Freshwater Inflow Needs of the Matagorda Bay System

Introduction

The Matagorda Bay system is the second largest estuary on the Texas Gulf Coast covering approximately 352 square miles. The abundant production of finfish and shellfish make this environmentally sensitive area important not only as a ecological resource, but also as a source of economically significant commercial and sports fisheries. Many factors contribute to this high natural productivity, but the most significant is an ample source of freshwater. Freshwater inflows are vital to the continued health of the natural ecosystems in and around the Matagorda Bay system.

To determine the freshwater inflow needs of the Matagorda Bay system, the LCRA entered into a cooperative agreement with TPWD, TWDB and TNRCC in 1993. The LCRA agreed to adapt or modify existing methods for estimating freshwater inflow needs used by the TPWD and TWDB and apply those methods to compute alternative freshwater inflow needs for the estuary. The participating state agencies provided technical assistance and advice to the LCRA.

Methodology for Estimating Freshwater Inflow Needs

This method involved the synthesis of three components: (1) development of statistical relationships between freshwater inflows and key indicators of estuarine conditions, (2) computation of monthly and seasonal freshwater inflows to optimize estuarine conditions subject to specific constraints at key estuarine locations and (3) evaluation of estuarine-wide salinity conditions to ensure conditions remain within desired limits.

The first major component is the development of statistical relationships for the varied and complex interactions between freshwater inflows and important indicators of estuarine ecosystem conditions. The key estuarine indicators considered are: salinity, species productivity, and nutrient inflows.

Statistical relationships were developed between seasonal freshwater inflows and biomass for nine finfish and shellfish species that are ecologically and economically important to the estuary. In general, most species demonstrated negative responses to freshwater inflows during winter months (November through February), and positive responses to freshwater inflows occurring from March through October.

The salinity conditions in upper Lavaca Bay and the eastern end of Matagorda Bay were found to be largely dependent on the freshwater inflows from the Lavaca and Colorado Rivers, respectively. These relationships were quantified into statistical relationships.

Similarly the nutrient inflows were related to total inflow to the estuary. A nutrient budget was prepared for the estuary which indicated that a minimum annual freshwater inflow of 1.7 million acre-feet was needed to replenish the estimated nutrient losses from the estuary.

The second essential process involves using the statistical functions noted above to compute optimal monthly and seasonal freshwater inflow needs. This is accomplished using the TWDB's Texas Estuarine Mathematical Programming (TXEMP) Model. TXEMP determines mathematically the best set of freshwater inflows needed to maximize specific conditions within the estuary while meeting a variety of limits on salinity, species productivity and nutrient inflows.

The third major component of the process of developing inflow needs is the simulation of the salinity conditions throughout the estuary using the TXBLEND estuarine hydrodynamic and salinity transport model developed by TWDB and modified by the LCRA. The simulated salinity is then compared to desired salinity ranges over broad areas of the estuary. If salinity is not within those ranges then constraints in TXEMP are modified to achieve the desired salinity.

Freshwater Inflow Needs

The freshwater inflow needs for the estuarine ecosystem associated with Matagorda Bay System were estimated for two levels of inflow needs: Target and Critical.

The Target inflows needs are the monthly and seasonal inflows that produced 98% of the maximum total normalized biomass for nine key estuarine finfish and shellfish species while maintaining certain salinity, population density and nutrient inflow conditions. The salinity condition requires that estimated salinity fall within predetermined monthly ranges preferred by most species. The productivity of any species must not be less than 80% of its historical average. Finally, the total inflow of nutrients are at least equal to the natural nutrient losses from the ecosystem. The 98 percent level of maximum biomass was selected for the target needs based on achieving the best tradeoff between productivity and freshwater inflows.

The Critical inflow needs were determined by finding the minimum the total annual inflow needed to keep salinity near the mouths of the Colorado and Lavaca Rivers at no more than 25 parts per thousand. These inflows needs are termed critical since they provide a fishery sanctuary habitat during droughts. From this sanctuary, the finfish and shellfish species, particularly oysters, could be expected to recover and repopulate the bay when more normal weather conditions returned.

The Target inflow need from all sources was calculated to be 2.0 million acre-feet per year (Table 1). Inflow needs from the Lavaca and Colorado Rivers were estimated at 346,200 and 1,033,100 acre-feet annually, respectively. The remaining contributing areas are estimated to provide an additional 620,700 acre-feet yearly.

The TXBLEND hydrodynamic and salinity transport model was used to simulate salinity conditions in the Matagorda Bay system with the Target inflow needs indicated in Table 1. The resulting simulated salinity regime was found to give acceptable salinity conditions throughout the estuary, thus the Target needs are anticipated to provide adequate salinity within the Matagorda Bay system.

A total annual freshwater inflow of about 287,400 thousand acre-feet was found to meet the Critical inflow needs (Table 2). Approximately 27,100 and 171,000 acre-feet yearly would be provided from the Lavaca and Colorado River basins, respectively, with the remaining annual inflow of 89,200 acre-feet coming from the other contributing from the other contributing drainage basins.


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