Simulation of daily flow pathways, tile-drain nitrate concentrations, and soil-nitrogen dynamics using SWAT

Authors: IKENBERRY, CHARLES - Iowa Department Of Natural Resources; SOUPIR, MICHELLE - Iowa State University; HELMERS, MATTHEW - Iowa State University; CRUMPTON, WILLIAM - Iowa State University; Arnold, Jeffrey; 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: 12/5/2017
Citation: Ikenberry, C.D., Soupir, M.L., Helmers, M.J., Crumpton, W.G., Arnold, J.G., Gassman, P.W. 2017. Simulation of daily flow pathways, tile-drain nitrate concentrations, and soil-nitrogen dynamics using SWAT. Journal of the American Water Resources Association. 53(6):1251-1266. https://doi.org/10.1111/1752-1688.12569.
DOI: https://doi.org/10.1111/1752-1688.12569

Interpretive Summary: Some of the most productive farm land in the U.S. was swamp until it was drained using subsurface tiles. Although the drains allow cultivation of these extremely productive lands, it also provides a direct conduit for nutrients and pesticides to reach water bodies. Reliable simulation models are needed for effective planning of nutrient reduction strategies. In this study, the Soil and Water Assessment Tool (SWAT) was tested on a small tile-drained watershed in Iowa. Model performance was evaluated for daily tile flow, nitrate concentration in the tile flow, and soil nitrogen dynamics. The model was able to simulate these processes with reasonable accuracy at a monthly time step, however, the model was not able to predict daily values satisfactorily. Many components of the soil nitrogen balance are highly variable in space and time, thus making it difficult to simulate accurately. Results from this study suggest: 1) soil nitrogen fluxes such as mineralization, denitrification, and plant uptake should be reported for all model studies, 2) physically based algorithms are still needed to improve model results, and 3) better model input parameterization is needed. Model improvement will continue to ensure effective planning of nutrient reduction strategies.

Technical Abstract: Tile drainage significantly alters flow and nutrient pathways 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 nitrate-nitrogen (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-drained watersheds, evaluate model performance for simulation of flow components and NO3-N concentration at daily intervals, and evaluate simulated soil-nitrogen dynamics. 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 best management practices (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 in N fluxes is extreme and better parameterization and constraint, through use of more detailed agronomic data, would also improve NO3-N simulation in SWAT.