An objective of the U. S. Geological Survey (USGS) Place-Based Studies (PBS) Program in south Florida is to develop accurate and reliable tools for simulating the hydrology of wetlands. Such tools are necessary to expand the capabilities of regional flow simulations, which are used for developing the Comprehensive Everglades Restoration Plan (CERP). Reliable detailed information is required to design public works needed to implement CERP concepts and to assure that these public works all function harmoniously to achieve the overall restoration goal, to "get the water right."

To achieve this objective, the USGS has initially focused on development of a dynamic wetland flow model that incorporates accurate land-surface elevations, all principal water-budget components (including precipitation and evapotranspiration) as well as all forces that significantly effect water flow (including vegetative flow resistance and wind drag on the water surface). Wide-ranging process-study and data-collection activities have also been undertaken on these and other parameters with the objective of eliminating as much subjectivity and empiricism as possible in the model-development process. The result of this intensive effort is the Southern Inland and Coastal Systems (SICS) model of the Taylor Slough region of Everglades National Park (fig 1). This area was chosen for initial model application because it is very representative of south Florida wetland

Figure 1. Boundaries for the Southern Inland and Coastal Systems (SICS) and Tides and Inflows in the Mangroves of the Everglades (TIME) models. Click for larger image.

settings; has a limited spatial extent; reasonably well-defined boundaries; and for its accessibility. Data-collection for model verification, research activities, and field parameter measurements could all be undertaken within reasonable time and cost constraints.

To minimize subjectivity in the SICS model calibration process, and thereby raise the level of confidence in model reliability, numerous process studies were undertaken to supply model input data sets and nearly eliminate the need for adjustment of empirical coefficients to obtain calibration. Additional studies included an investigation of evapotranspiration in various wetland environments, which indicated that it is possible to regionally define the Priestly-Taylor coefficient as a function of solar radiation and water depth. Using this formulation of and a least squares fit of , water depth, and solar radiation to field measured evapotranspiration, the SICS model computes cell-by-cell evapotranspiration rates using only regional solar radiation data as input. Although approximated by the least squares fit, this computation of evapotranspiration provides a robust and practical method for defining an important parameter that can be difficult to quantify.

The frictional resistance coefficient is a crucial surface-water parameter that usually is estimated and adjusted in the calibration process. This is because little real data usually exists to determine friction effects. However, in the SICS model area, where frictional resistance in wetland flow is a function of vegetation type, airborne and ground methods were used to delineate vegetation types and density in the study area. Another study used a laboratory flume and field measurements to relate vegetation type to frictional resistance. Thus, frictional resistance terms are directly defined spatially in the model without need for adjustment. Through the variety of vegetation types, the average Manning's n values range from 0.38 to 0.46 for typical flow depths.

Because the large-scale topography is flat, small variations in land-surface elevation can substantially affect the wetlands flow regime. An extensive data set of land- surface elevations was collected in the SICS area by the U.S. Geological Survey. Because land based surveying is difficult in wetland terrain, helicopter-borne methods were used. Reproducibility within several centimeters was shown for this application.

Additional research projects have yielded data for wind-friction effects on flow, distributions of ground-water inflows, salinity boundaries, coastal creek outflows, and wetland flow velocities. Using this information, the model performed well in the initial simulation, successfully matching measured coastal creek flows and unit discharges measured by mobile velocity meters in the wetlands. This success indicates that the input parameters developed from the process studies were probably accurate and correctly defined their hydrodynamic effects.

A more extensive simulation indicated the ability to match measured water levels at stations in the wetlands; matching within 0.3 foot most of the time. An uncertainty analysis was performed, and the coastal creek frictional resistances were identified as having the least certainty, followed by wind friction terms and ground-water inflows. These findings have motivated further efforts that include; multiple stage and flow stations on Taylor River to compute the creek frictional coefficient, laboratory research on wind friction in wetlands with emergent vegetation, and coupling of the vertically averaged, dynamic, two-dimensional surface-water code SWIFT2D with the ground-water flow and variable-density transport model SEAWAT.

The SICS model is useful for testing management scenarios for restoration. Figure 2 shows the model-produced effects of increasing flow at Taylor Slough Bridge by 50 percent. The model results indicate that the volumes from local rainfall have a greater effect on water levels than do flows at Taylor Slough Bridge. For this particular scenario, the effects of Taylor Slough Bridge flows on water levels are only significant in the northern areas, where there is a noticeable shift in the location of the 1.0-foot water-level contour (fig. 2). To develop a model for observing long-term phenomena, a simulation with a 2-year duration was created. This time span allows: (1) adequate output data to supply water-level data for the Across Trophic Levels System Simulation (ATLSS) model of biological processes in the Everglades; and (2) improved tracking of waters entering the study area to their destinations.

Although results of the initial SICS modeling efforts are very encouraging, an important limitation exists in that interactions between ground-water and surface-water were approximated. These interactions are important even in areas remote from drainage features and canals. Therefore, these processes were incorporated into SICS via the explicit coupling of SWIFT2D with SEAWAT, which allows for density-dependent transport of constituents. This coupled model is referred to as Flow and Transport Linked Overland-Aquifer Density-Dependent Simulator (FTLOADDS); it simulates flow and salinity exchange between the surface water and ground water.

FTLOADDS will also be used for the Tides and Inflows to the Mangroves of the Everglades (TIME) model, which covers essentially all the non-urban area south of Tamiami Trail extending to the western coast (fig. 1). The western areas have received far less field study and hydrologic data collection than the SICS study area. New data collection for the western area includes: Continuous acoustic measurements of coastal flows and wetland flow velocity, airborne mapping of topography, and collection of ground-water levels and salinity. The reliable hydrodynamic simulation of surface-water and ground-water flow and transport in the major part of Everglades National Park and part of Big Cypress National Park, using the TIME model, is important to evaluate potential impacts of restoration alternatives. This larger simulation area also is more useful as a source of input to the ATLSS model. The ground-water/surface-water coupling created for the SICS model and the expanded model area and data collection effort for the TIME model will result in an important numerical tool for answering restoration questions. It will allow resource managers and planners to examine the effect of restoration alternatives with greater confidence prior to expensive physical and operational changes to the water management system.

(This abstract was taken from the Greater Everglades Ecosystem Restoration (GEER) Open File Report (PDF, 8.7 MB))

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