State Methodology for Determination of Needs in the Major Estuaries
Freshwater inflow studies within the State have been guided by Section 11.147, Texas Water Code, which
defines beneficial inflows as
"a salinity, nutrient, and sediment loading regime adequate to maintain an
ecologically sound environment in the receiving bay and estuary system that is necessary for the
maintenance and productivity of economically important and ecologically characteristic sport
or commercial fish and shellfish species and estuarine life upon which such fish and shellfish are
dependent."
As written, this includes a foundation for developing the management goals and scientific studies
upon which the recommendations are based.
Management Goals of the Methodology
Legislative directives calling for the development of freshwater inflow recommendations
for Texas' bays and estuaries can be distilled down to two management goals:
Ensuring the maintenance and productivity of economically important and ecologically
characteristic sport or commercial fish and shellfish, and
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Ensuring the maintenance of estuarine life upon which such fish and shellfish are dependent.
The State Methodology addresses the first goal, "maintenance of ... fish and shellfish",
by setting a management goal to achieve more than 70% of historical average harvests or abundances
of important fish and shellfish. This requirement is set in the TxEMP optimization model (Element 6
described below) as a lower constraint.
The State Methodology addresses the second goal, "maintenance ... of estuarine life upon
which such fish and shellfish are dependent", in the less explicit final check of needs
(Element 7 described below). In this step, salinity levels resulting from the inflow solutions provided
by TxEMP are examined at important locations within the estuary. This step ensures that the
recommended inflows will provide conditions favorable to maintaining the fish, shellfish, and
estuarine life upon which they are dependent, i.e., an ecologically healthy system.
Scientific Basis of the Studies
The State Methodology was designed to answer the question: How much water is needed to
provide a beneficial inflow? In order to do this, a great deal of information is
needed. While complete descriptions of the State Methodology for determining inflow needs to the
major estuaries are provided in the Longley (1994) report and in Powell
et al. (2002), here we present only a brief summary of the seven components or elements
of the State Methodology.
Data Collection/Hydrographic Surveys - Field studies designed to collect physical
measurements (water level, velocity, etc.) of the bays and the mixing of fresh and seawater
within them for modeling purposes. They include both short-term intensive inflow studies and
longer-term water quality and tide gaging data collection efforts conducted by the TWDB and
contracted to other agencies or universities. Complete hydrographic surveys provide the data
needed to calibrate the TxBLEND hydrodynamic and salinity models (Element 2). Long-term flow
and salinity data sets are used to set constraints and to develop statistical relationships
between flow and salinity. These constraints and equations are used in the TxEMP optimization
model (Element 6).
Hydrodynamic & Salinity Modeling - The TxBLEND hydrodynamic and
salinity transport model allows simulation of circulation and salinity patterns within the estuary
that affect ecological health. Model inputs include bay bathymetry, tides, rainfall, evaporation,
wind, and freshwater inflows from rivers and adjacent watersheds. The model is used to
predict changes in circulation and salinity for various inflow scenarios.
Sediment Analyses - Studies relating the input of sediment to
estuaries with the inflow of fresh water. Complete sediment analyses provide constraints
for the optimization model that directly or indirectly translate into a minimum freshwater
inflow requirement to satisfy the sediment needs for maintaining river deltas and
wetland habitats.
Nutrient Analyses - Studies relating the input of nutrients, primarily
nitrogen, to estuaries with the inflow of freshwater. The purpose is to estimate nutrient
needs for maintaining a positive balance in the estuarys nutrient budget. A nutrient
analysis therefore depends on building a nutrient budget for the estuary and has a variety
of components that represent gain and loss terms. In general, a nutrient budget is complete
when a nutrient constraint can be defined. A nutrient constraint is the flow required to
input enough nutrients to the system so there is no net annual deficit in the nutrient balance
between gains and losses.
Fisheries Analyses - Analyses which estimate fisheries needs for
maintaining long-term average production (harvest) or population levels of commercially
important and ecologically characteristic species within an estuary. Fisheries data are
used to develop equations relating fisheries harvest or abundance to freshwater inflows.
