2009 SAC freshwater inflow regime methods

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, 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 Texas Commission on Environmental Quality (TCEQ) rulemaking. Senate Bill 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. Each Basin and Bay Expert Science Team (BBEST) is charged with developing recommendations for both an instream flow regime and for a complete freshwater inflow regime to protect a �sound ecological environment� and to maintain the productivity, extent, and persistence of key aquatic habitats in bays and estuaries. Instream flow regime requirements have been addressed previously (SAC, 2009a),1 by the Texas Environmental Flows Science Advisory Committee (SAC). The focus of this document is on bay and estuary inflows. This regime will have to be developed recognizing the inherent variability in weather and inflow conditions that have contributed to and sustained these productive estuarine ecosystems over time. Freshwater inflow serves a variety of important functions to coastal estuarine ecosystems by creating and preserving low-salinity nurseries, transporting sediments, nutrients, and organic matter downstream, and affecting estuarine species movements and reproductive timing (Longley 1994, Montagna et al. 2002; SAC, 2004). This document provides background information and discussion of various methods that can be used to develop freshwater inflow recommendations for Texas bays and estuaries. While a few germane references to the literature are made, this document is not intended to be a tutorial on the physics and ecology of estuaries, nor on the range of modeling techniques of potential application. Rather, it attempts to present a succinct summary of methods that are presently sufficiently developed and suitable for application to Texas estuaries, for consideration by the Basin and Bay Expert Science Teams (BBESTs). For detailed background information on estuaries and the coastal environment, the 2004 Science Advisory Committee (formed under Senate Bill 1639) report (SAC, 2004) and citations therein should be consulted. Emphasis here is placed upon delineating the basic approaches of available methods, identifying the necessary supporting data and analyses, and stating their strengths and weaknesses. Section 2 reviews briefly the guidance offered by previous state scientific advisory committees regarding what constitutes a �sound ecological environment� and how that might apply in the bay and estuary context, particularly with regard to flow regimes as noted in Senate Bill 3. Section 3 identifies various sources of available hydrologic, abundance, habitat, salinity and water quality data for Texas bays and estuaries and discusses existing tools that constitute the �State Methodology�, and how they have been used for evaluating these data in the context of establishing appropriate freshwater inflow needs. Various methods for using the available data to develop freshwater inflow recommendations are described in Section 4, with key decision points involved in selecting and applying the various inflow methodologies briefly highlighted along with each method�s strengths and weaknesses. Other considerations, including the role of nutrient and sediment delivery in sustaining the ecological environment of bays and estuaries and the issue of how instream flow recommendations might be integrated with freshwater inflow recommendations in a particular basin, are discussed in Section 5. Finally, SAC observations and recommendations regarding information presented in the document and how freshwater inflow recommendations for the bays and estuaries could be established within the scope and timeframe of Senate Bill 3 are summarized in Section 6. References cited in the text of the document are listed in Section 7, and a list of contributors is presented in Section 8.