Improving nitrate load simulation of the SWAT model in an extensively tile-drained watershed

JungJin Kim; Younggu Her; Rabin Bhattarai; Hanseok Jeong

doi:10.1016/j.scitotenv.2023.166331

Improving nitrate load simulation of the SWAT model in an extensively tile-drained watershed

Sci Total Environ. 2023 Dec 15:904:166331. doi: 10.1016/j.scitotenv.2023.166331. Epub 2023 Aug 16.

Authors

JungJin Kim¹, Younggu Her², Rabin Bhattarai³, Hanseok Jeong⁴

Affiliations

¹ Institute of Environmental Technology, Seoul National University of Science and Technology, Seoul, Republic of Korea.
² Department of Agricultural and Biological Engineering/Tropical Research and Education Center, Institute of Food and Agricultural Sciences, University of Florida, Homestead, FL, United States.
³ Department of Agricultural and Biological Engineering, University of Illinois at Urbana-Champaign, Urbana, IL, United States.
⁴ Institute of Environmental Technology, Seoul National University of Science and Technology, Seoul, Republic of Korea; Department of Environmental Engineering, Seoul National University of Science and Technology, Seoul, Republic of Korea. Electronic address: hanjeong@seoultech.ac.kr.

PMID: 37595899
DOI: 10.1016/j.scitotenv.2023.166331

Abstract

Subsurface drainage systems are effective management practices employed to remove excess soil water, thereby improving soil aeration and crop productivity. However, these systems can also contribute to water quality issues by enhancing nitrate leaching and loads from agricultural fields. The Soil and Water Assessment Tool (SWAT) is commonly used to assess nitrate loads and long-term water quality impacts from agricultural watersheds. However, the current SWAT model oversimplifies nitrate transport processes by assuming a linear relationship between nitrate concentrations in tile flow and soil nitrate content. It also neglects the time lag between nitrate loading and transport with the flow. This study aimed to enhance the accuracy of nitrate load prediction by revising the subsurface drainage routine in the SWAT model. The revised routine was tested using flow and nitrate load measurements from a typical tile-drained watershed in east-central Illinois, U.S. The results demonstrated that the revised SWAT nitrate routine outperformed the current one in simulating nitrate transport at field and watershed scales. The revised routine improved the nitrate load prediction from an "unacceptable" to a "satisfactory" or "good" rating on the field scale. A sensitivity analysis conducted using the revised nitrate module showed the parameters directly associated with transpiration, groundwater discharge to the reach, the lag time of tile flow, and channel flow hydraulics were the most sensitive in nitrate load simulation. In addition, different tile depth scenarios were modeled to evaluate variation in the amount of surface runoff, tile flow, and nitrate loads by the surface flow and tile flow. The results of tile configuration scenarios agreed with understanding the tile flow process. The test results demonstrated the potential of the revised SWAT nitrate module as a tool to accurately evaluate the effects of tile drainage systems on water quality.

Keywords: Calibration; Nitrate; SWAT; Sensitivity analysis; Tile drainage.