It is not possible to accurately estimate the capacity of the Edwards reservoir. A more meaningful figure for water availability is annual recharge or refilling of the reservoir. From 1934 to 1973 the average yearly recharge was 556,700 acre-feet. The primary recharge mechanism in the Edwards is the Balcones fault zone, an area of porous limestone through which surface water enters the underground reservoir. Rivers and streams crossing the fault zone lose a large part of their water to the aquifer.
Water literally disappears into the large cracks and crevices in the streambeds.
About three-fifths of the water entering the reservoir comes from streams in the western counties of Medina, Uvalde, and Kinney. Discharge from the Edwards Underground Reservoir is through wells and springs. About 38 percent of the discharge, averaging 400,000 acre-feet a year, is from more than 4,000 wells. Natural discharge from the reservoir occurs in six major springs and numerous smaller ones. Flows from the largest springs feed into the San Antonio and Guadalupe rivers.