Freshwater Inflow Recommendation for the Nueces Estuary

Executive Summary

This report summarizes studies performed by Texas Parks & Wildlife Department (TPWD) in accordance with Texas Water Code 11.1491 to recommend freshwater inflow targets which sustain the unique biological ecosystems characteristic of an "ecologically sound and healthy" Nueces Estuary. Methods for determining the quantity and quality of freshwater inflows (FWI) needed to maintain biological productivity of Texas’ estuaries were developed by the State Bays and Estuaries Research Program [consisting of the Texas Water Development Board (TWDB) and TPWD] under Texas Water Code 16.058. These methods, relying on computer optimization and hydrodynamic modeling, predict a minimum freshwater inflow (termed the MinQ flow) and maximum harvest inflow (the MaxH flow) for each estuary. In this report, the MaxH target flow predicted by modeling studies is critically evaluated for its effectiveness in maintaining historical fisheries production and wetland habitats in the Nueces Estuary. For this analysis, fisheries-independent sampling data from the TPWD Coastal Fisheries Resource Monitoring Program and wetland maps from the TPWD Coastal Studies Program are used to evaluate the computer simulation results.

Review of TWDB/TPWD Modeling Results

As presented in the Appendix, the Estuarine Mathematical Programming or Optimization Model (known as TxEMP) was used to compute a range of flows necessary to maintain an “ecologically sound and healthy” environment within the Nueces Estuary. In addition to maximum harvest inflow, TxEMP also identified both the minimum (MinQ = 115,600 acre-ft/year) and maximum (MaxQ = 167,100 acre-ft/year) annual inflows that satisfied all the modeling constraints. The model predicted that maximum fisheries harvest flow (MaxH) occurred at 138,500 acre-ft/yr, with a specific distribution of monthly inflows. Despite a 16.6% volumetric difference between annual MinQ and MaxH target flows, the difference appeared small (only 7.3%) in total commercial fisheries harvest predicted between the two cases (1.992 vs. 2.149 million pounds for MinQ vs. MaxH, respectively). MaxH flow produced slightly higher harvests of red drum, black drum, spotted seatrout, and brown shrimp, but slightly decreased amounts of blue crab. Under MinQ and MaxH scenarios, brown shrimp dominated total harvests, which were 26.7% and 36.6% higher, respectively, than the TxEMP target (70% of mean historical commercial harvest).

TPWD Studies Verifying Biological Responses to Target Inflows

TPWD performed two types of verification analyses on the computed FWI targets: 1) Seasonal salinity gradients predicted by the hydrodynamic model were evaluated for any measurable biotic effects; and 2) Fisheries-independent relative abundance data were correlated with historical hydrologic regimes, thereby allowing comparisons of species abundance under observed inflow regimes to that under modeled inflows.

Salinity Gradient Effects and Time Series Analysis of MinQ vs. MaxH Flows

Geographic Information System (GIS) techniques were used to compare salinity gradient maps from the hydrodynamic model (TXBLEND) output under optimized MaxH or MinQ inflows. Two different hydrologic regimes in Nueces Bay were examined: MinQ or MaxH inflows with tides, winds, and temperatures from the 1988-1989 DRY regime; and MinQ or MaxH inflows with tides, winds, and temperatures from the 1991-1992 WET, cooler period. Salinity change analysis was performed by overlaying monthly MinQ and MaxH salinity maps for each regime, producing salinity difference maps between each target flow condition. Locations of critical marsh nursery habitat in the Nueces Bay delta region (approximately 2000 ha of regularly flooded salt marsh) were given special consideration in this evaluation. Results documented the almost complete lack of a typical estuarine salinity gradient in the system under both WET and DRY regimes, with salinities > 30 ppt found year-round within lower Nueces Bay proper. Slight salinity differences (< 2.0 ppt) were evident between the MaxH and MinQ cases at a few locations in parts of upper Nueces Bay, primarily during May and June.

Time-series analyses were performed on the salinity data from the TXBLEND model at two key sites (model nodes) in the Estuary, to determine how often model constraints for salinity were exceeded under target FWI flows. Both MinQ and MaxH cases exceeded the model salinity constraints in the upper bay by 5 to 6 ppt on many days during DRY weather years (39% vs. 27%, MinQ vs. MaxH); while during the WET years, few exceedances were observed (5% vs. 2%, MinQ vs. MaxH). Overall, salinity values predicted by MinQ and MaxH flows under DRY conditions exceeded salinity constraints most severely during the spring and late summer, a cause of concern for the critical delta nursery area.

Low- vs. High-Inflow Analysis of Coastal Fisheries Monitoring Data

Because a substantial salinity gradient is lacking within the Corpus Christi Bay system, statistical techniques were used to establish direct correlations between FWI hydrology and fisheries organisms. Simple linear regression, unpaired t-tests, and GIS analyses were used to correlate historical inflows with natural abundance of eight target species in Nueces Estuary: white and brown shrimp, blue crab, Gulf menhaden, Atlantic croaker, bay anchovy, spot, and striped mullet. TPWD Coastal Fisheries monitoring data covering the period 1978 to 1997 were analyzed and statistical correlations were determined between seasonal freshwater inflows and the relative abundance (measured by catch per unit effort, or CPUE) of young animals caught in bag seines. Arc/Info GIS plots (overlays) were also developed with the observed bag seine catch rates and contoured salinity data from the Coastal Fisheries database to determine spatial relationships between species abundance and corresponding habitat locations within the Estuary.

