Water-budget analysis of the Medina and Diversion Lake system, with estimated recharge to the Edwards aquifer and the upper zone of the Trinity aquifer, Bandera, Bexar, and Medina Counties, Texas, 1955–2022

The U.S. Geological Survey—in cooperation with the San Antonio Water System and the Edwards Aquifer Authority—used data collected during four different periods (March 1955–August 1964, October 1995–September 1996, March 2001–June 2002, and March 2017–October 2022) as part of a new study to refine previously derived relations between the altitude of the water surface of Medina Lake and recharge to the Edwards aquifer and the upper zone of the Trinity aquifer in the form of seepage losses from Medina Lake and the immediately downstream Diversion Lake. Any seepage losses that occur within the conservation pools of Medina and Diversion Lakes infiltrate the Edwards aquifer and the upper zone of the Trinity aquifer as recharge. To quantify recharge to the Edwards aquifer and the upper zone of the Trinity aquifer from Medina and Diversion Lakes, daily water budgets were used to calculate monthly and annual recharge (method 1). A new statistical analysis culminated in a new log-log weighted least-squares (WLS) regression equation that relates recharge from Medina and Diversion Lakes to the Medina Lake stage. Recharge estimates obtained by using the new log-log WLS regression equation (method 2), as well as the recharge estimated by using a method published in 1978 (referred to as the “Puente method”) (method 3), were compared with the calculated recharge during March 2017–September 2022. During March 2017–September 2022, the WLS estimated recharge was 224,310 acre-feet, 0.5 percent less than the calculated recharge of 225,400 acre-feet. The Puente method estimated recharge was 342,080 acre-feet, about 52 percent more than the calculated recharge. The analysis of the three methods indicates that WLS estimated recharge provides a more accurate accounting of actual recharge to the Edwards aquifer and the upper zone of the Trinity aquifer compared to the Puente method.