The majority of recharge to the Edwards Aquifer originates as precipitation over the Drainage Area and Recharge Zone. The EAA maintains a joint funding agreement with the U.S. Geological Survey (USGS) to provide surface recharge estimates for eight of the nine delineated drainage areas 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 for each drainage basin for the period of record from 1934 through 2018 is based on USGS calculations. Estimated annual recharge has ranged from a low of 43,700 acre-feet, at the height of the drought of record in 1956, to a high of 2,486,000 acre-feet in 1992. In 2018, total estimated recharge was 1,100,000 acre-feet, which is significantly greater than the average annual recharge of 709,000 acre-feet.
Annual reports summarizing historical surface recharge data can be found on the Hydrologic Data Reports page.
Some recharge may also occur as interformational flow from adjacent aquifers. Although interformational flow is difficult to quantify, it probably accounts for less than 10 percent of recharge on average, but may be a more significant portion of the total recharge during drought years. An ongoing interformational flow study will attempt to better quantify this portion of recharge.