Supplemental graphs for "Comparison of Two Tile-Drain Methods in SWAT via Temporal and Spatial Testing for an Iowa Watershed"

Tile drainage is a common practice to increase agricultural productivity in regions with high water tables. Ecohydrological models, such as the Soil and Water Assessment Tool (SWAT), are key tools used to simulate these subsurface drainage structures and their corresponding management effects on water balance. This study is an investigation of the application of two different SWAT tile-drain calculation methods (empirically and physically-based) in simulating the streamflow of the Des Moines River Basin (DMRB), central Iowa, USA. The model was calibrated for the DMRB outlet and validated across 24 monitoring stations. We found that both empirical function and physically-based methods have satisfactory model performance for monthly and daily calibration (NSE > 0.5; KGE > 0.6; Pbias < ±25%). The daily validation process showed that the physically-based method had more accurate spatial variability in representing the hydrological processes and a better representation of the annual tile-drain flow. The findings in this study reinforce the importance of spatial validation in assessing model performance and ensuring its suitability for practical applications.