These equations are at the core of the TxEMP optimization model and are used to determine
minimum flows needed to achieve a particular fisheries target.
5a. Fisheries Harvest Analyses - Harvest analyses produce species-specific equations relating
annual commercial harvest (biomass) of fish and shellfish with freshwater inflow to an estuary.
A complete analysis includes a set of equations for a select number of representative species
for which harvest data is available through the Texas Parks and Wildlife Department and other
published sources.
5b. Fisheries-Independent Analyses - Fisheries analyses produce species-specific equations
relating species abundance (as catch per unit effort or as density) with freshwater inflow,
for commercially and recreationally important fish and shellfish. A complete analysis
includes a set of equations for representative species for which data is available through
the Texas Parks and Wildlife Department Coastal Fisheries Monitoring Program.
Freshwater Inflow Optimization Modeling - This analysis relies on the Texas Estuarine
Mathematical Programming model (TxEMP) which uses information from the hydrographic surveys
including: seasonal inflow hydrology constraints, salinity-inflow equations and salinity
constraints (Element 2); sediment and nutrient constraints (Elements 3 & 4); and, fisheries-inflow
equations and biomass/harvest constraints (Element 5). A complete analysis generates products
that include performance curves relating harvest or abundance to annual inflow as well as the
monthly distribution of flows for a given annual inflow. Annual performance curves are continuous from
a MinQ value, representing the absolute minimum inflows necessary to meet the flow, salinity,
harvest, and biological constraints, to an inflow representing maximum harvest value (MaxH),
and finally to a maximum inflow (MaxQ) that satisfies all constraints. The purpose of the
optimization approach is to provide solutions which meet all specified constraints, limits,
and stated management objectives with a minimum amount of freshwater inflows
Verification of Needs - A final check of TxEMP results serves to evaluate
the inflow solutions and provide assurance that the estimated needs will maintain
ecological health and productivity. There are two kinds of checks, graphic examination
of the simulated monthly pattern of salinity from a hydrodynamic and conservative
transport salinity model (TXBLEND) and daily checks of salinities at selected nodes
that are also generated by the TXBLEND model. This step is complete when the calculated
salinity patterns in the monthly or daily simulations do not violate the salinity constraints
more frequently than a predefined level of acceptance.
Frequently Asked Questions about the State Methodology
Why use commercial harvest (fisheries-dependent) data?
Commercial harvest data are commonly used in fisheries management, although with known
and accepted limitations. Much of the information is self-reported by fishermen and
subject to certain biases, such as fishing effort and market price. However, when the
bay and estuary studies were started in the late 1980s, reported harvest was the only
data set that had been collected for a long enough period (since 1959) to be useful.
Consequently, the earlier inflow studies are based on commercial harvest data.
Can TPWD Coastal Fisheries monitoring data be used instead of harvest data?
TPWDs Coastal Fisheries Monitoring database, which contains information on species
abundance, distribution, and seasonality, has been used to check model results based
on harvest data. However, more recent freshwater inflow studies (e.g., Sabine Lake (2005),
Laguna Madre (2004), and Matagorda Bay (2006)) have been able to use this source of fisheries-independent
data to develop abundance-inflow regression equations based on species abundance.
TPWDs monitoring program began in 1976, and so the database now spans enough years to
encompass statistically meaningful natural variation. The TPWD monitoring data is complex,
and naive use of the data can lead to misleading conclusions if the sampling and data
reporting protocols are not fully understood. Because the program aims to characterize
species distributions and abundances throughout an estuary, sampling effort is randomly
distributed and is not concentrated in areas where capture success may be highest (in
contrast to commercial harvests). Additionally, TPWD uses a range of gear types which
target different habitats and life-stages. Therefore, while both the harvest and TPWD
monitoring data sets may be valid measures of estuarine productivity, they differ in detail.
Abundance values from TPWD monitoring samples can be expected to differ from commercial
harvest due to differences in collection methods and data reporting.
The fishery equations used by the agencies for some of the target species do not
appear to have strong correlations (r2) between inflow and harvest/abundance. Is this
a problem?
