2009 SAC geographic scope of instream flow recommendations

Date

4/3/2009

Authors

Brandes, Robert
Heitmuller, Franklin
Huston, Robert
Jensen, Paul
Kelly, Mary
Manhart, Fred
Montagna, Paul
Ward, George
Wiersema, James

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Abstract

The purpose of this paper is to address the geographic scope of environmental flow regime recommendations for the Senate Bill 3 (SB 3) process. As defined by SB 3, an environmental flow regime means a schedule of flow quantities that reflect seasonal and yearly fluctuations that typically would vary geographically, by specific locations in a watershed, and that are shown to be adequate to support a sound ecological environment and to maintain the productivity, extent, and persistence of key aquatic habitats in and along the affected water bodies and receiving coastal bay system. The question of geographic scope for the SB 3 process is a matter of recommending the number and spatial distribution of locations where flow regime recommendations will be developed. This document does not address implementation of flow recommendations. In contrast to the SB 3 process, environmental assessments of water rights applications are currently conducted on a case by case basis. Numerous sources are utilized to determine whether special conditions, including streamflow restrictions, are necessary to satisfy environmental concerns. If streamflow restrictions are recommended, an effort is made to tie the restrictions to a nearby active USGS gage. However, this is not always possible or practical. For a diversion or project on an ungaged stream, an effort is made to locate an active gage with at least 20 years of flow data in the same watershed or an adjacent watershed. In addition to proximity, factors such as stream characteristics, ecological characteristics, hydrological characteristics (e.g. whether the gage and project location are influenced by wastewater discharges or reservoir releases), and drainage area size may be taken into consideration in selection of an appropriate gage. Once a gage has been selected and a historical period of record determined, streamflow restrictions for the gaged location are calculated based on median flow values for each month of the year. The resulting monthly restrictions are then prorated to the project location using a drainage area ratio. If streamflow restrictions are placed in the permit, the permittee is responsible for developing a method or installing a reference device to measure the appropriate flow value. There are a number of issues that should be considered in determining the geographic scope of instream flow recommendations for the SB 3 process. For example, flow recommendation locations should consider, and be compatible with, the river segments identified by the Texas Instream Flow Program (TIFP) for Senate Bill 2 (SB 2) studies. In addition, biologic, hydrologic, and geomorphic information, water quality segments, basin management subdivisions and water availability could play a role in these location determinations. There are a broad range of aquatic ecosystems in Texas� rivers and the study methodology for instream flow recommendations may need to be customized for specific river systems (TCEQ et al., 2008). Thus for each river basin, the choices may be different. The document includes general information that could be used to segment a river basin for purposes of determining the number of flow recommendation locations that might be needed to characterize an environmental flow regime in a particular basin. Available information includes the spatial scale units adopted by the TIFP and other general information, such as hydrology, geomorphology, biology and water quality that are available for all river basins and should be considered in location determinations. The document also describes basin specific information that could be added to the general information, or used to enhance this information, using the Trinity River Basin as an example application. The summary outlines a general process for determining geographic scope.

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Citation

Science Advisory Committee. 2009b. Geographic Scope of Instream Flow Recommendations. Report # SAC-2009-02, April 3, 2009. Available at https://www.tceq.texas.gov/assets/public/permitting/watersupply/water_rights/eflows/geographicscope.pdf

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