between 25 and 55 million acre-feet of water may be present in the Edwards aquifer, only a small portion of this water is practically or legally available for use. Storage is the difference between recharge (inputs) and discharge (outputs) from the Edwards Aquifer. Annual storage can be negative during dry years with high water use and positive during wet years with relatively low water use. A long-term negative imbalance between recharge and discharge in an aquifer may lead to the depletion of the available water in the aquifer. Annual storage between 1955 and 2012 estimated from data provided by a continuing program between the U.S. Geologic Survey and the Edwards Aquifer Authority ranged from -633,000 to +1,653,000 acre-feet per year. The average storage during this period was 37,000 acre-feet per year. Inputs (recharge) Water mainly enters the Edwards Aquifer in two ways: it either falls as precipitation and percolates directly into the aquifer, or it enters as streamflow flowing through the Recharge Zone. The Recharge Zone occurs along the Balcones Fault Zone where the Edwards Plateau drops steeply and meets the Gulf Coastal Plain. Here, highly fractured limestones are exposed at the Earth's surface, which allow rain and streamflow to infiltrate directly into the aquifer. The Contributing Zone, which occurs on 5,400 mi2 (14,000 km2) of the Edwards Plateau (Texas Hill Country), collects precipitation and streamflow that drain to the Recharge Zone. Major streams draining the Contributing Zone include Cibolo Creek, Helotes Creek, Barton Creek, and Onion Creek. Water is unable to percolate into the aquifer in the Contributing Zone because much of the underlying geology is impermeable. Average precipitation in the region is around 30 inches per year. Only precipitation that falls on the contributing area is available for infiltration. With a contributing and recharge area of over 6,650 mi2, the mean annual volume of precipitation that is available for recharge is 10,660,000 acre-feet (13.15 cubic kilometers), or the equivalent of 5.3 million Olympic sized swimming pools. The average annual recharge rate between 1934 and 2013 is estimated to be 699,000 acre-feet (median annual recharge is 556,950 acre-feet), or only 6% of the total inputs to the system. Outputs (discharge) Water from the Edwards Aquifer is discharged in two ways: it is either pumped from wells (well discharge) or it leaves as stream outflow (spring discharge). The Edwards Aquifer Authority (EAA) and the United States Geological Survey (USGS) have