Evaluation of Existing and Modified Wetland Equations in the SWAT Model

Abstract

The ability to accurately simulate flow and nutrient removal in treatment wetlands within an agricultural, watershed-scale model is needed to develop effective plans for meeting nutrient reduction goals associated with protection of drinking water supplies and reduction of the Gulf of Mexico hypoxic zone. The objectives of this study were to incorporate new equations for wetland hydrology and nutrient removal in Soil and Water Assessment Tool (SWAT), compare model performance using original and improved equations, and evaluate the ramifications of errors in watershed and tile drain simulation on prediction of NO 3 -N dynamics in wetlands. The modified equations produced Nash-Sutcliffe Efficiency values of 0.88 to 0.99 for daily NO 3 -N load predictions, and percent bias values generally less than 6%. However, statistical improvement over the original equations was marginal and both old and new equations provided accurate simulations. The new equations reduce the model's dependence on detailed monitoring data and hydrologic calibration. Additionally, the modified equations increase SWAT's versatility by incorporating a weir equation and an irreducible nutrient concentration and temperature coefficient. Model improvements enhance the utility of SWAT for simulating flow and nutrients in wetlands and other impoundments, although performance is limited by the accuracy of inflow and NO 3 -N predictions from the contributing watershed. Editor's note: This paper is part of the featured series on SWAT Applications for Emerging Hydrologic and Water Quality Challenges. See the February 2017 issue for the introduction and background to the series.(KEY TERMS: wetlands; nitrate-nitrogen; tile drainage; SWAT; nutrient reduction.) Ikenberry, Charles

Simulation of Daily Flow Pathways, Tile‐Drain Nitrate Concentrations, and Soil‐Nitrogen Dynamics Using SWAT

“…This suggests that parameterization may not be transferable across watersheds with similar characteristics, and also that models calibrated at larger scales may not accurately reflect hydrology and nutrient transport at small watershed (e.g., drainage district) scales, as noted by Baffaut et al (2015). These limitations are especially important in cases where the model is intended to help locate, design, and/or estimate NO 3 -N removal capabilities of water quality BMPs, as indicated by impacts on NO 3 -N simulation wetlands at the outlet of these case study watersheds (Ikenberry et al, 2017).…”

“…The two watersheds simulated in this study each drain to Conservation Reserve Enhancement Program (CREP) wetlands located in the Des Moines Lobe ecoregion in north-central Iowa (Ikenberry et al, 2017). The 309-ha KS watershed is located in Story County, Iowa, at the headwaters of a first-order tributary to Squaw Creek, a HUC-12 watershed in the South Skunk River basin.…”

A study of nitrogen fate and transport in agricultural landscapes at the field, wetland, and watershed scales

“…Paired with sustained water monitoring, watershed modeling can quantify nutrient reduction progress and guide conservation practice implementation and management. While dozens of watershed models now exist, relatively few perform nutrient fate and transport simulations (Imhoff et al, 1983;Hansen et al, 2009;Li et al, 2010;Ikenberry et al, 2017). Therefore, the major goal of this thesis was to develop a water quality model encompassing the major processes of the nitrogen cycle and couple to a physically-based hydrologic model to perform watershed nitrogen fate and transport simulations to evaluate the effectiveness of wetlands for reducing agricultural NO3-N loads in Iowa under variable hydrologic conditions to help achieve Gulf Hypoxia water quality goals.…”

“…SWAT was used to assess changes in land use and agricultural management on stream NO3-N loading in two large Iowa watersheds (9400-16175 km 2 ; Jha et al, 2007;Schilling and Wolter, 2009). More recently, Ikenberry et al (2017) evaluated the flow and nutrient removal algorithms for wetlands in SWAT using two small case study wetlands in Iowa (drainage areas less than 3.1 km 2 ). The revised equations provided similar or improved performance of simulated daily wetland outflows and NO3-N removal and required less calibration compared to the original wetland equations.…”

“…Watershed models of this type decompose the landscape into subbasins and river reaches defined by spatially averaged (lumped) flow and water quality parameters, use conceptual approaches to represent subsurface flow processes, and generally rely on more empirically-based methods to describe flow and water quality processes. Ikenberry et al (2017) used the Soil and Water Assessment Tool (SWAT) to accurately simulate daily wetland outflow and NO3-N removal in two small Iowa constructed wetlands (drainage areas < 3.1 km 2 ). The model performed well at predicting daily wetland outflow and NO3-N removal when measured inflow and NO3-N concentration were imposed, while performance was limited by the accuracy in predicting inlet conditions with the full watershed simulations.…”