Estimated Recharge to the Edwards Aquifer in the
San Antonio Area, Texas, 1934–2007

Method for Estimating Recharge

Reference

Figure

Table

The Edwards aquifer is one of the most permeable and productive aquifers in the United States and is the major source of public water supply for Bexar, Comal, Hays, Medina, and Uvalde Counties. The Edwards aquifer also supplies large quantities of water to agriculture, business, and industry in the region. The major artesian springs of the Edwards aquifer provide water for recreational activities, businesses, and downstream users, and provide habitat for several threatened or endangered species.

The areas pertinent to the recharge process are the catchment area and the recharge zone (fig. 1). The catchment area comprises the parts of stream basins upstream from the recharge zone. Streams in the catchment area generally gain water (hence the term “catchment area”) as they flow toward the recharge zone. As the streams flow across the recharge zone, which is essentially the outcrop of the Edwards aquifer, they lose substantial amounts of water through infiltration into the highly permeable, faulted and fractured rocks of the outcrop. Direct infiltration of rainfall also occurs in the interstream areas of the recharge zone.

Figure 1.  Gaged stream basins and ungaged areas pertinent to estimation of annual recharge to the Edwards aquifer in the San Antontio area, Texas (modified from Puente, 1978, fig. 1).

Method for Estimating Recharge

Annual recharge estimates (table 1) are based on data collected from a network of U.S. Geological Survey (USGS) streamflow-gaging stations at sites upstream and downstream from the recharge zone (fig. 1) and on assumptions related to applying runoff characteristics from gaged areas to ungaged areas (Puente, 1978). Runoff is affected by the distribution of rainfall, which historically has been estimated from a network of rain gages, some operated by the USGS, some by the Edwards Aquifer Authority (EAA), and some by the National Weather Service. Since 2004, the distribution of rainfall has been estimated from calibrated NEXRAD (NEXt-generation weather RADar) data provided by the EAA. The annual estimation of recharge to the Edwards aquifer is part of a continuing program of the USGS in cooperation with the EAA.