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Estimates of annual recharge to the Edwards aquifer in the San Antonio area, Texas, have been made by several investigators as part of the long-term studies of the aquifer by the U.S. Geological Survey under agreements with several local cooperators .

The method of estimating annual recharge is based on data collected from a network of stream-gaging stations and on assumptions related to applying the runoff characteristics from gaged areas to ungaged areas.

The basic approach is a water-balance equation, in which recharge within a stream basin is the difference between measured streamflow above and below the infiltration area of the aquifer plus the estimated runoff in the zone that includes the infiltration area. Recharge in the Medina River basin also includes seepage losses from Medina Lake and Diversion Reservoir.

Gaging stations are located above and below the infiltration area in five of the nine basins that have .been delineated. Gaging stations above and below the infiltration area are located in parts of the other four basins. Parts of two basins have stations only at points below the infiltration area, and parts of the other two basins have no gaging sites at all.

The principal errors in the estimates of annual recharge are related to errors in estimating runoff in ungaged areas, which represent about 30 percent of the infiltration area. The estimated long-term average annual recharge in each basin, however, is probably representative of the actual recharge because the averaging procedure tends to cancel out the major errors.