South Central Texas Regional Water Planning Group
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General Assumptions for Applications of Hydrologic Models

Following are general assumptions for applications of hydrologic models in the evaluations of water supply options and regional water plans for the South Central Texas Regional Water Planning Group. Pertinent exceptions to, or clarifications of, these general assumptions will be included in the scope of work for each option presently identified for study.


  • Full exercise of surface water rights.
  • Edwards Aquifer pumpage of 400,000 acft/yr with Critical Period Management rules.
  • Subordination of all senior Guadalupe River hydropower permits to Canyon Reservoir.
  • Annual effluent discharge / return flows reported for 1988 with SAWS direct reclaimed water use of 35,000 acft/yr.
  • Operation of power plant reservoirs (Coleto Creek, Braunig, and Calaveras) subject to authorized consumptive uses at the reservoir, with makeup diversions as needed to maintain full conservation storage subject to instream flow constraints and/or applicable contractual provisions.
  • Delivery of GBRA's full contractual obligations from Canyon Reservoir to point of diversion in all years. Uncommitted balance of Canyon Reservoir currently authorized annual diversions, and additional diversions proposed under an amendment presently before TNRCC, to be diverted near Lake Dunlap.
  • Desired San Antonio River flows at Falls City gage of 55,000 acft/yr. Minimum desired instream flows under current SAWS/SARA/CPS agreement included.
  • Application of Environmental Water Needs Criteria of the Consensus Planning Process in consideration of water potentially available for diversion and/or impoundment as part of a new water supply project.
  • Operation of Choke Canyon Reservoir/Lake Corpus Christi (CCR/LCC) System subject to Phase 4 (maximum yield) policy and TNRCC Agreed Order regarding freshwater inflows to the Nueces Estuary.
  • Historical Edwards Aquifer recharge estimates developed by HDR.
  • Applicable rules of groundwater management districts will be included to the extent possible.
  • Period of record for simulations: Guadalupe - San Antonio River Basin (1934-89, Critical Drought = 1950s), Nueces River Basin (1934-96, Critical Drought = 1990s), Colorado River Basin (1941-65, Critical Drought = 1950s).


Hydrologic Models expected to be applied include, but are not limited to:

  • Guadalupe - San Antonio River Basin Model (HDR)
  • Nueces River Basin Model (HDR)
  • Lower Nueces River Basin & Estuary Model (HDR)
  • Guadalupe - San Antonio River Basin Water Availability Model (WRAP) (TNRCC/HDR)
  • Nueces River Basin Water Availability Model (WRAP) (TNRCC/HDR)
  • Colorado River Daily Allocation Program (DAP) (LCRA)
  • Edwards Aquifer (Balcones Fault Zone) Model GWSIM4 (TWDB)
  • Carrizo-Wilcox Aquifer Model (TWDB/LBG-G/HDR)
  • SIMYLD, RESOP, & SIMDLY (TWDB/TDWR)
  • OASIS (WRMI/UT)