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This memorandum presents various concepts for a second generation of environmental flow criteria for the Guadalupe – San Antonio River Basin to be used in the Trans-Texas Water Program. In Phase 1 of the Trans-Texas Water Program, preliminary environmental criteria as outlined in a document entitled “Trans-Texas Water Program Environmental Assessment” were used to determine water potentially available for most of the supply alternatives. Criteria for Instream Flows, Freshwater Inflows to Bays and Estuaries, and New Reservoirs were used to determine when surplus flows could be obtained by run-of-the-river diversions and when inflows to proposed reservoirs could be impounded. The results of the Phase 1 analyses for proposed reservoir projects generally showed that sufficient firm yield remained after honoring the criteria to consider the reservoir projects potentially viable with respect to firm yield. However, the results of the Phase 1 studies for most run-of-the-river diversion projects indicated that very little water, if any, would be available during drought, leading to the conclusion that run-of-the-river diversion projects are essentially infeasible with respect to firm yield under the existing preliminary Trans-Texas Environmental Criteria….

Major components of this evaluation of alternative environmental criteria included: 1) Preliminary statistical analyses to identify monthly flow-frequency relationships and drought contingency targets and triggers over a range of streamflow moving average durations; 2) Enhancement of the Guadalupe – San Antonio River Basin Model (GSA Model) to track streamflow moving averages and incorporate normal and drought monthly flow targets; 3) Performance of water availability analyses to assess potential effects on water supply alternatives; and 4) Presentation of modified streamflow statistics which reflect the effects of diversion of water available under alternative environmental criteria. Each of these components is addressed in the following sections of this memorandum. Evaluation of potential biological effects of implementation of environmental criteria is not within the scope of this study.