5.2.4 Cumulative Effects

Sophisticated hydrologic models have been employed to quantify the cumulative effects of implementation of the South Central Texas Regional Water Plan through the year 2050. These cumulative effects are quantified through long-term simulation of natural hydrologic processes including precipitation, streamflow, aquifer recharge, springflow, and evaporation as they are affected by human influences such as aquifer pumpage, reservoirs, diversions, and the discharge of treated effluent. Cumulative effects of plan implementation on the Edwards Aquifer are measured against a baseline representative of full utilization of proposed permits prorated to a total of 400,000 acft/yr subject to Critical Period Management Rules without any additional recharge enhancement projects. Edwards Aquifer simulations with implementation of the Plan do not reflect the activation of available Management Supplies as may be necessary to offset Edwards Aquifer pumpage reductions necessary to maintain springflow. The baseline for consideration of effects on streamflow reflects the baseline for the Edwards Aquifer, full utilization of existing water rights, and treated effluent discharge representative of current conditions. Cumulative effects of plan implementation on Carrizo and Simsboro Aquifer levels are measured against a baseline of projected local pumpage.

The potential cumulative effects of plan implementation on Comal Springs discharge from the Edwards Aquifer are shown in Figure 5.2-26 for a 56-year historical simulation period. Springflows would increase much of the time and particularly in the summer due to Edwards Recharge – Type 2 Projects (L-18a) and SAWS Aquifer Storage & Recovery (ASR) Program in southern Bexar County (SCTN-1a), respectively. However, springflow increases would be offset to some degree by increased pumpage closer to the springs associated with Edwards Irrigation Transfers (L-15) and Irrigation Demand Reduction (Conservation) with Transfer (L-10 Irr.). As shown in Figure 5.2-27, simulated San Marcos Springs discharges would increase substantially because the Edwards Recharge – Type 2 Projects (L-18a) include a recharge enhancement dam on the Blanco River with pumped diversions to the outcrop in the Upper San Marcos River watershed. Overall pumpage from the Edwards Aquifer would increase (Figure 5.2-28) due to potential EAA authorizations for recharge recovery (see Appendix C in Volume III) pursuant to development of the Edwards Recharge – Type 2 Projects (L-18a). Figure 5.2-29 shows

simulated water levels at key monitoring wells in Uvalde, Medina, and Bexar Counties with implementation of the Plan. Percentages of time under Critical Period Management in Uvalde and Medina Counties would be less with the Plan than for baseline conditions.

The potential cumulative effects of phased implementation of water management strategies involving pumpage from the Carrizo Aquifer are summarized in Figures 5.2-30 through 5.2-36. Figure 5.2-30 shows the projected pumpage from Wilson, Gonzales, and Bastrop Counties associated with the following water management strategies: Carrizo Aquifer–Wilson & Gonzales (CZ-10C); Carrizo Aquifer – Gonzales & Bastrop (CZ-10D); and Schertz-Seguin Water Supply Project (SSWSP). Projected drawdown associated with CZ-10C and SSWSP is referenced to simulated 1994 aquifer levels and shown in plan view in Figure 5.2-31 along with monitoring well locations for the simulated well hydrographs presented in Figures 5.2-32 through 5.2-35. Note that projected drawdown shown in these figures is a result of both projected local demands and the development of two water management strategies in the Plan. Drawdown associated with CZ-10D in northern Gonzales County and southern Bastrop County, in addition to that associated with projected local demands, is shown in Figure 5.2-36.

Simulated cumulative effects of implementation of the Simsboro Aquifer (SCTN-3c) strategy in Milam, Lee, and Bastrop Counties are summarized in Figures 5.2-37 through 5.2-39. Projected drawdown associated with SCTN-3c between years 2000 and 2050 is shown in plan view in Figure 5.2-37. Figures 5.2-38 and 5.2-39 illustrate the simulated incremental effects on Simsboro Aquifer levels associated with local demands and mining operations (baseline) and the implementation of the Plan for the Aluminum Company of America (Alcoa) and San Antonio City Public Service (CPS) well fields.

Potential cumulative effects of implementation of the South Central Texas Regional Water Plan on streamflows at selected locations in the Guadalupe – San Antonio River Basin are summarized in Figures 5.2-40 through 5.2-42. Streamflow comparisons for the Guadalupe River at Cuero (Figure 5.2-40) and the San Antonio River at Falls City (Figure 5.2-41) indicate that streamflows are expected to increase with full implementation of the Plan. Increased streamflow at Cuero will be primarily due to Edwards Recharge – Type 2 Projects (L-18a) and the associated increases in Comal and San Marcos springflow. Note that average annual freshwater inflows to the Nueces Estuary will be reduced by approximately three percent due to enhanced recharge associated with Edwards Recharge – Type 2 Projects (L-18a). Increased streamflow at Falls City will be a direct result of net projected increases in treated effluent discharge associated with increasing water use and expansion of SAWS Recycled Water Program in Bexar County. Figure 5.2-42 shows increased streamflows (as compared to the baseline) in the Guadalupe River at the Saltwater Barrier in 2050. This is particularly evident during low streamflow periods.

Potential effects of implementation of the South Central Texas Regional Water Plan on streamflows in the Colorado River at Bay City are summarized in Figure 5.2-43. Results of statistical analyses of simulated streamflows from each of two potential Regional Water Sharing Alternatives proposed by the LCRA are presented in Figure 5.2-43. The Plan includes diversions from both Bastrop and Bay City totaling 150,000 acft/yr, which is the same annual diversion from the Colorado River as simulated by LCRA. Median streamflow in months during which irrigation use is limited or non-existent (October through March) may be reduced by more than 300 cfs once this management strategy is fully implemented in 2050.