Flow Model

In order to quantify the effects of facies distributions on the hydrologic system at Hays, a detailed groundwater flow model was developed based on the hydrogeologic model, incorporating three variable-thickness layers. The flow model was implemented using the popular MODFLOW simulator. Since the hydraulic conductivities of the hydrostratigraphic units are poorly known at Hays, these were treated as adjustable parameters during model calibration. While connectivity and thickness of the high-permeability sand facies (Qal) was expected to be the dominant control on flow, the likely impact of additional effects of such as non-uniform recharge, surface water interaction, and semi-confined conditions in the Qal was unknown. The flow model results illustrate the strong impact of Qal thickness on groundwater Darcy velocity beneath Hays (Fig. 12), with secondary effects from variable recharge and semi-confined Qal.

Figure 12: Modeled per-element discharge, Qal layer, as reported by MODFLOW. Longest arrow represents a velocity $\sim$ 5 m/d, mean velocity in Qal $\sim$ 1 cm/d. Select figure to view full-sized version.