Watershed Scale Nitrate-N Abatement of Instream Wetlands: An Appraisal Using the Soil and Water Assessment Tool

Abstract

HighlightsSWAT used to address watershed scale nitrate-N abatement of instream wetlands (ISWs).Experimental ISW results were incorporated into the watershed modeling framework.SWAT successfully captured and reproduced ISW impact on nitrate-N at sub-basin level.Scenarios of ISWs implementation were simulated, effects on nitrate-N export were evaluated.Results show ISWs can be used as conservation structures aimed at enhancing water quality.Abstract. In watersheds under high agricultural production, nitrate nitrogen (nitrate-N) pollution often originates from intensive application of fertilizers and animal manure to croplands. Surface runoff and nitrate-N export from farmlands contributes to the pollution of nearby reaches which flow into the watershed stream network. Experimental studies reported significant nitrate removal capacities of constructed instream wetlands (ISWs). However, cases of large-scale implementations of ISWs are uncommon, probably due to a paucity of watershed-scale studies which highlight the influence of ISWs on riverine water quality. To elucidate the ISWs nitrate-N abatement potential at the watershed scale, the Soil and Water Assessment Tool (SWAT) was used to model nitrate-N export in a highly agricultural watershed located in the Coastal Plain of North Carolina. SWAT was first calibrated and validated for streamflow and for nitrate-N export using data collected from the inlet and outlet of an experimental instream wetland. The validated SWAT model was used to simulate a decade of nitrate-N export under two scenarios: 1) watershed with ISWs implemented; and 2) watershed without ISWs. The results of the case study indicated that a watershed-wide implementation of ISWs is likely to curtail annual nitrate-N export by 49%. The study also evaluated cases where ISWs are implemented in selected percentage of sub-basins across the watershed. The outcomes show higher increments of nitrate-N curtailment when ISWs are implemented in the first top agricultural sub-basins. Hence, implementation of ISWs on selected sub-basins can mitigate nitrate-N from non-point sources and enhance water quality in the watershed’s stream network. Keywords: Runoff, Croplands, Instream wetland, Nitrate-N export, Denitrification, SWAT model, Watershed.