A Comparison of Alternative Methods for Prioritizing Buffer Placement in Agricultural Watersheds for Water Quality Improvement

Two methods have been developed for identifying locations in watersheds where vegetative buffers would yield greater benefit for reducing agricultural nonpoint source pollution. The methods were compared and contrasted to evaluate the degree to which they identified the same or different locations within a watershed. The two methods included the terrain analysis-based topographic index, TI, (Qiu, 2009) and three soil survey-based indexes, STE, WTE, and GI (Dosskey et al. 2006). Both methods were applied to agricultural land in the 144 km Neshanic River watershed in New Jersey. Results showed that each index associated higher pollution risk and mitigation potential to a different part of the landscape: TI to sites having larger contributing area and lower slope with coarser shallower soil; WTE to sites having lower slope with coarser soil; and STE to sites having steeper slope with finer-textured soil. TI typically identifies riparian zones and swales that are prone to saturate and generate overland runoff. The STE and WTE identify sites where conditions are better for a buffer to trap pollutants in infiltration-excess runoff from up-gradient fields. Both TI and GI ostensibly identify sites where a shallow water table is more likely to occur, but correspondence between these two indexes was only modest. There is strong potential for the complementary use of these indexes. First, terrain analysis could be used to improve STE and WTE indexes by providing site-specific slope information and by enabling adjustment for spatial variation in runoff contributing area. Second, each index identifies sites that have high pollution mitigation potential, but for different reasons. Vegetative buffers could be apportioned among the different index-identified sites in a way that treats those most promising locations to mitigate pollutant loading to the Neshanic River.