This research aims at improving the performance of regional groundwater models by incorporating high-resolution elevation data into the head-dependent packages of MODFLOW. Model code specific to the evapotranspiration package (EVT) of MODFLOW was modified to account for the variability in elevation data and to effectively delineate the evapotranspiration (ET) simulated region at user-specified digital elevation model (DEM) resolution. The regional groundwater model of the Rincon Valley-Mesilla Basins (NMOSE-2007 flow model) was improved and considered to evaluate and validate the developed code. The base DEM of the study area is smoothened and aggregated to various resampled resolutions that are integer divisors of NMOSE-2007 flow model resolution for use with ET simulation. A gradual decrease in ET outflow is observed when the variability in elevation is eliminated across the grid cell. Also, changes in cumulative ET outflow (as a fraction of total outflow) at different resampled grids followed a similar trend during the simulation. The computational cost is high for the models simulated at fine resolution, whereas the simulation accuracy is low for the models simulated at coarse resolution. To select the optimum resampled DEM resolution to simulate the ET component of groundwater for use with the NMOSE-2007 flow model, a chi-square test of homogeneity was performed at 5 and 10% significance levels by considering computational cost and simulation accuracy as the base parameters. Results of the statistical analysis concluded that simulating ET component at 80.467-m resolution and integrating the outflows to model cell (402.336 m) resolution would significantly improve the performance of the flow model without compromising on the computational cost. © 2014 American Society of Civil Engineers.