Modeling nutrient removal using watershed-scale implementation of the two-stage ditch

Western Lake Erie Basin (WLEB) is the most intensively farmed region of the Great Lakes. Because of the flat topography and poorly-drained soils many farmers rely on drainage management practices (e.g., subsurface tile drainage, ditch channelization) to maintain productive agriculture. However, these practices also facilitate the delivery of excess nutrients and sediments to Lake Erie, which have been linked to recurring harmful algal blooms (HABs) and associated environmental degradation. Implementation of inset floodplains in formerly channelized waterways via the two-stage ditch can improve water quality but the efficacy has been tested using only implementation in short reaches. Watershed models are critical tools for assessing watershed-scale implementation and as such can guide effective management. We evaluated the effectiveness of the two-stage ditch in improving water quality in the River Raisin Watershed (RRW), a major subbasin in the WLEB, combining empirical measurements for nutrient reductions from two-stage ditches across the Midwest with output from a Soil Water Assessment Tool (SWAT) model. We modeled two-stage implementation in 25, 50, and 100% of headwater reaches in the RRW, and found that the practice could reduce total annual NO3−-N export by 2, 5 and 10%, respectively. The two-stage was even more effective at reducing total phosphorus (TP) export, which was reduced by 12, 20 and 31%, respectively. Compared to other conservation practices, nutrient reduction efficiency for the two-stage ditch was good, both in terms of percent load reduction and cost, but watershed-scale adoption will be required in order to achieve significant nutrient reductions as called for by policymakers.