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

Authors: CHRISTOPHER, SHEILA - University Of Notre Dame; TANK, JENNIFER - University Of Notre Dame; MAHL, URSULA - University Of Notre Dame; YEN, HAW - Texas Agrilife Research; Arnold, Jeffrey; TRENTMAN, MATT - University Of Notre Dame; SOWA, SCOTT - Nature Conservancy; HERBERT, MATTHEW - Nature Conservancy; ROSS, JARED - Nature Conservancy; White, Michael; ROYER, TODD - Indiana University

Submitted to: Ecological Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/29/2017
Publication Date: N/A
Citation: N/A

Interpretive Summary: Some of the lost productive farmland in U.S. was once wetlands and swamps until the land was drained with subsurface tiles and drainage ditches. However, these drainage practices provide a direct delivery system for excess nitrogen and phosphorus to water bodies. The Western Lake Erie Basin was once the Great Black Swamp and now relies heavily on drainage management, which has been blamed for harmful algal blooms in the lake. One management option currently being tested to reduce nutrient loads into Lake Erie is the 2-stage ditch. A 2-stage ditch consists of a small ditch inset to a wide second stage that traps nutrients. In this study, we developed a model for nutrient loss in a 2-stage ditch and incorporated it into the Soil and Water Assessment Tool (SWAT). The model showed that nitrogen loads could be reduced by 10% and phosphorus loads by 30% if all ditches in the watershed had 2-stage ditches installed. This provides policymakers with a management option that could potentially provide significant nutrient reductions.

Technical Abstract: 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.