SWATDRAIN, a new model to simulate the hydrology of agricultural Lands, model development and evaluation

• A new model called SWATDRAIN was developed by integrating DRAINMOD into SWAT model.
• Hydrology of model was tested in a small tile drained watershed in Ontario, Canada.
• Model simulated water balance components for this agricultural watershed adequately.
• Model provides alternative tile flow simulation and drainage design tool for SWAT.

In this study, a new model was developed by fully incorporating the DRAINMOD model into the Soil and Water Assessment Tool (SWAT). In this modeling approach, surface flow is simulated using SWAT model and subsurface flow is estimated using the DRAINMOD model. The newly developed model, referred to as SWATDRAIN, has the potential to perform simulations of multiple scenarios such as controlled drainage, subsurface irrigation and wastewater treatment to determine cost-effective water management at the watershed scale. The model was evaluated for Green Belt Watershed, located in southern Ontario. Measured tile drainage and water table depth data from this watershed were used to evaluate the capability of the new model to simulate water balance components for this tile-drained agricultural watershed. Simulations were carried out over the period 1991–1993; 1991 and 1992 data were used for model calibration and 1993 data were used for validation. During both calibration and validation periods, SWATDRAIN simulated the watershed hydrologic response, the water table depth, and tile flow very well. Model accuracy statistics for monthly and daily water table depth over the validation period were, respectively, 0.86 and 0.70 for Coefficient of Determination, 0.11 and 2.90 for Percent Bias, and 0.80 and 0.67 for the Nash Sutcliffe Efficiency. Model accuracy statistics for events, monthly and daily tile drainage over the validation period were, respectively, 0.86, 0.88 and 0.70 for R2, 11.7, 13.26 and 13.26 for Percent Bias, and 0.82, 0.86 and 0.69 for Nash Sutcliffe Efficiency. This clearly demonstrates that integrated DRAINMOD approach in SWAT provides an effective water management tool for tile-drained watersheds.