THE CHALLENGES OF SIMULATING THE DEWATERING OF THE HIGH PLAINS AQUIFER IN TEXAS AND SETTING AQUIFER MANAGEMENT GOALS WHEN SPECIFIC-CAPACITY AND OTHER HYDROGEOLOGIC DATA REMAIN SPARSE

The High Plains aquifer is an extensive unconfined aquifer supplying public-water supply systems and agriculture across the U.S. High Plains. The Ogallala Formation is the most ubiquitous part of the aquifer. More than 70 percent of the aquifer's extent in parts of eight states is under regulatory jurisdiction. Most states claim ownership of groundwater but issue permits for water withdrawal. Groundwater in Texas is a property right that can be separated from surface rights. Groundwater conservation districts (GCDs) are Texas' preferred method of groundwater management and 12 districts cover the most productive part of the High Plains aquifer in the state. Most GCDs were created to control the depletion of storage in the aquifer. Another GCD duty is to certify for the IRS the annual depletion tax credit claimed by landowners for their property's decreased saturated thickness.

During the past 50 yr, irrigation was 70 to >90 percent of water use from the aquifer and irrigation rates locally were 20× recharge rates. GCD-issued permits for irrigation-water production typically were ~0.3 m³/m²/yr for 10²- to 10³-ha areas. Water levels fell by >0.6 m/yr and saturated thickness decreased by half. Demand for municipal water, however, is projected to increase 1.5× from 2000 through 2060. The traditional irrigation-permit rate is now applied to municipal-water-production permits for >10⁵-ha-area projects. This creates challenges for improving hydrogeologic conceptual models and data quality that must be met to make reasonable predictive simulations of aquifer dewatering. There could be, for example, a 4× range in specific yield between depositional facies beneath different properties. A GCD management goal limiting average rates of decline of saturated thickness to a small annual percent (e.g., 1.25 %) could impact monetary value of water rights in the High Plains aquifer in Texas except for major uncertainty in the geospatial distribution of this hydrogeologic property.