Comparison of indexes for prioritizing placement of water quality buffers in agricultural watersheds

Five physically based, spatially distributed, empirical indexes were compared for the degree to which they identified the same or different locations in watersheds where vegetative buffers would function better for reducing agricultural nonpoint source pollution. All five indexes were calculated on a 10m×10m digital elevation grid on agricultural land in the 144-km² Neshanic River watershed in New Jersey. The indexes included the topography-based Wetness Index (WI) and Topographic Index (TI) and three soil survey-based indexes (sediment trapping efficiency [STE], water trapping efficiency [WTE], and groundwater interaction [GI]). Results showed that each index associated higher pollution risk and mitigation potential to a different part of the landscape. The WI and TI identified swales and riparian areas where runoff converges, whereas STE and WTE identified upland sites. The STE and WTE lack the fine scale of slope resolution and the accounting for convergent runoff patterns that can be important for properly locating buffers in some watersheds. The GI index indicates the existence of a shallow water table but the correspondence with WI- and TI-identified sites was only modest. For watersheds where pollutant loading is generated by both saturation-excess (emphasized by TI and WI) and infiltration-excess processes (emphasized by STE and WTE), the indexes could be complementary. However, techniques would be needed for properly apportioning priority among sites identified by each index.