Although there are biological reasons for presuming a strong role of inflows
in the life history of many species, inflows are only one of a number of important factors.
Because these equations reflect only the natural variation as explained by a single factor
(inflow), and not by other important factors (such as temperature), correlations (r2)
between inflow and harvest may not be particularly high. We would not expect
to obtain a very high correlation unless inflow was the predominate influence on a species.
Nonetheless, only statistically significant (p<0.05) equations are included in the TxEMP
analysis. Equations with moderate r2 values are not flawed,
but they allow us less certainty in the answers produced. TxEMP incorporates both the answers
from the equations and the uncertainties associated with those answers.
Should a non-linear fisheries regression equation be used in place of the linear fisheries equation?
We will continue to explore the best way to quantify correlations and causal relationships
between inflows and estuarine responses. The TxEMP model can accept a variety of model forms.
At this time we may still lack, to some extent, the amount of data required to support more
complex models.
How does TxEMP come up with a solution?
A further description of TxEMP is provided in Longley 1994; here
instead is a summary.
TxEMP is a nonlinear, stochastic, multiobjective mathematical programming model that seeks a
combination of inputs which produce the most favorable (optimized) result. TxEMP uses an
algorithm to satisfy many, often conflicting requirements as it produces a solution to a
problem. The user specifies what system responses to optimize (the objective functions),
such as harvest level, and provides data on the driving inputs, such as monthly inflows.
The user then sets requirements or limits (constraints) which determine how much the
algorithm can change the driving variables and still be acceptable. Some requirements
are limits of biological function; some are limits of resource availability. Since the
problem is one of determining an estuarys response to inflow, limits and targets are
expressed as functions of inflow to the best of our knowledge. TxEMP finds a solution
through repeated trials of input combinations, recalculating responses and testing for
violation of constraints within each iteration of the model. In this way, the model
tunes the levels of monthly inflows to find a combination which will produce the best
outcome. TxEMP generates and tests a large number of inflow schedules and amounts.
TxEMP then produces a curve displaying a range of feasible solutions between alternative
desired states. Each point along the curve represents a combination of monthly inflows
which produces a result that is viable (not violating any constraints). Specific points
along the curve can be identified as optimal solutions satisfying particular goals.
Is it necessary to set model constraints on inflow, salinity, and harvest?
Yes, constraints are needed to keep the model from producing physically unreachable or
ecologically undesirable solutions. Also on a practical level, the model should deal
only with inflows that are in the range of management options. In water-use permitting,
water volumes pertinent to establishing the permit are related to the availability and
frequency of particular inflows. Likewise, planning for future inflows to the estuary
may require acknowledging the historical range of inflow.
Harvest constraints similarly reflect a management goal for the system that is based on
historical data. For example, we may choose to set a constraint which will maintain >75%
of historical harvest (lbs. per annum) according to the historical proportions of the target
species in the estuary.
Salinity constraints - Salinity constraints define one set of upper and lower limits
on the TxEMP solution. They are based on the statistical characteristics of salinity within
the estuary combined with known salinity preferences and tolerance limits of the target species.
Salinity constraints help to ensure that the TxEMP solutions are reasonable and that the management
goals are achieved.
Inflow Constraints - specified as monthly upper and lower bounds and/or seasonal
(bimonthly) upper and lower bounds. Upper bounds on monthly inflows generally are set at
the median historical monthly flow, while lower bounds are set at the 10th percentile flows.
Harvest (Fisheries) Target - A constraint set so that harvest or catch as calculated
by the model is at least some percentage of the historical average (70% and 80% have been used).
Biomass Ratio (or Relative Abundance) - A constraint set to produce solutions which
include harvest/catch for a set of species that reflect the historically observed proportions.
Salinity and Harvest Probability - Constraints can be set on the probability that
the solution produced meets the salinity or harvest criteria. When probabilities are defined
to be high, the model may be very limited in the range of solutions which can be explored.
