Perennial vegetation impacts on stream discharge and channel sources of sediment in the Minnesota River Basin

Abstract

While sediment loads in streams draining agricultural landscapes have traditionally been attributed to field sources via overland flow and surface erosion, recent research has identified important nonfield sources of sediment in the Minnesota River Basin. We evaluate the impacts of altered vegetation on water budget and sediment loading in Seven Mile Creek, an agricultural watershed with important nonfield sources of sediment. Using the Soil and Water Assessment Tool (SWAT) and monitoring data for different locations, we developed an approach to account for different sediment sources and show that nonfield sediment export is strongly affected by changes in stream discharge, which is affected by vegetation impacts on water yield. This is important because many watershed-scale models are calibrated to sediment loads at the watershed outlet based on the assumption that all sediment is derived from field sources. This can lead to overestimation of erosion from field sources and unrealistic expectations for watershed-scale reductions based on some conservation practices. In Seven Mile Creek watershed under current conditions, approximately 76% of the sediment delivered to the watershed outlet originates from nonfield sources (streambanks and ravines). Relative to the current landscape, increases in perennial grasses or switchgrass (Panicum virgatum L.) cause decreases in water yield owing to differences in evapotranspiration (especially in the spring). As streamflow decreases, sediment export is disproportionately reduced from nonfield sources. These results demonstrating an interaction between evapotranspiration and streambank/ravine erosion are important for showing that nonfield sources of sediment can be important in some watersheds, and that realistic approaches for improving water quality may need to include strategies for managing water quantity.