Medina Lake Leakage to the Edwards Aquifer: A Better Estimate

By Richard N. Slattery, William H. Asquith and John D. Gordon, U.S. Geological Survey, with the San Antonio Water System. Fact Sheet 2017–3008, February 2017.

Figure 1: the upper Medina River basin, with Medina and Diversion lakes on the Edwards and Trinity aquifer outcrops. USGS.

The lakes and the aquifer

In 2016, the USGS and the San Antonio Water System began refining estimates of how much groundwater flows out of Medina and Diversion Lakes in south-central Texas. When full they hold about 255,000 and 2,555 acre-feet.

Most recharge to the Edwards aquifer comes from streams seeping into its outcrop — the recharge zone. But water leaking from the Medina/Diversion Lake system has long been known as another potentially important source.

Earlier estimates

• 1978: Puente published methods for estimating monthly and yearly recharge from the lakes.
• October 1995–September 1996: a USGS study measured short-term recharge rates and cut the uncertainty in monthly estimates (Lambert and others, 2000).
• 2017: Slattery and Miller built detailed water budgets for 1955–1964, 1995–1996 and 2001–2002 — periods when budget components were fairly stable, with little influence from rain, storm runoff or storage changes — and used linear regression to relate lake level to outflow.

The reanalysis

In 2016 the USGS reanalyzed the lake-level and outflow data in the R statistical software (version R-3.3.2); the data and scripts are published online (Asquith and Slattery, 2016).

• Result: a new log-log weighted least-squares regression — the "preferred regression equation" — published as an appendix to Slattery and Miller (2017).
• Better fit: it matches average outflows and lake levels better than the earlier equation.
• Mind the uncertainty: 75- and 90-percent prediction intervals give the range in which actual outflow at a given lake level is likely to fall that share of the time (Helsel and Hirsch, 2002).

Figure 2: average groundwater outflow against average Medina Lake stage for 1955–1964, 1995–1996 and 2001–2002, with earlier linear fits, the preferred log-log fit and its prediction intervals. The equations apply for lake levels from 963 to 1,064.2 feet. USGS.

Sources

Based on Refining Previous Estimates of Groundwater Outflows from the Medina/Diversion Lake System, San Antonio Area, Texas, by Richard N. Slattery, William H. Asquith and John D. Gordon, USGS Fact Sheet 2017–3008, U.S. Geological Survey; a work of the United States government in the public domain. The underlying analysis is Slattery and Miller, Scientific Investigations Report 2004–5209 (version 1.1, 2017), with data and scripts at doi:10.5066/F7ZS2TNF. Both figures are taken from the fact sheet's PDF, which the import had left out.