Nutrient and Sediment Constraints These constraints are expressed in terms of
inflows. To ensure a minimum supply of beneficial nutrients and sediments, constraints can
be set on the inflows.
In the context of the TxEMP optimization model, which is more important to
assuring maximum fisheries harvests: total volume of inflow or timing of inflows to the bay?
TxEMP produces results in terms of monthly inflows, which are then summed to an annual
total inflow. Therefore, timing of inflows is an integral part of the solution. From an
ecosystem perspective, both volume and timing are important. An otherwise beneficial inflow
volume could hurt productivity if the timing was dramatically skewed. Many estuarine species
depend on the right seasonal sequence of high and low inflows for critical life history stages.
How are the flow recommendations produced by TxEMP checked?
TxEMP produces an inflow schedule and an annual inflow volume. The main way these are
evaluated is to use the inflow schedule product as an input to the TWDB salinity and hydrodynamic
circulation model of the estuary and then to evaluate how such an inflow affects the salinity
gradient. The salinity regime in the bay produced by the model is inspected by technical staff
familiar with the system. The salinity regime also is compared to the distribution of important
estuarine species and habitats inside the bay, as determined by TPWD monitoring. The salinity
regimes simulated for habitat features such as oyster reefs, wetlands, and fishery nursery
areas are examined. TPWD technical staff examines the salinity gradient produced by TxEMP
results for correspondence with preference zones of the species in each area of the estuary.
Completed Freshwater Inflow Studies and Supporting Work
Coast-wide Studies
TWDB. 1967. A new concept: Water for Preservation of Bays and Estuaries. Report No. 43.
Prepared by Lockwood, Andrews, and Newman, Inc. for the Texas Water Development Board, Austin, Tx. 39 pp.
TDWR. 1982. The influence of freshwater inflows upon the major bays and estuaries of the Texas Gulf
Coast: Executive Summary. LP-115 (second edition). Texas Department of Water Resources, Austin, Tx. 51pp.
Longley, W.L., ed. 1994. Freshwater inflows to Texas bays and estuaries: ecological relationships and
methods for determination of needs. Texas Water Development Board and Texas Parks and Wildlife Department,
Austin, Tx. 386pp.
Powell, G.L., J. Matsumoto and D.A. Brock. 2002. Methods for determining minimum freshwater inflow needs
of Texas bays and estuaries. Estuaries 25(6b): 1262-1274.
Sabine Lake (Sabine-Neches Estuary)
TDWR. 1981. Sabine-Neches Estuary: A study of the influence of freshwater inflows. LP-116.
Texas Department of Water Resources, Austin, Tx. 213pp.
Kuhn, N.L. and G.Chen. 2005. Freshwater inflow recommendation for the Sabine Lake Estuary of Texas and
Louisiana. Texas Parks and Wildlife, Austin, Tx. 71pp.
Galveston Bay (Trinity-San Jacinto Estuary)
TDWR. 1981. Trinity-San Jacinto Estuary: A study of the influence of freshwater inflows. LP-113. Texas Department of Water Resources, Austin, Tx. 491pp.
Lee, W., D. Buzan, P. Eldridge, and W. Pulich, Jr. 2001. Freshwater inflow recommendation for the Trinity-San Jacinto Estuary of Texas. Texas Parks and Wildlife, Austin, Tx. 59pp.
Brock, D.A., Solis, R.S., and Longley, W.L. 1996. Guidelines for Water Resources Permitting: Nutrient Requirements for Maintenance of Galveston Bay Productivity.
Batchelor, M.E., and C.G. Guthrie. 2008. Galveston Bay Freshwater Inflow Re-Study. An investigation of producitivity-inflow relationships.
Matagorda Bay (Lavaca-Colorado Estuary)
TDWR. 1980. Lavaca-Tres Palacios Estuary: A study of the influence of freshwater inflows.
LP-106. Texas Department of Water Resources, Austin, Tx. 349pp.
Martin, Q., D. Mosier, J. Patek, and C. Gorham-Test. 1997. Freshwater inflow needs of the Matagorda
Bay system. Lower Colorado River Authority, Austin, Tx. 229pp.
