MATAGORDA BAY FRESHWATER INFLOW NEEDS STUDY Lower Colorado River Authority Texas Commission on Environmental Quality Texas Parks and Wildlife Texas Water Development Board August 2006 EXECUTIVE SUMMARY Matagorda Bay Freshwater Inflow Needs Study Introduction The primary purpose of this study is to reassess the freshwater inflow needs for Matagorda Bay based on more than eight years of new data collected since the completion of the 1997 Freshwater Inflow Needs Study. The earlier study was based on five years of data collected after the U.S. Army Corps of Engineers (USACE) opened a diversion channel in 1991 from the Colorado River into Matagorda Bay to increase freshwater inflows entering into the bay. The current study also reviews and modifies some of the 1997 study methodologies and assumptions. The results of this study indicate that higher freshwater inflows are needed to achieve the Target and Critical inflow needs than indicated in the 1997 study. This is largely due to the availability of additional, more variable data collected over a longer period of time. Both the 1997 study and the current study represent a joint effort of the Lower Colorado River Authority (LCRA), the Texas Parks and Wildlife Department (TPWD), the Texas Water Development Board (TWDB), and the Texas Commission on Environmental Quality (TCEQ). Each study partner was represented on the study advisory committee and guided by a joint memorandum of agreement and scope of work (See Appendix A). A representative of the Lavaca-Navidad River Authority also attended the advisory committee meetings. The advisory committee meetings were open to the public and development of the study involved public meetings and workshops, a stakeholder process and public review and comment on the draft study. Improvements in the Current Study The current study builds and improves upon several areas of the 1997 study. Flow, salinity, and biological productivity data have been extended beyond what was used in the1997 study and are ii applied here. This includes the availability and use of independent fisheries data collected by TPWD rather than the reliance on commercial harvest data. As a result, improved equations have been developed for relating flow to salinity and to productivity. Biological equations for shrimp have also been improved resulting from more careful consideration of the influence of inflows on shrimp during the juvenile life stage. Sensitivity analyses were conducted on the TXEMP optimization model to provide better insight on the influence of constraints and other factors on the model results. As a result, limits on inflow and biological constraints were revised from the 1997 study. In addition to developing Critical and Target Flows as in the 1997 study, the current study presents options for use of TXEMP model results to develop intermediate flow solutions for use as a transition between the Critical and Target inflow needs. Model calibration of the TXBLEND hydrodynamic and salinity transport model benefited from additional data, and multi-year simulations were conducted in this study and provide better confidence in the model behavior. The 1997 report identified a number of areas that would benefit from additional study. However, some areas could not be extended in this study due to lack of additional new data, including nutrients (related to primary productivity, offshore concentrations, loadings, and sediment), benthic dynamics and sedimentation needs of Lavaca Bay and East Matagorda Bay. In addition to the need to continue developing additional data and analysis, more information is needed on existing habitat conditions and their relationship to biological productivity. A comprehensive assessment of factors that may affect the overall ecological health of the bay is also being conducted as a part of the LCRA-SAWS Water Project (LSWP) Bay Health Study. The improvements and modifications to the State Methodology being done for this study in combination with the more comprehensive look at overall bay health and productivity by the LSWP Bay Health Study are expected to contribute to the development of improved methods and tools for assessing freshwater inflow needs for Matagorda Bay. iii Target Flows and Critical Flows Beginning with the 1997 study, two freshwater inflow needs (FINS) have been identified: the “Target” freshwater inflow need based on the application of the State Methodology; and the “Critical” freshwater inflow need. The “Target” freshwater inflow need seeks to optimize selected estuarine species productivity; the “Critical” freshwater inflow need is intended to provide a fishery sanctuary habitat during drier periods from which finfish and shellfish species are expected to recover and repopulate the bay when more normal weather conditions return. The monthly Target flow needs estimated by the current study by source are summarized in Table ES.1. Monthly