Estuary Circulation & Salinity Models

Senate Bill 137 (1975), House Bill 2 (1985), Senate Bill 683 (1987), and other legislative directives called for the Texas Water Development Board (TWDB) to maintain a data collection and analytical study program focused on determining freshwater inflow needs which are supportive of economically important and ecologically characteristic fish and shellfish species and the estuarine life upon which they depend. More recent legislative directives, Senate Bill 1 (1997) and Senate Bill 3 (2007), also direct TWDB to provide technical assistance in support of regional water planning and development of environmental flow regime recommendations, which include consideration of coastal ecosystems. As part of this effort, TWDB developed and maintains an operational three-dimensional (3D) hydrodynamic and salinity transport model—the Texas BAYCAST—which is capable of producing high-resolution, dynamic simulations of estuarine and near-shore currents and salinity patterns. Because these model simulations are a function of freshwater inflow, tides, wind, and precipitation/evaporation, it is an essential tool to use when evaluating changes in bathymetry (dredging, reef building, storm impacts), water level, and freshwater inflow management (reduced volume, altered timing of inflows, increased salinity).

The complexity of the Texas coast offers unique challenges to numerical modeling. As well as mathematical and computational limitations, the accuracy of hydrodynamic models is also constrained by the quality and availability of data required to calibrate the model and run simulations. A detailed description of the data needed to run the model is given in the “Model Inputs” section below. In an effort to improve modeling capabilities, the TWDB has developed in-house software, modified external software, and funded research to improve our physical understanding of Texas' estuaries upon which our models are based. TWDB also funds data collection programs and collaborates with other agencies to obtain the data required for these models.

Texas BAYCAST

In order to accurately resolve the currents and transport patterns inside and near Texas estuaries, which are geographically complex and span a wide range of shapes and sizes, TWDB relies on the Semi-implicit Cross-scale Hydroscience Integrated System Model (SCHISM), which is a state-of-the-art, open-source, community-supported modeling system principally developed by Dr. Joseph Zhang at the Virginia Institute of Marine Science (VIMS). In addition to being highly efficient and computationally sophisticated, the SCHISM modeling system is three-dimensional, which is particularly important in coastal zones where salinity and thermal stratification can be major drivers of estuarine processes. In combination with the unstructured grid used by the model, which can efficiently span a large range of spatial sizes and resolutions, SCHISM allows the TWDB to accurately resolve and simulate all coastal water bodies in Texas in full 3D, including all major and minor estuaries, navigation channels, and near-shore environments.

The operational implementation of SCHISM for Texas, the BAYCAST, was developed as part of a collaboration between TWDB staff (Drs. Kyle Wright and Amin Kiaghadi) and researchers at Texas A&M University at Galveston (Drs. Jiabi Du and Kyeong Park). Over an approximately two-year period beginning in 2023, the model grid, bathymetry, parameters, and settings were iteratively designed, refined, and tested. The model was calibrated and validated with observational data from hundreds of monitoring stations measuring water level, currents, salinity, and temperature across the coast. In total, the model has been tested against millions of individual observations, and while some uncertainty is unavoidable, the model has demonstrated a very high level of accuracy. TWDB staff are working on applying results from the Texas BAYCAST to improve our understanding of environmental flows, coastal flooding, and pollutant transport and tracking, and remain focused on improving the model accuracy and predictive capacity in the future. With the BAYCAST, the Texas coastline now has a state-of-the-art modeling system, the outputs of which are provided by TWDB for free for research and educational purposes.

Model Inputs and Dependencies

While the SCHISM software can accurately resolve the movement of water and nutrients inside of the model domain, the model still needs to be provided information about flows of matter and energy into the domain from the land, ocean, or atmosphere. The availability, locations, and quality of these “boundary condition” datasets can often be the bottleneck for accurately simulating estuarine processes, and a significant amount of the software development that went into the BAYCAST operational system is dedicated to downloading and pre-processing these input datasets to ensure reliability. A few of the key data sources on which the BAYCAST relies are listed below.