LCRA. 2006. Matagorda Bay Freshwater Inflow Needs Study. Lower Colorado River Authority, Austin, Tx. 280pp.
San Antonio Bay (Guadalupe Estuary)
TDWR. 1980. Guadalupe Estuary: A study of the influence of freshwater inflows. LP-107. Texas
Department of Water Resources, Austin, Tx. 344pp.
Pulich, Jr., W., W.Y. Lee, C. Loeffler, P. Eldridge, J. Hinson, M. Minto, and D. German. 1998.
Freshwater inflow recommendation for the Guadalupe Estuary of Texas. Coastal Studies Technical Report
No. 98-1. Texas Parks and Wildlife, Austin, Tx. 100pp.
Aransas Bay (Mission-Aransas Estuary)
TDWR. 1981. Nueces and Mission-Aransas estuaries: A study of the influence of freshwater inflows.
LP-108. Texas Department of Water Resources, Austin, Tx. 381pp.
Chen, G.F. (Appendix by TWDB). 2010. Freshwater inflow recommendation for the Mission-Aransas estuarine system. Texas Parks and Wildlife, Austin, Tx. 120pp.
Corpus Christi Bay (Nueces Estuary)
TDWR. 1981. Nueces and Mission-Aransas estuaries: A study of the influence of freshwater inflows.
LP-108. Texas Department of Water Resources, Austin, Tx. 381pp.
Pulich, Jr., W., J. Tolan, W.Y. Lee, and W. Alvis. 2002. Freshwater inflow recommendation for the
Nueces Estuary. Texas Parks and Wildlife, Austin, Tx. 95pp.
Laguna Madre Estuary
TDWR. 1983. Laguna Madre Estuary: A study of the influence of freshwater inflows. LP-182.
Texas Department of Water Resources, Austin, Tx. 286pp.
Tolan, J.M., W.Y. Lee, G. Chen, and D. Buzan. 2004. Freshwater inflow recommendation for the
Laguna Madre Estuary System. Texas Parks and Wildlife, Austin, Tx. 114pp.
Status of the Bays and Estuaries Program - September 2008
There are seven major and five minor bay and estuary systems along the Texas Gulf Coast.
Study of the major estuaries is essentially complete and agency staff now are collecting data
and beginning modeling work on the minor estuaries. TWDB continues to work in cooperation with
the TPWD on minor estuaries in Texas with the goal of developing flow recommendations for these
systems. Work is ongoing in the Cedar Lakes/San Bernard Estuary and the Rio Grande Estuary.
Modeling and data needs for the minor estuaries require that new and innovative approaches be
developed and tested. In particular, data needs exist and are being addressed in coastal marshland
habitats, ubiquitous in Texas' minor estuaries, where inundation and dewatering must be monitored
and modeled.
Progress has been made in the review and update of work previously completed for the 2001 Trinity-San
Jacinto Estuary (Galveston Bay) freshwater inflow study. This review process began in 2007 in
collaboration with TPWD, TCEQ, and input from interested stakeholders. Statistical reanalysis
of the earlier work and extension of the analysis is nearing completion, with results expected this
fiscal year.
Data Sources for Freshwater Inflow Studies
Texas Water Development Board Estuary Monitoring Data - salinity, pH, DO, water temperature, water level
Texas Water Development Board Estuarine Hydrographic Studies - water temperature,
salinity, pH, DO, water level, water velocity, water flowrate, wind velocity
Texas Water Development Board Evaporation and Precipitation - monthly average evaporation, precipitation, wind speed, air temperature
Texas Water Development Board Coastal Hydrology - freshwater inflows to Texas bays and estuaries
United States Geological Survey (USGS) Streamflow - historical daily streamflow for rivers in Texas
Texas Coastal Ocean Observation Network (TCOON) - Coastal tides - NOAA-standard water levels
Texas Parks and Wildlife Department (TPWD) Coastal Fisheries Monitoring Program - biological sampling
using multiple gear types (upon request)
National Weather Service (NWS)Meteorological data - rainfall, wind velocity, air temperature
