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This report presents the results of hydrologic and hydraulic analyses for the Medina Lake and Medina Diversion Lake reservoir system. This engineering analysis was conducted in order to determine the availability of surface water from these reservoirs under various operating scenarios for the purpose of recharge to the Edwards Aquifer. Also included in the project objective was an engineering analysis of the hydraulic capacity of each of these facilities under a variety of flood conditions.

The Medina Lake inflows, recharge and seepage curves and historical diversions were used in the “reported diversions” Scenario 1 with a starting elevation at 1,064.1 feet NGVD. The resulting end-of-month elevations and contents were then compared to the actual historical values for Medina Lake given in Tables 3-2 and 3-3 (TWC, 1980 and US WEST, 1988). A plot of the end-of-month elevations in Figure 3-3 shows that the actual elevation values for the 1940 to 1957 period were lower than the elevations given by the computer simulation for Scenario 1.

This result was investigated further by re-running Scenario 1 with the starting elevation at the historical level in January 1940. The results were similar to those above. However, since the elevations for the historical and simulated cases were very close for the 1957 to 1986 period (when the inflows and canal diversions were based on more reliable, gaged information), it was concluded that the recharge and seepage  relationships discussed in Section 3.2 were providing remarkably close results to those observed historically, and could be adopted for use without further evaluation.

The Medina Lake inflows were then verified using a statistical correlation between the Guadalupe River Comfort gage and the Medina River Pipe Creek gage for the 39.5 years of data available at both gages. This analysis predicted a linear relationship between the flows at these gages with an R-squared value of 0.72. Since the average inflows calculated using the statistical equation discussed in this paragraph were larger than the average inflows calculated using the drainage area calculations described above, the original inflows were judged to be the most conservative and appropriate for use in this study. In addition, no justification could be found for lowering the initially computed inflow values.

One of the primary differences between the reservoir operations discussed in this report and the actual historical operation was the criteria used for diversions. Historically, the diversions were made for actual year-to-year irrigation requirements and were not directly dependent on the lake elevation for either the diversion rate or amount. When the scenarios evaluated in this report were operated, the diversion rates were based on the beginning of month elevation. In EH&A’s opinion, the historical diversions through the Medina Canal during the 1940-1955 time period were much higher than those reported by BMA. Therefore, this discrepancy in the historical diversions was assumed to be the source of the end-of-month elevation discrepancy found using Scenario 1 (vs. actual historical) and Scenario 1 was discarded and not used for further analyses.

Scenarios 2 through 12 were then performed as described in Section 8.0 in order to evaluate the differences and impacts which can be attributed to the various different operating criteria evaluated in Section 8.0 for recharge purposes.

The conclusions which may be drawn from the results of the yield model are as follows:

1) The optimum reservoir system operation for recharge is to maximize the recharge during wet periods, sustain a constant level of recharge during normal years and to accept no recharge diversions during dry years. This type of operation was illustrated by scenarios 9, 11 and 12 without any correction for leakage.

2) Enhancement of natural recharge by ceasing diversions and maximizing the storage in the reservoir did not, comparatively speaking, prove to be a viable alternative, as demonstrated by Scenario 2. The natural recharge increased by an amount of 8,528 acre-feet per year over the recharge computed by EH&A to have occurred historically.

3) There was no annual demand which could be withdrawn from the reservoir system through the critical drought of record without significant shortages, i.e., there is no firm yield. Even with zero diversions, Medina Lake was drawn down below the outlet levels for 17 consecutive months during the 1949 to 1957 drought, due to evaporation, natural recharge and leakage. This is demonstrated by Scenario 3.

4) Leakage correction at the Diversion Dam will net an additional 4,500 acre-feet per year when the recommended minimum flows are provided downstream.

5) Leakage correction at the Main Dam will not increase the amount of water available for recharge. The Main Dam leakage rate at full level is approximately equal to the Diversion Lake recharge rate, diversions and minimum flows.

6) The flows to the San Antonio Bay were not significantly affected by the different scenarios tested. The maximum difference in bay inflows was observed between Scenario 2, No Diversions, and Scenario 10, Artificial Recharge/Minimum Flows/Leakage Correction. Scenario 2 had a bay inflow of 1,552, 271 ac-ft per year while Scenario 10 had a bay inflow of 1,517,214 ac-ft per year. The difference of 35,057 ac-ft per year represents only 2.26% of the Scenario 2 inflows.

7) The hydroelectric system generation capability as assumed by Prodek in their analyses may be affected by altering the system operating criteria to a recharge operation specifically, due to significant differences in methodologies employed by Prodek in their studies and those developed in this study, Prodek’s analysis may give higher flows being available for hydroelectric power generation than this study would support even with no alteration in the system operating criteria. Also, the impact will vary greatly, depending upon which scenarios of operation are compared.

8) The natural recharge and leakage curves for Medina Lake and Diversion Lake given in Figures 3-1 and 3-2, respectively, should be adopted by the Edwards Underground Water District for future calculations of natural recharge from the lakes. Further studies should be performed including additional stream gaging as described in Appendix B.