Target flows range from 480,100 acre-feet in May to 111,700 acre-feet in August. Compared to the 1997 study results, these results represent a maximum monthly increase of 267,600 acre-feet of freshwater inflows in the month of February to a maximum monthly decrease of 181,500 acre-feet of water in the month of May needed to deliver the Target inflow necessary to optimize selected estuarine species productivity. Table ES.1 Monthly Target Flow Needs by Source. Colorado Lavaca Other Total Monthly Month (1,000 ac-ft) (1,000 ac-ft) (1,000 ac-ft) (1,000 ac-ft) January 205.6 77.0 37.2 319.8 February 194.5 68.9 44.5 307.9 March 63.2 15.6 42.3 121.1 April 60.4 30.3 51.1 141.8 May 255.4 139.4 85.3 480.1 June 210.5 86.0 80.2 376.7 July 108.4 29.2 66.4 204.0 August 62.0 18.3 31.4 111.7 September 61.9 37.3 107.2 206.5 October 71.3 42.9 100.7 214.9 November 66.5 23.0 47.4 136.9 December 68.0 24.9 35.7 128.7 During the 1997 study, 25 parts per thousand (ppt) was established as the desired maximum salinity level during drought or extended dry periods. This criterion was based on review of species tolerances with particular attention to the Eastern Oyster and its predators. The Eastern Oyster was iv chosen not only because it is a commercially important species, but because it cannot migrate to areas with lower salinity ranges to avoid predators. The 25 ppt salinity level is also the approximate dividing line between brackish estuarine waters and more saline marine waters and, thus, an appropriate threshold to use in determining whether estuarine species may become significantly affected. Based on additional data and analysis, Critical inflow needs for the Colorado River increase from 14,260 acre-feet of water per month to 36,000 acre-feet of water per month over those calculated in the 1997 study. Lavaca-Navidad Critical inflow needs increase from 2,260 acre-feet of water per month to 4,290 acre-feet per month. Corresponding increases would also be needed from the other coastal basins that feed into West Matagorda Bay. The primary reasons for the increases in Critical and Target inflow needs from the 1997 study include the following: • More accurate statistical relationships based on additional data collected over a longer period of time since the USACE diversion channel was completed in 1991. For example, data was collected on a more continuous basis and data collection included fewer gaps and was subject to less frequent equipment failures. • More variability of wet and dry years and corresponding salinities was captured by the statistical relationships; and • Increased monthly inflow constraints were selected for use in the TXEMP modeling. Historic flows in the 70th percentile replaced the historic monthly mean used in the 1997 study or the monthly median flow (50th percentile monthly or median monthly historic flow), which has been widely used in other Texas estuaries. This constraint increase was necessary to allow the model to be solved without exceeding the selected salinity and biological productivity constraints. Consistent with the 1997 study, the 2006 update continued to use the same salinity regimes and rationale for Critical and Target inflows, measurement locations, State optimization and hydrodynamic and conservative transport models (TXEMP and TXBLEND), target species (with v one exception), and the relative weighting of species, as the 1997 study. In contrast to the 1997 study, the 2006 study also explored a range of intermediate inflow regimes that may be considered as a transition from Target to Critical inflow regimes. Both the 1997 and the 2006 studies used the State Methodology, with some modifications. The State Methodology includes the use of the TXEMP model, a general purpose optimization model to find a minimum or maximum function (e.g., biological productivity) that satisfies constraints that are imposed (hydrology, salinity, biological). TXEMP results are bounded by the constraints. The State Methodology also includes the use of the TXBLEND model, a hydrodynamic and conservative transport model specific to water bodies such as lakes or bays. The conservation transport aspect of the model looks at the movement of salt through the water body. The TXBLEND model is used to verify that the inflow regime results identified by the TXEMP model as meeting all constraints will also provide the desired salinity regime throughout the Matagorda Bay system. While the logic and equations of the optimization model itself are built on mathematical and engineering analyses, application of the model requires the inclusion of operative constraints, limits, and resource management objectives. These objectives are based more on policy than science and engineering. Other policy decisions include: the indicator species to be used, the relative weighting of the species; and the selection of inflow-response equations and inflow constraints (Longley, ed., 1994).