Incorporation of a New Shallow Water Table Depth Algorithm into SWAT2005

Abstract

The fluctuation of the shallow water table depth (wtd) is important for planning drainage systems at the plot-, field-, and watershed-scale because its proximity to the ground surface impacts farm machine trafficability, crop development, agricultural chemical transport, soil salinity, and drainage. Therefore, it is important for hydrologic models to accurately simulate wtd. The goals of this study were to: (1) develop and incorporate a new wtd algorithm into the Soil and Water Assessment Tool model (SWAT Release 2005), a continuous-time, physically based, watershed-scale hydrologic model, in order to improve the prediction of the wtd; and (2) evaluate the wtd prediction improvement using measured wtd data for three observation wells located within the Muscatatuck River basin in southeast Indiana. The Modified DRAINMOD wtd simulation approach, based on the DRAINMOD wtd prediction approach, was developed and incorporated into SWAT2005. SWAT2005 was calibrated and validated for wtd for the three observation wells, and the wtd prediction performance of the Modified DRAINMOD approach was compared to those of three other wtd routines used in SWAT. Based on the simulation results, the Modified DRAINMOD approach yielded the best wtd prediction performance, as indicated by the highest average daily calibration and validation Nash-Sutcliffe efficiency (NSE) values of 0.64 and 0.41, respectively, and correlation coefficient (R) values of 0.81 and 0.65, respectively, and the lowest percent bias (PBIAS) values of -13% and -3%, respectively, and root mean square error (RMSE) values of 0.41 m and 0.59 m, respectively, for the three observation wells. This implies that the Modified DRAINMOD approach within SWAT2005 improved the prediction of wtd. Enhanced wtd prediction is anticipated to increase the simulation accuracy of watershed hydrologic processes and water management components such as tile drainage.