Modeling the effects of riparian buffer zone and contour strips on stream water quality

The use of contour and riparian buffer strips planted with perennial vegetation has been found to improve surface water quality by reducing NO3-N and sediment outflow from cropland to a river. Modeling such a system to compare alternative layout and different strip sizes often faces challenges in flow routing scheme. The hillslope scheme in the Soil and Water Assessment Tool (SWAT) offers the flexibility of allowing the flow from a crop area to be routed through a buffer and/or contour strip, in which a thin sheet flow represents more closely the natural condition of a watershed. In this study, the SWAT model was applied to the Walnut Creek watershed and the hillslope option was used to examine the effectiveness of contour and riparian buffer strips in reducing NO3-N outflows from crop fields to the river. Numerical experiments were conducted to identify potential subbasins in the watershed that have high water quality impact, and to examine the effects of strip size and location on NO3-N reduction in the subbasins under various meteorological conditions (dry, average and wet). Variable sizes of contour and riparian buffer strips (10%, 20%, 30% and 50%, respectively, of a subbasin area) planted with perennial switchgrass were used to simulate the effects of strip size on stream water quality. Simulation results showed that a filter strip having 10–50% of the subbasin area could lead to 55–90% NO3-N reduction in the subbasin during an average rainfall year. Strips occupying 10–20% of the subbasin area were found to be more efficient in reducing NO3-N when placed along the contour than that when placed along the river. Varying the area and location of the contour and buffer strip affects NO3-N outflow and crop yields as well since it takes the land out of production. The size of the filter strip has economic implications in deciding how much land area to dedicate to prevent NO3-N loss to a desired limit or vice versa. The results of this study can assist in cost-benefit analysis and decision-making in best management practices for environmental protection.