Recharge to and Discharge from the Edwards Aquifer in the San Antonio Area, Texas, 1997

-- R.N. Slattery, J.T. Patton, and D.S. Brown

In cooperation with the Edwards Aquifer Authority

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, EAA, 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 (1) spring-discharge data collected by the USGS; (2) pumpage data for public water supply, industry, and the military reported by pumpers to the Texas Water Development Board, EAA, and USGS; (3) pumpage data for irrigation wells obtained from the Nueces-Frio-Sabinal Soil and Water Conservation District, Medina Valley Soil and Water Conservation District, and U.S. Department of Agriculture, in conjunction with irrigated-acreage data supplied by the U.S. Department of Agriculture; (4) irrigated-acreage estimates from the Bexar-Medina-Atascosa Counties Water Control and Improvement District No. 1; and (5) pumpage for domestic supply, stock, and miscellaneous use estimated by the USGS. Average daily and total annual discharge by county and by water use ( table 3) are computed by the USGS. (No irrigation discharge for Bexar, Medina, and Uvalde Counties for 1997 is included in tables 2 and 3 because data to estimate that discharge are not available.)

Recharge
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.

The estimated annual recharge for 1997 is 1,135,000 acre-feet (acre-ft). The estimated annual recharge for 1934-97 ( table 1 ) ranges from 43,700 acre-ft in 1956 to 2,486,000 acre-ft in 1992. The average and median estimated annual recharge for 1934-97 are 676,000 and 547,100 acre-ft, respectively.

Discharge
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 1997 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 (which was not estimated for 1997). The remaining discharge (primarily from wells in Bexar County) in 1997 supplied industry, domestic uses, stock, and miscellaneous uses.

The estimated annual discharge (excluding irrigation discharge for Bexar, Medina, and Uvalde Counties) from wells and springs during 1997 is 684,700 acre-ft. The estimated annual discharge from wells and springs for 1934-97 ( table 2 ) ranges from 388,800 acre-ft in 1955 to 1,130,000 acre-ft in 1992. The 1934-97 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 (325,200 acre-ft) accounted for about 85 percent of spring discharge during 1997. The remaining spring discharge was from Hueco Springs in Comal County, San Pedro 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 1934-97 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 363,700 acre-ft and the median is 372,600 acre-ft.

Reference
Puente, Celso, 1978, Method of estimating natural recharge to the Edwards aquifer in the San Antonio area, Texas: U.S. Geological Survey Water-Resources Investigations Report
78-10, 34 p.

Information on technical reports and hydrologic data
related to this and other studies can be obtained from:
Subdistrict Chief
U.S. Geological Survey
435 Isom Road, Suite 234
San Antonio, TX 78216
Phone: (210) 321-5200
FAX: (210) 530-6008
Email: gbozuna@usgs.gov

U.S. Geological Survey in Texas