Recharge

Discharge

Reference

Table 1

Table 2

Table 3

The Edwards aquifer is the sole source of public water supply for more than 1 million people in the San Antonio area and supplies large quantities of water for agriculture, industry, and the military. The dissolutioned, faulted limestone aquifer is the major source of water for Bexar, Comal, Hays, Medina, and Uvalde Counties. The annual compilation of estimates of recharge to and discharge from the Edwards aquifer is part of a continuing program of the U.S. Geological Survey (USGS) in cooperation with the Edwards Aquifer Authority (EAA).

Annual recharge estimates (table 1) are based on data collected from a network of streamflow- and rainfall-gaging stations (operated by the USGS and National Oceanic and Atmospheric Administration) and on assumptions that the runoff characteristics of gaged areas relate to ungaged areas (Puente, 1978). Annual discharge estimates (table 2) are compiled from spring-discharge data collected by the USGS and from annual water use reports submitted to the EAA by Edwards aquifer users. Unreported water use was estimated by the EAA. Water well discharges for 1999 do not include estimates of discharge for domestic uses, stock, and miscellaneous uses.

Recharge to the Edwards aquifer is derived mainly from seepage into the aquifer from streams that cross the outcrop of the aquifer and direct infiltration of precipitation on the outcrop. The watershed areas used for estimating recharge to the Edwards aquifer in the San Antonio area (fig. 1) have been modified slightly from the areas described by Puente (1978) to reflect existing data-collection sites. Recharge in the Guadalupe River Basin is not included because the net recharge to the aquifer in this basin is negligible (Puente, 1978). The watershed areas are based on surface- and groundwater divides.

Discharge from the Edwards aquifer is by wells and springs. The major discharge from wells primarily is in Bexar, Medina, and Uvalde Counties. Most of the well discharge in Bexar County in 1999 was for public water supply and the military. Some well discharge in Bexar County and most of the well discharge in Medina and Uvalde Counties was for irrigation. The remaining discharge (primarily from wells in Bexar County) in 1999 was for industry.

The estimated annual discharge from wells and springs during 1999 is 884,000 acre-ft. The estimated annual discharge from wells and springs for 1934–99 (table 2) ranges from 388,800 acre-ft in 1955 to 1,130,000 acre-ft in 1992. The 1934–99 estimated annual discharge from wells ranges from 101,900 acre-ft in 1934 to 542,400 acre-ft in 1989.

Discharge from San Marcos and Comal Springs (378,200 acre-ft) accounted for about 83 percent of spring discharge during 1999. The remaining spring discharge was from Hueco Springs in Comal County, San Pedro and San Antonio Springs in Bexar County, and Leona Springs in Uvalde County. Discharge from Leona Springs includes underflow from the Edwards aquifer into gravels of the Leona Formation along the stream. The 1934–99 estimated annual discharge from springs ranged from 69,800 acre-ft in 1956 to 802,800 acre-ft in 1992; the average for the period is 366,700 acre-ft and the median is 375,600 acre-ft.