Assessing Nitrate Loss and Nitrous Oxide Emissions in a Typical Tile-Drained Field and Watershed in the Midwest USA

Tile drainage, a common agricultural practice in the Midwest USA, improves soil aeration and crop yields but also contributes to environmental challenges, such as nitrate (NO₃) loss and nitrous oxide (N₂O) emissions. To address these issues, ecohydrological models are essential for understanding the intricate hydrological and biogeochemical processes in tile-drained watersheds and for evaluating management strategies. Recent enhancements to the Soil and Water Assessment Tool (SWAT) have incorporated Century-based soil carbon and nitrogen cycling processes, along with N₂O emission algorithms, improving its ability to simulate nitrogen cycling and greenhouse gas emissions at the watershed scale. In this study, the Century-based soil carbon and nitrogen cycling module was integrated with two tile drainage modules within SWAT to enhance its simulation capabilities. The enhanced model was first evaluated at the Kelley experimental site, using observed data on drainage discharge, NO₃ loss, and N₂O emissions influenced by cover crops, corn-soybean rotation, and fertilization. Subsequently, the model was applied to simulate NO₃ loss and N₂O emissions in Iowa’s South Fork Watershed (SFW) using an ensemble modeling approach. This approach tested eight scenarios combining the two nitrogen modules, the two tile drainage modules, and calibration variations. Results showed that all eight scenarios effectively simulated daily stream flow but underestimated daily NO₃ load due to the underrepresentation of peak flows. Most models performed well at the monthly scale for both stream flow and NO₃ load. The ensemble modeling results aligned with prior studies, suggesting that ensemble approaches can reduce prediction uncertainties and address equifinality issues. However, the study emphasizes the need for additional data collection to improve the accuracy of denitrification and N₂O emissions simulations, especially in tile-drained systems. This research advances ecohydrological modeling for tile-drained watersheds, offering insights into improving water quality and reducing greenhouse gas emissions under agricultural management practices.