Hydrologic Monitoring of Medina and Diversion Lakes, San Antonio Area

The Edwards aquifer is one of the most permeable and productive carbonate-rock (limestone and dolostone) aquifers in the United States and is the major source of public water supply for more than 1.85 million people in Bexar County. Most recharge to the Edwards aquifer occurs as seepage from streams as they cross the outcrop (recharge zone) of the aquifer. Groundwater outflows from the Medina and Diversion Lake (hereafter referred to as the Medina/Diversion Lake system) have also long been recog­nized as a potentially important additional source of recharge. A better understanding of the interaction between the Medina/Diversion Lake system and the Edwards aquifer, as well as estimating the recharge to the Edwards aquifer from these lakes, would aid water resource decision makers in better managing the systems.

The objective of this study is to develop a better understanding of the estimated groundwater recharge and quantify the associated uncertainty for the Medina/Diversion Lake system to the Edwards aquifer. The scope of the study is the Medina/Diversion Lake system, which includes Medina Lake and Diversion Lake, as well as the catchment area upstream of the USGS gaging station Medina River near Rio Medina, TX (08180500).

Phase 1:  Data Analysis

Earlier estimates of groundwater recharge were based on an ordinary least squares regression analysis. In Phase 1 of the study, three different regression methods were tested in an effort to identify a regression equation that more accurately models the relation between groundwater recharge from the Medina/Diversion Lake system and Medina Lake stage. These included:
   (1) a linear-linear ordinary least-squares (OLS) regression equation,
   (2) a linear-linear weighted least-squares (WLS) regression equation, and
   (3) a log-log weighted least-squares (log-log WLS) regression equation.

Results of these analysis are published as Appendix 1 to SIR 2004-5209, A water-budget analysis of Medina and Diversion Lakes and the Medina/Diversion Lake system, with estimated recharge to Edwards aquifer, San Antonio area, Texas. The results are also summarized in FS 2017-3008, Refining Previous Estimates of Groundwater Outflows from the Medina/Diversion Lake System, San Antonio Area, Texas.

Phase 2: Data Collection

Preliminary findings of Phase 1 of the study have identified periods where the relation between Medina Lake stage and groundwater recharge estimates are not well defined. Additional hydrologic data are needed to address data gaps and to validate the statistical relation developed as part of Phase 1.

To accomplish this, hydrologic data will be collected to complete the overall water-budget for the Medina/Diversion Lake system. The water-budget equation incorporates measurable components of inflow and outflow to solve for unknown gains or losses from the lake system. The measurable components include: surface water inflow and outflow, precipitation, evaporation, and change in storage. The unknown gains and losses are represented by the residual of all the measurable components—or water that is not accounted for. The overall water-budget equation for the Medina/ Diversion Lake system can be written as follows:

GWout = P – E + SWin – SWout – ΔS,

Where:
   GWout is groundwater outflow (loss from the lake system, assumed to recharge the groundwater system);  
   P is precipitation on the lake;
   E is evaporation from the lake surface;
   SWin is surface-water inflow to the lake;
   SWout is surface-water outflow from the lake; and
   ΔS is change in lake storage.

• Precipitation on the lake (P) and evaporation from the lake (E) will be measured at USGS gaging station Medina Lake Meteorological Station near Rio Medina, TX (293355098560601)
• Surface-water inflow (SWin) to the lake will be measured at two USGS gaging stations: Medina River above English Crossing near Pipe Creek, TX (08178980) and Red Bluff Creek at FM 1283 near Pipe Creek, TX (08179110)
• Surface-water outflows (SWout) from the lake will be measured at USGS gaging station Medina River near Rio Medina, TX (08180500)
• change in lake storage (ΔS) will be measured using a stage-storage rating for the USGS gaging stations Medina Lake near San Antonio, TX (08179500) and Diversion Lake near Rio Medina, TX (08180010)