Shallow Water Table Depth Algorithm in SWAT: Recent Developments

Abstract

Knowledge of the shallow water table depth (wtd) is crucial in many studies, including determination of optimum irrigation and drainage management systems for agricultural production, farm machine trafficability, and water quality due to agricultural chemical transport and soil salinity. Therefore, it is essential for hydrologic models to accurately simulate wtd. A shallow wtd algorithm, herein called the modified DRAINMOD approach, that relates drainage volume (vol) to wtd, was recently incorporated into the Soil and Water Assessment Tool (SWAT) model. In the modified DRAINMOD approach, wtd is computed as a function of vol and a water table factor (wt_fctr), which converts vol into wtd. The constant wt_fctr is currently a calibration parameter. However, at the watershed scale where there are many fields (hydrologic response units, HRUs), it is difficult if not impossible to determine an optimum wt_fctr value for each HRU through the calibration process. The objectives of this study were to: (1) revise the modified DRAINMOD wtd algorithm in SWAT so that wt_fctr is automatically computed by the model as a function of soil physical properties in order to eliminate determination of wt_fctr through the calibration process, and (2) evaluate the revised modified DRAINMOD wtd algorithm within SWAT using measured wtd data for three observation wells located in forest fields within the Muscatatuck River basin in southeast Indiana. On average, the calibrated wt_fctr yielded daily NSE values of 0.64 and 0.41, PBIAS values of -13% and -4%, and RMSE values of 0.41 m and 0.59 m during the calibration and validation periods, respectively, for the three observation wells. The automatically computed variable wt_fctr yielded daily NSE values of 0.66 and 0.58, PBIAS values of 4% and 10%, and RMSE values of 0.40 m and 0.50 m during the calibration and validation periods, respectively, for the same observation wells. Based on these model performance results, there were no significant differences between the wtd simulated using the manually calibrated constant and the automatically computed variable wt_fctr values. The automatically computed variable wt_fctr will enable the revised modified DRAINMOD shallow wtd algorithm to be used at the watershed scale.