ANIMATING THE POTENTIOMETRIC SURFACE OF A HEAVILY IRRIGATED AQUIFER

Researchers at the Illinois State Water Survey have recently applied head-specified models to understand the influence of transient drivers (withdrawals, stream stages, storage, and recharge) on aquifer heads and flow directions. The head-specified modeling approach involves assigning transiently changing groundwater level and stream stage observations as constant heads in a finite-difference MODFLOW model and calibrating to reported pumpage over a large regional area. This approach has been shown to provide insight into deep confined flow systems. However, in unconfined systems with dedicated monitoring wells, the approach fails without also assigning a transiently changing areal flux. This flux is a combination of all unsimulated sources (recharge) and sinks (low-order streams, groundwater withdrawals), and can be calculated through an iterative approach that eliminates flow from constant head cells. The approach was tested in Mason County, IL, a single-layer sandy aquifer system that is heavily irrigated during the summer months. The resulting potentiometric surface honored the governing differential equations of groundwater flow. Since the model was transient (with daily stress periods), it created a series of potentiometric surfaces that simulate two large rivers (the Illinois and Sangamon) reverting between gaining and losing conditions. This same result could not be achieved using standard kriging approaches. The average areal flux assigned to the head-specified model is also consistent with the values assigned to an independently developed flow model of the region, at least during the spring months when the areal flux is analogous to recharge.