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Introduction 2 Purpose and scope 2 Methods of study 4 Acknowledgments 4 Descriptionof irrigation data collected 4 Selected references 9 Supplemental information 10

The regional specific yield in the recharge area of the Edwards and associated limestones, the principal aquifer in the San Antonio, Texas, area, is estimated to be about 0.025. This estimate is based on annual differences between recharge and discharge and on the averages of annual water-level changes in 10 observation wells in and near the outcrop area. HYDROGEOLOGY OF THE AREA The Edwards and associated limestones o2 Early Cretaceous age consist of the Comanche Peak, Edwards, and Georgetown Limestones (fig. 1). These light-gray limestones form a single hydrologic unit and generally hard, dense, and fine grained. They are characterized by numerous solution-enlarged openings at various depths. The total thickness of the limestones is 400 to 500 feet in the artesian area and about 400 feet in the water-table or recharge area. The artesian part of the aquifer is confined by the Del Rio Clay above and the Glen Rose Limestone below. In the San Antonio area, the limestone aquifer dips south and southeast towards the Gulf of Mexico at a rate of 10 to 20 feet per mile. The Balcones Fault Zone, an area of moderate to intense block faulting within the study area, is characterized by high-angle normal faults, downthrown to the south or southeast, with displacements oL as much as 500 feet (fig. 2). The faults provide passageways for circulating ground water, which in turn enlarges the openings by dissolving the limestone. The effective porosity (precentage of total pore volume occupied by interconnected interstices available for fluid flow)