Watershed-scale modeling of the water quality effects of cropland conversion to short-rotation woody crops

The conversion of cropland to the production of woody biomass, or short-rotation woody crops (SRWCs), has the potential to provide an economic alternative to Midwestern farmers, while simultaneously offering an environmental dividend in the form of reduced erosion and nutrient pollution of streams. However, notwithstanding a wealth of plot-scale and anecdotal data suggestive of these benefits, there are few watershed-scale integrated analyses on which to base regional policy decisions regarding incentives to convert fields to SRWCs. This study applied a field-scale runoff, sediment and nutrient transport model (Agricultural Drainage and Pesticide Transport, ADAPT) to a simulation of 10, 20 and 30% cropland conversion to SRWCs, grown on a 5-year rotation, in a representative Minnesota River sub-watershed. While the generation of a highly precise simulation would require extensive calibration of the model, its application with parameters previously calibrated to neighboring, similar watersheds provided reasonably robust results that indicated real differences resulting from cropland conversion. At the highest conversion level, mean annual runoff was reduced by up to 9%, sediment loads by 28% and nitrogen (N) loads by 15%, although total phosphorus (P) loads increased by 2% relative to the no-SRWC scenario. However, the relative benefits of conversion at the field level were contingent on soil type, drainage status and the alternative crop. These differences provide useful insights with respect to the targeting of possible conversion incentives.