Watershed scale nitrate-N abatement of instream wetlands: An appraisal using the soil and water assessment tool

Authors: Sohoulande, Clement; Szogi, Ariel; Stone, Kenneth - Ken; Novak, Jeffrey

Submitted to: Applied Engineering in Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/2/2020
Publication Date: 6/1/2020
Citation: Sohoulande Djebou, D.C., Szogi, A.A., Stone, K.C., Novak, J.M. 2020. Watershed scale nitrate-N abatement of instream wetlands: An appraisal using the soil and water assessment tool. Applied Engineering in Agriculture. 36(3):387-397. https://doi.org/10.13031/aea.13736.
DOI: https://doi.org/10.13031/aea.13736

Interpretive Summary: In watersheds under high agricultural production, nitrate pollution often originates from intensive application of fertilizers and animal manure to croplands. The nitrate exported from farmlands flows into nearby streams and contribute to pollution at the watershed level. Experimental studies reported that constructed instream wetlands (ISWs) can significantly remove nitrate, but the large-scale impact of their implementation are not well investigated. To elucidate this large-scale impact of ISWs on water quality, the Soil and Water Assessment Tool (SWAT) was used to model nitrate export in a highly agricultural watershed located in the Coastal Plain of North Carolina. The model was calibrated and validated based on data collected from an experimental instream wetland. The validated SWAT model was used to simulate 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 reduce annual nitrate export by 49%. The study also evaluated cases where ISWs are implemented in selected percentage of sub-basins across the watershed. The outcomes show the implementation of ISWs in the first top agricultural sub-basins have a higher impact on water quality.

Technical 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.