The need for tools to scientifically examine the hydrology of the coastal wetlands in southeastern Everglades National Park has led the U. S. Geological Survey (USGS) to develop the Southern Inland and Coastal Systems (SICS) numerical model. SICS is an application of the SWIFT2D two-dimensional hydrodynamic model, with necessary modifications for the study area.

The development of SICS began with the realization that the only way to generate an adequate model input data set was to have well-defined empirical data. For this purpose, and to better define study area hydrology, a series of process studies were implemented to examine important parameters. The model was developed while these studies were in progress and updated whenever new data were obtained.

During the preliminary model development, topography data were based on sparse ground-based surveys. Subsequent helicopter-assisted mapping of the SICS area provided a higher resolution grid of elevation data that is based on a more accurate vertical datum. The addition of these data improved the model’s representation of ponding and flows. Representation of evapotranspiration (ET) was improved using data from field energy-budget stations. Research projects also have yielded additional data for wind-friction effects on flow, salinity boundaries, coastal creek outflows, and wetland flow velocities.

The basic modeling approach was to create an input data set consisting almost entirely of field data, using fewer assumptions, and to assess the model’s response. The direct use of the process study results included simple spatial interpolation of land elevation data within the model grid, application of regional ET equations derived from the field study, and assignment of frictional resistance terms for defined vegetation types in the study area. These uncalibrated data produced a model that reproduced the model-area hydrology well.

The model has various uses, including estimating flows at the coast, delineating ponding areas, and tracking flow paths of input waters. This last use is illustrated in figure 1

Figure 1. Location of waters from Taylor Slough Bridge and L-31W on October 14, 1996. Click for larger image.

which shows water entering the SICS area from Taylor Slough Bridge and L-31W canals, and flowing through the wetlands. On October 14, 1996, Taylor Slough Bridge waters have reached Joe Bay, but L-31W waters have not.

After the development of the first model version, several unknown parameters existed that required estimation. Although the wetland frictional resistance had been defined by the field studies, the friction coefficients for the coastal creeks were not researched. These values were adjusted to obtain modeled flows at the creeks that matched those measured as part of a process study. Another parameter with high uncertainty is the quantity and spatial distribution of the ground-water leakage. Estimates were made based on seepage measurements and the total mass balance of the model.

To increase the amount of empirical data in these two areas, further studies are being undertaken. At one of the coastal locations where discharge is measured (Taylor River) an additional upstream site has been added to obtain a water level slope. These slope data, along with measured discharge, can be used to derive a frictional resistance term. A more extensive study involves the coupling of the SWIFT2D model with a variant of the ground-water flow model MODFLOW. This variant, SEAWAT, allows for density-dependent flow, a requirement with the Florida Bay saltwater interface. The coupling is accomplished using the main routine FTLOADDS (Flow and Transport in a Linked Overland-Aquifer Density Dependent System). For a given timestep, SWIFT2D computes surface-water flow, stage, and ground-water leakage based on the previous timestep’s ground-water heads. SEAWAT then computes the ground-water heads while accounting for this leakage rate.

In the coupled model, recharge and ET for the ground water, as well as the surface water, are computed by SWIFT2D. If the surface-water condition is wet for a particular cell, the computed recharge and ET is applied to the surface water, and the computed leakage is applied to the ground water. If the surface-water condition is dry for the cell, the recharge and a computed ET rate, rather than leakage, are applied to the ground water. This method provides a continuity of recharge and ET between the SWIFT2D and SEAWAT models as surface wetting and drying occurs. The unique ability of the coupled model to hydrodynamically represent flows as well as water levels makes it a valuable tool for understanding the Everglades/Florida Bay interface.

(This abstract was taken from "Programs and Abstracts - 2001 Florida Bay Science Conference". (PDF, 6.8 MB))

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