A HYDROGEOLOGIST'S PERSPECTIVE ON DESIRED FUTURE CONDITIONS FOR THE AQUIFERS OF TEXAS

Groundwater conservation districts in Texas are charged with developing the desired future conditions of the state's aquifers. A desired future condition amounts to a management goal such as maintaining a certain minimum amount of spring flow or a certain minimum water level or level of water in storage. A desired future condition is primarily a policy decision, but hydrogeology provides the bounds to that decision as well as the resulting amount of water that can be pumped. The first question for decision makers to answer is: How do you want to manage your aquifer? If the answer is sustainably, then the subsequent focus can be on acceptable levels of spring and base flows and/or static water levels such as what has been done by districts for the northern segment of the Edwards (Balcones Fault Zone) Aquifer. If the answer is planned depletion, then the focus can be on water remaining in the aquifer or a set amount of maximum annual water level decline, such as what is being considered by districts for the Ogallala Aquifer. If the answer is a combination of approaches (planned depletion phased into sustainability), then the focus can be on the appropriate parameters as discussed above. Dipping clastic aquifers offer some challenges to setting desired future conditions. These aquifers have confined and unconfined zones that respond much differently to pumping and climatic conditions. In addition, the unconfined zone generally controls how water levels decline in the confined zone with the potentiometric surface generally hinging off of the downdip edge of the outcrop. Regardless of management goals and aquifer setting, the presumed location of pumping also affects how much can be pumped to attain the desired future condition. This uncertainty, as well as the uncertainty in our numerical and analytical tools, requires an adaptive management approach where the amount that can be pumped—the managed available groundwater—can be adjusted to reflect an increased understanding of the aquifer, the reality of the magnitude and location of future pumping, and how the aquifer actually responds to that pumping. Fortunately, the joint planning process among districts that will result in desired future conditions allows a revisiting of the desired future conditions on at least a five-year cycle.