ~10 m resolution USGS DEMs in ArcGIS to create flow accumulation rasters for >30 active agricultural fields in the Coastal Plain and Piedmont regions of Virginia. We extract total flow accumulation at the field margin from the flow accumulation raster, and determine the drainage area and the fraction of the total field area drained for each cell along the margin. Drainage area, and presumably field runoff, is concentrated along the field margin at certain discrete points on every field we have measured. On the studied fields, the margin cell with the greatest drainage area captures from ~10% to ~50% of the total area of the field, and the top five cells capture ~40% to ~90%. A field survey of one of the studied sites revealed evidence for concentrated surface runoff and channelization within the buffer at seventeen of the top twenty-six drainage points along the margin identified in the GIS analysis. This suggests the GIS analysis is a valid predictive tool for identifying points of concentrated flow, although more field surveys are needed to confirm. Plots of field margin points ranked by drainage area show a logarithmic pattern for all studied fields. We suggest this reflects patterns inherent in surface topography; therefore, widely disseminated runoff necessary for buffer effectiveness may be rare within the Chesapeake Bay watershed.