2009 SAC hydrologic methods of instream flow recommendations working draft

Environmental flows, which include flows in rivers and streams and freshwater inflows to bays and estuaries, have not been addressed uniformly in water development project planning and permitting in Texas. Senate Bill 3 (SB 3), passed by the Texas Legislature in 2007, set out a new regulatory approach to protect such flows through the use of environmental flow standards developed through a local stakeholder process culminating in Texas Commission on Environmental Quality (TCEQ) rulemaking. SB 3 directed the use of an environmental flow regime in developing flow standards and defined an environmental flow regime as a schedule of flow quantities that reflects seasonal and yearly fluctuations that typically would vary geographically, by specific location 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. This document provides an overview of how hydrologic data may be used in the identification of instream flow recommendations pursuant to the requirements of SB 3. As such, it describes one piece of the collaborative process envisioned by SB 3 for the identification of flows to maintain a sound ecological environment in rivers and streams.1 Other disciplines such as biology, geomorphology, and water quality, although not discussed directly in this report, also warrant specific attention to ensure that instream flow recommendations are based on the broadest set of information available. It is important to recognize that the provisions of SB 3 dealing with environmental flows are structured specifically to provide a mechanism for protecting certain levels of flow for environmental purposes while at the same time allowing for the use of surface water to meet other needs, including human water needs. The discussion in this document pertains only to the scientific aspects of establishing appropriate environmental flow requirements for river and streams and does not consider the needs of other users or uses for which surface water flows may be required. Section 2 of this document provides background information on relevant legislation, flow regime concepts, and hydrologic data. Section 3 highlights resources and methods that can be used to generate instream flow recommendations using hydrologic data. Section 4 introduces decisions that must be made when using hydrologic data to define flow recommendations.2 Clarifying examples are provided throughout this document to provide context to the reader. Such examples are solely intended to illustrate the types of factors that could be considered and should not be construed as recommendations. This document originally was prepared by the Texas Parks and Wildlife Department (TPWD) at the request of the SB 3 Science Advisory Committee (SAC), with comments from the Texas Water Development Board (TWDB) and TCEQ. Members of the SAC have reviewed, edited, and expanded the document and have provided recommendations regarding the application of the information and procedures presented in the document pursuant to the requirements of SB 3.