Mean CPUE was compared for LOW- and HIGH-inflow years, with catch data separated according to actual, seasonal surface inflows over the 19-year period. Differences between HIGH- and LOW-inflow years were based on the cumulative surface inflow reaching the estuary during the seasonal periods of peak occurrence of each species. HIGH and LOW inflows were separated using, as a cutoff, the cumulative monthly values of MaxH flows (which is 89,200 acre-ft for the April through July period). Statistical analyses confirmed that shellfish (brown shrimp, white shrimp, and blue crab) and finfish species (Atlantic croaker) differed significantly in average relative abundance between LOW- and HIGH-flow years, as shown by significantly higher catch rates under HIGH flows (i.e., inflows higher than cumulative MaxH of 89,200 acre-ft), compared to LOW flows (i.e., inflows lower than MaxH). This is interpreted as meaning that observed production of these four species continues to increase with inflows two- and three-fold higher than this seasonal MaxH value. Although results for the four remaining finfish species (bay anchovies, Gulf menhaden, striped mullet, and spot) were not statistically significant, all except menhaden exhibited the trend of higher relative abundance in HIGH-flow as compared to LOW- flow years.

Evaluation of MaxH Flows in relation to Pulsed Historical Hydrology Cycles

When historical inflows from 1941 to 1996 are compared to the MinQ and MaxH target values, the results reveal significant patterns. The monthly quantities for MaxH equal the monthly historical median values in 8 out of 12 months; six of these months are March through August. These median values (the 50th percentile inflows) are, in fact, the upper hydrology bounds (inflow constraints) allowed in the solution of the TxEMP model. This is important because Nueces Estuary historically receives inflows in a highly pulsed or episodic mode. For example, between 1977 and 1997, 64 % (or 9 out of the 14 such large pulses of inflow greater than 100,000 ac-ft per month) occurred in the critical spring-summer months of May and June. Thus, seasonally-required median MaxH target flows are quite significant in that they overlap with these actual May-June monthly pulses.

Various water diversion projects (e.g., Rincon Bayou channel diversion, Allison wastewater treatment plant discharge, etc.) are considered examples of water management solutions with potential to enhance productivity of upper Nueces Bay by increasing the inundation regimes of the Nueces delta. Based on information compiled for the Bureau of Reclamation Rincon Bayou diversion project, predicted MaxH flows were evaluated for effectiveness in producing necessary inundations of the Nueces Delta. In the absence of the Rincon Bayou diversion, MaxH monthly inflows during May and June (approximately 37,000 acre-ft per month or 1,215 acre-ft per day) are much lower than the amount calculated by Bureau of Reclamation actually needed to cause overbanking into the upper Rincon Bayou (approximately 4,170 ac-ft per day). Once the Rincon Bayou diversion project is implemented, the river's minimum flooding threshold is lowered from 1.64 m (5.4 ft mean sea level) to approximately 0.0 m mean sea level. This water project will allow not only more frequent diversions of freshwater into the upper delta; it also will provide daily, bi-directional non-riverine flow events, such as winds and tides, to force water exchanges between the upper delta and the river. Cumulative MaxH flows for May and June, if delivered in a pulsed release over 3-4 weeks, could supply 74,000 acre-ft (at a rate of 2,600–3,500 acre-ft per day) to the delta. Thus, with the diversion project in operation, the cumulative MaxH flows could supply a sufficient inflow pulse to inundate the Nueces delta during critical spring/summer months and maintain productivity of this sensitive nursery area.

Target Inflow Recommendation

TPWD recommends that:

  1. a minimum of the cumulative monthly, April through July, MaxH inflows (equivalent to 89,200 acre-ft) be delivered to Nueces Estuary during the late spring/early summer season as the FWI target protective of the biological needs of this estuary;
  2. this cumulative spring/summer inflow be delivered in one or two pulsed events during the April through July period to the critical nursery habitats in the Nueces Bay delta region; and 3) monthly MaxH flows be supplied all other months of the year, except during the fall season (September through November) of years when cumulative spring/early summer MaxH flows have not occurred. In the latter case, cumulative MinQ target flows for these three months (27, 510 acre-ft total) should be provided to maintain refugia in the extreme upper bay, delta, and tidal portion of the Nueces River.

This cumulative spring MaxH flow essentially mimics the pulsed pattern of historical hydrology characteristic of Nueces Estuary inflows. Delivery of spring pulses of this magnitude correlates with higher historical catches demonstrated for important target fishery species (blue crab, brown and white shrimp, and Atlantic croaker). These critical, pulsed spring flows appear to maintain the estuarine wetlands located in the delta and to provide upper estuary nursery habitat conditions when estuarine-dependent species are actively recruiting into the Bay. Dryer conditions during the peak summer months (July through September) are expected to occur naturally, and fishery species dependent upon the estuary at those times (e.g., white shrimp) can tolerate suboptimal conditions if the estuary is provided with adequate inflows earlier in the year.

The entire report can be downloaded by clicking on its title: Freshwater Inflow Recommendation for the Nueces Estuary media download(PDF 432.4 KB)


Information:

Many other people contributed to the final production of this report. We thank especially: Jeffrey Williams of TPWD for compiling special datasets from the Coastal Fisheries database; various persons for reviewing report drafts (TPWD staff: Dave Buzan, Cindy Loeffler, Nathan Kuhn; TWDB staff: David Brock, William Longley, Junji Matsumoto; Univ. of Texas Marine Science Institute, Paul Montagna); and those too numerous to name who engaged us in stimulating discussions about freshwater inflow issues.

This work was supported by funding from the Sport Fish Restoration Program (U.S. Fish and Wildlife Service) under Federal Aid Project F-37-TA and by funding from the Water Research and Planning Fund of the Texas Water Development Board

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