Recharge to the Edwards Aquifer originates as precipitation over the Contributing Zone and Recharge Zone of the Aquifer or as interformational flow from adjacent aquifers. The EAA maintains a joint funding agreement with the U.S. Geological Survey (USGS) to provide surface recharge estimates for eight of the nine major drainage basins with streams that flow on to the Edwards Aquifer Recharge Zone (Figure 1). Recharge is estimated using a water-balance method that relies on precipitation and streamflow measurements across the region. Based on the USGS methodology, the Guadalupe River basin does not appear to provide significant recharge to the Edwards Aquifer, so recharge is not estimated for that drainage basin by the USGS.
Estimated annual recharge by drainage basin for the period of record from 1934 through 2016 is based on USGS calculations. Estimates of total annual recharge ranged from 43,700 acre-feet at the height of the drought of record in 1956 to 2,486,000 acre-feet in 1992. In 2016, total estimated recharge was 1,221,100 acre-feet, which is significantly greater than the mean annual recharge of 706,500 acre-feet. The median annual recharge for the period of record is 557,800 acre-feet. The median value represents the amount of annual recharge that has a 50 percent chance of being exceeded in any given year.