TR-513 Daily Water Availability Model for the Brazos River Basin and Brazos-San Jacinto Coastal Basin

Authors: Ralph Wurbs

The monthly Brazos WAM consists of the generalized Water Rights Analysis Package (WRAP) and input data for the Brazos River Basin and adjoining coastal basin from the statewide TCEQ Water Availability Modeling (WAM) System. The Brazos WAM simulates water resources development, allocation, management and use in accordance with the over 1,200 water rights permits in effect for the basin. Operations of 680 reservoirs are simulated. The daily version of the Brazos WAM documented by this report was created by converting the monthly WAM to daily, adding routing parameters for 67 selected river reaches, flood control operations of 19 Corps of Engineers reservoirs, and SB3 environmental flow standards at 19 gage sites. The hydrologic period-of-analysis was updated to extend from January 1940 through December 2017. Monthly naturalized stream flows at 77 primary gaged control points are distributed to over 3,000 ungauged control points within the simulation. Monthly naturalized flows are disaggregated to daily based on daily pattern hydrographs at 58 gaging stations. This report accompanies the WRAP simulation input files for daily and monthly versions of the Brazos WAM and relevant auxiliary data files.

The Brazos WAM represents the inaugural application of the expanded daily modeling capabilities incorporated in the July 2018 and May 2019 versions of WRAP. The Brazos case study development and application of a daily WAM contributed to improvements in the generalized WRAP modeling system. The first half of this report focuses on development of the daily Brazos WAM. Latter chapters explore case study comparative analyses of the various features of the simulation model and alternative options for performing different tasks. Monthly WRAP/WAM modeling is complex, and daily modeling is much more complex. This report provides guidelines and sets of recommended optional methods for developing manageable and effective strategies for employing the daily modeling system that are generally applicable for any river basin.

Daily WRAP/WAM modeling and analysis capabilities can significantly contribute to various types water management endeavors. The work documented by this report focuses on improving capabilities for incorporating Senate Bill 3 (SB3) environmental flow standards (EFS) in the TCEQ WAM System. A strategy is demonstrated in which daily instream flow targets for SB3 EFS are computed and summed to monthly quantities within the daily SIMD simulation for input to the monthly SIM simulation model. The monthly SIM simulation model is applied with the SB3 EFS modeled as instream flow IF record water rights with targets defined as target series TS records stored in an input file. Both a daily WAM dataset and a monthly WAM dataset with SB3 EFS added in this manner accompany this report.

Different strategies for employing the expanded WAM will be useful for different types of applications. With the strategy explored in this report, after SB3 EFS targets are established with the daily WAM, routine modeling applications employ the monthly WAM. SB3 EFS set-asides are incorporated in the monthly WAM, appropriately reducing the quantities of stream flow available for further appropriation by junior water users. The daily WAM can be employed directly in many other types of studies with input data varied in alternative daily SIMD simulations to explore various water management strategies and issues. For example, the daily model can be applied directly in the formulation, evaluation, and improvement of environmental flow standards to assess capabilities (reliabilities) of satisfying proposed alternative sets of flow standards. The daily simulation modeling capabilities can also support various types of studies in which operation of reservoirs during and after floods is a significant concern.