Transactions of the ASABE Vol. 54(5): 1705-1711 2011 American Society of Agricultural and Biological Engineers ISSN 2151-0032 1705 SHALLOW W ATER T ABLE DEPTH ALGORITHM IN SWATā : RECENT DEVELOPMENTS D. N. Moriasi, J. G. Arnold, G. G. Vazquez‐Amábile, B. A. Engel 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. Keywords. DRAINMOD, Simulation, SWAT, Watershed, Water table depth. ccurate simulation of shallow water table depth is important in determining optimum management systems for farm machine trafficability, agricul‐ tural production, and water quality due to agricul‐ tural chemical transport and soil salinity. The Soil and Water Assessment Tool (SWAT) model (Arnold et al., 1998; Arnold and Fohrer, 2005) is a continuous‐time, physically based, watershed‐scale model developed to predict the impact of land management practices on water, sediment, and agricul‐ tural chemical yields in watersheds with varying soil, land use, and management conditions over time. SWAT has been successfully used to evaluate nonpoint‐source water resource problems for a large variety of water quality applications Submitted for review in January 2011 as manuscript number SW 8995; approved for publication by the Soil & Water Division of ASABE in August 2011. The authors are Daniel N. Moriasi, ASABE Member, Research Hydrologist, USDA‐ARS Grazinglands Research Laboratory, El Reno, Oklahoma; Jeffrey G. Arnold, ASABE Member, Supervisory Agricultural Engineer, USDA‐ARS Grassland Soil and Water Research Laboratory, Temple, Texas; Gabriel G. Vazquez‐Amábile, ASABE Member, Professor, Graduate School, University of La Plata, Buenos Aires, Argentina; and Bernard A. Engel, ASABE Member, Professor and Head, Department of Agricultural and Biological Engineering, Purdue University, West Lafayette, Indiana. Corresponding author: Daniel N. Moriasi, USDA ARS Grazinglands Research Laboratory, 7207 W. Cheyenne St., El Reno, OK 73036; phone: 405‐262‐5291, ext. 263; fax: 405‐262‐0133; e‐mail: daniel.moriasi@ars.usda.gov. globally (Gassman et al., 2007). The shallow water table depth (wtd) simulation methods available in SWAT are the SWAT‐M, SWAT2005, and the recently incorporated modi‐ fied DRAINMOD approach (Moriasi et al., 2009). In the SWAT‐M approach (Du et al., 2005), a restrictive layer, which simulates a confining layer and is used as the maxi‐ mum wtd, is set at the bottom of the soil profile (Moriasi et al., 2009). Beginning with the bottom soil layer, the soil pro‐ file above the confining layer is allowed to fill with water to field capacity. When the bottom soil layer reaches field ca‐ pacity, additional water is allowed to fill the profile from the bottom of the soil layer upward, from which the height of the water table above the restrictive layer and hence the wtd from the ground surface is computed . This approach, which is based on antecedent climate, serves as the master soil per‐ colation component. The SWAT2005 routine computes wtd using 30‐day moving summations of precipitation, surface runoff, and ET (Neitsch et al., 2002). The SWAT‐M and SWAT2005 approaches do not require calibration. The modi‐ fied DRAINMOD algorithm relates drainage volume (vol) with wtd (Moriasi et al., 2009). Water table depth is computed as a function of vol and the vol‐to‐wtd converter (wt_fctr j ), a factor that converts vol into wtd for hydrologic response unit (HRU) j. The conversion factor wt_fctr j is currently a calibra‐ tion parameter. In a previous study, the wtd prediction performance of the modified DRAINMOD approach was tested using wtd data measured at three observation wells located in Storm Creek A