Drainage intensity representation in the SWAT model – A case study in the Maumee River Watershed, USA

Agricultural subsurface drainage is a major driver of field-to-watershed-scale hydrologic and water quality processes in the US Midwest. However, the true extent and intensity of subsurface drainage are largely unknown. Therefore, most watershed-scale hydrologic models in this region rely on simplified drainage representations. This study refined the representation of drainage intensity in a field-scale Soil and Water Assessment Tool (SWAT) model and evaluated its effects on hydrologic and water quality responses at watershed and field scales. Drainage intensity was refined for the Maumee River Watershed, USA, using soil-specific drain spacing and depth recommendations from the Ohio Drainage Guide. At the watershed scale, refined drainage intensity significantly improved monthly nitrogen and phosphorus load predictions (Nash-Sutcliffe efficiency ≥ 0.4) but had limited impacts on stream discharge. Annual nitrate (NO 3 - -N) and total nitrogen (TN) loads decreased (10–20%), while dissolved reactive phosphorus (DRP) and total phosphorus (TP) increased (5–15%), due to greater surface runoff (6%) caused by wider drain spacings (9–18 m) and shallow drain depths (90 cm). Field-scale hydrologic responses showed greater variability with changes in drain spacing and saturated hydraulic conductivity. Narrower drain spacings (9–11 m) increased subsurface drainage discharge and reduced surface runoff, while wider spacings (15 and 18 m) increased drainage discharge primarily due to higher hydraulic conductivity. Reducing drain depth from 100 to 90 cm had negligible effects on annual hydrologic responses. Drainage discharge and surface runoff were more sensitive to increases in the lateral to vertical hydraulic conductivity ratio (LATKSATF) in fields with lower hydraulic conductivity. These results highlight the importance of accurately representing drainage intensity and soil properties in SWAT applications for heavily subsurface-drained agricultural watersheds.