Evaluation of existing and modified wetland equations in the SWAT model

Authors: IKENBERRY, CHARLES - Iowa Department Of Natural Resources; CRUMPTON, WILLIAM - Iowa State University; Arnold, Jeffrey; SOUPIR, MICHELLE - Iowa State University; GASSMAN, PHILIP - Iowa State University

Submitted to: Journal of the American Water Resources Association
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/23/2017
Publication Date: 12/1/2017
Citation: Ikenberry, C., Crumpton, W.G., Arnold, J.G., Soupir, M.L., Gassman, P.W. 2017. Evaluation of existing and modified wetland equations in the SWAT model. Journal of the American Water Resources Association. 53(6):1267-1280. https://doi.org/10.1111/1752-1688.12570.

Interpretive Summary: Some of the most productive farmland in the U.S. was wetland 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 phosphorous to water bodies. The Soil and Water Assessment Tool (SWAT) has been widely used to predict the impact of management on nitrate losses from agricultural fields. However, few studies have evaluated the models ability to simulate nitrogen dynamics such as tile losses, denitrification, mineralization, crop uptake, and soil NO3 storage. SWAT was applied to a small tile drained watershed in Iowa and was able to meet accepted performance criteria for tile flow and nitrate loads. It was also determined that improvements to algorithms for soil nitrogen processes are needed and more detailed research data of the complete nitrogen budget would improve SWAT simulations.

Technical Abstract: The drainage significantly alters flow and nutrient pathways in small watersheds and reliable simulation at this scale is needed for effective planning of nutrient reduction strategies. The Soil and Water Assessment Tool (SWAT) has been widely utilized for prediction of flow and nutrient loads, but few applications have evaluated the model’s ability to simulate pathway specific flow components or NO3-N concentrations in tile-drained watersheds at the daily time step. The objectives of this study were to develop and calibrate SWAT models for small, tile drain watersheds, evaluate model performance for simulation of flow components and NO3-N concentration at daily intervals, and evaluate simulated N dynamics such as denitrification, mineralization, crop uptake and soil NO3 storage. Model evaluation revealed that it is possible to meet accepted performance criteria for simulation of monthly total flow, subsurface flow (SSF), and NO3-N loads while obtaining daily surface runoff (SURQ), SSF, and NO3-N concentrations that are not satisfactory. This limits model utility for simulating BMPs and compliance with water quality standards. Although SWAT simulates the soil N-cycle and most predicted fluxes were within ranges reported in agronomic studies, improvements to algorithms for soil-N processes are needed. Variability of N fluxes is extreme and better parameterization and constraint, through use of more detailed agronomic data, would also improve NO3-N simulation in SWAT.