An approach for using soil surveys to guide the placement of water quality buffers

Vegetative buffers may function better for filtering agricultural runoff in some locations than in others because of intrinsic characteristics of the land on which they are placed. The objective of this study was to develop a method based on soil survey attributes that can be used to compare soil map units for how effectively a buffer installed in them could remove pollutants from crop field runoff. Three separate models were developed. The surface runoff models for sediment and for dissolved pollutants were quantitative, based mainly on slope, soil, and rainfall factors of the Revised Universal Soil Loss Equation (RUSLE), and were calibrated using the Vegetative Filter Strip Model (VFSMOD) for a standard buffer design and field management. The groundwater model categorized map units by the presence or absence of suitably-shallow groundwater and hydric conditions for interaction with the root zone of a buffer. The models were applied to a ~65 km2 (~25 mi2) agricultural watershed in northwestern Missouri. Data acquisition, calculations, and map production utilized the Soil Survey Geographic Database (SSURGO). For surface runoff, soil survey-based values correlated strongly with corresponding VFSMOD estimates for sediment (R2 = 0.94) and dissolved pollutant trapping efficiency (R2 = 0.83) for a wide range of soil, slope, and rainfall conditions. A strong negative correlation between trapping efficiency and field runoff load was indicated. Mapped results revealed large differences in buffer capability for surface runoff across the test watershed (21 to 99 percent for sediment and seven to 47 percent for dissolved pollutants). Trapping efficiency for dissolved pollutants was much smaller than for sediment in every map unit. Lower values of trapping efficiency were associated with map units where runoff loads are higher and where a buffer will trap greater loads of sediment, but smaller loads of dissolved pollutants, than in units with higher values. Comparative rankings can be adjusted somewhat for site conditions that depart from the reference conditions, and recalibration may be desired to better account for them. For groundwater, the confluence of hydric conditions and shallow water table occurred only in the highest reaches of the test watershed, but a buffer can also interact with groundwater in most upland and riparian locations due to the prevalence of a seasonally shallow water table. By this approach, soil surveys may be used as a screening tool to guide planners to locations where buffers are likely to have a greater impact on water quality and away from those where impact is likely to be small.