River Inflows

Rivers provide the dominant source of freshwater inflow to Texas bays and estuaries. Inflow datasets are primarily derived from measurements of streamflow and water temperature from the U.S. Geological Survey (USGS) stream gages in gaged watersheds along the coast. For smaller ungaged watersheds that lack real-time measurements of streamflow, inflows are approximated from either historical measurements or hydrological attributes of the stream (e.g. watershed size). When forecasting future conditions, streamflow estimates are derived from hydrological models, such as the National Water Model (NWM), a joint collaboration between the National Oceanic and Atmospheric Administration (NOAA) and National Weather Service (NWS). In the future the model will also incorporate the TWDB Coastal Hydrology modeling datasets which integrate diversion and return data from water users.

Meteorology

The movement of the water surface is heavily influenced by atmospheric conditions, such as wind speed, pressure, and temperature. In addition, the combination of precipitation and evaporation can lead to changes in the volume of freshwater in select areas, which can also drive water movement or changes in salinity. All of these meteorological parameters related to air conditions, precipitation, and solar radiation are provided by the National Centers for Environmental Prediction (NCEP) and used as inputs to drive the model. When simulating past conditions, the model utilizes outputs from the North American Regional Reanalysis (NARR) product, which uses modeling and observational data assimilation to derive past atmospheric conditions. When forecasting future conditions, weather forecasts are extracted from either the Global Forecast System (GFS) or the North-American Mesoscale (NAM) models.

Tides and Oceanic Data

The model includes the influence of both astronomical tides and broader oceanic circulation currents in the Gulf. Astronomical tides are computed from the Finite Element Solution (FES2014) global tide atlas for the largest harmonic components near the Texas coast, which are provided by AVISO. Oceanic currents, as well as salinity and temperature along the ocean boundary, are supplied by the HYbrid Coordinate Ocean Model (HYCOM), a global oceanic model that simulates hydrodynamics in the deep ocean but does not resolve the coastal or estuarine environment at the resolution of the Texas BAYCAST.

In locations where the USGS lacks measurements of water temperature, proximal observation stations from CO-OPS and TCOON are used for fluvial temperature inputs. In addition, CO-OPS and TCOON provide the largest collection of water level and currents observations used for daily model validation to ensure that model outputs are meeting benchmarks for high accuracy.

Bathymetry

While not updated as frequently as other model inputs, high-quality bathymetry data is one of the most essential variables for accurately resolving hydrodynamics near the coast. The bathymetry of the Texas BAYCAST is derived from NOAA's National Centers for Environmental Information (NCEI) Continuously Updated Digital Elevation Model (CUDEM) mosaic, which integrates the best available bathymetric datasets together to form a seamless model of bed elevation in the coastal zone. In locations where newer bathymetry data is available—in particular, in locations where the TWDB has recently funded contracts for acquiring updated bathymetry data—the CUDEM digital elevation model (DEM) has been updated to reflect those newer datasets. Elevations are all converted to NAPDG2022, which is the vertical datum used in the model.

Model Outputs

All simulation outputs from the Texas BAYCAST are freely provided for research and educational purposes on the TWDB modeling server. Output files are all formatted as archival NetCDF files following CF and ACDD metadata and naming conventions. Simulation results are split apart by day, and the temporal coverage of outputs available on the server will continue to expand (both forwards and backwards in time) as new simulations are completed and validated. In order to accommodate a wide variety of users and stakeholders, there are currently six standard data products which are provided for each simulation day, offering hydrodynamic or water quality outputs in 3D or 2D (surface or depth-averaged) options. For more information on data products, visit the documentation page on the modeling server.

Legacy TxBLEND Model

TxBLEND was the primary hydrodynamic model used by TWDB between the 1990's and summer 2025 when it was replaced in operational usage by the Texas BAYCAST model. TxBLEND was a two-dimensional (depth-averaged) finite element model, which was based on the BLEND model developed by Dr. William Gray of Notre Dame University and modified by TWDB engineers for use in the shallow bays and estuaries of Texas. TxBLEND was used in past state freshwater inflow studies and by all of the Texas Senate Bill 3 Basin and Bay Expert Sciences Teams in their effort to develop freshwater inflow recommendations for Texas estuaries. In addition, TWDB maintained operational TxBLEND models for most of the major bays in Texas to provide forecasts of water currents for oil spill response. With the launch of the new 3D modeling system, which is significantly more accurate, computationally efficient, and higher resolution than the legacy model, TxBLEND has since been retired due to the difficulty of maintaining and updating the legacy code. Archived TxBLEND code and model outputs remain available upon request.

Selected TWDB Modeling Reports