APPENDIX III

NATURAL SYSTEMS HYDROLOGIC MODELS OF THE SOUTH FLORIDA SYSTEM

General Concepts and Status

Models that approximate water flows and hydropatterns of the South Florida ecosystem prior to drainage were developed to provide a quantitative description of the predrainage hydrologic system.

The "natural system" models were versions of models of the present hydrologic system that had been established using existing hydrologic data. To create the natural systems versions, all of the water management features (canals, levees, and water control structures) were removed. Simulations from these models can be compared to simulations from correlary models of the present system to gain perspective on how patterns of surface and groundwater flow, water depth, surface water coverage, and water table elevations have been changed by drainage control structures and regulatory operations. To facilitate this comparison, the two simulations are run with the same time series of rainfall data. Comparisons of this information can provide guidelines for restoring a more natural hydrologic system to support ecosystem recovery.

A preliminary natural system model for the Everglades from the south shore of Lake Okeechobee to the mangrove zone of Everglades National Park and including the eastcoast ridge has been developed jointly by hydrologists at Everglades National Park and the South Florida Water Management District (Fennema et al. 1993). The model extends from the eastern Florida coast, including the east coast ridge, to State Road 29, and therefore includes the eastern part of the Big Cypress. This is a spatially explicit landscape model on a 3.2-km (2-mi) grid (1.6-km grid inside Everglades National Park). The model generates both groundwater and surface water flow from one grid to the next based on water surface elevation differences using empirical equations of hydrology. Volume and depth are also calculated for both ground and surface water in the model. This model was developed from the South Florida Water Management model (SFWMM) that is used by the District in routing and water management planning.

The natural system model described in Fennema et al. (1993), although it includes the Everglades Agricultural Area, uses existing surface elevations and therefore, produces conservative estimates of flow because surface elevations have changed, particularly in the EAA, due to the subsidence and oxidation of peat soils that occurred with drainage. Decreases have been on the order of about 5 ft since the predrainage state. Projects are currently underway at the District and the Park to estimate surface elevations prior to drainage and to replace the elevation data in the model with more realistic values. This project is expected to be completed by the end of 1993 (Fennema pers. comm.).

The model described in Fennema et al. (1993) does not include Lake Okeechobee, but rather, accepts flows from Lake Okeechobee, based on historical stage estimates. A Lake Okeechobee module is presently under development by the South Florida Water Management District and should be completed by the end of 1993 (Fennema pers. comm., should try to confirm with the District).

A goal of the present modeling effort is to get realistic outflow estimates for the predrainage condition not only for Florida Bay but also for South Biscayne Bay. A refined sub-model with a finer grid is being developed for the southern part of South Florida that will be especially useful for this effort. This model will be able to simulate spatially explicit outflows to South Biscayne Bay and to Florida Bay. This model should be very valuable in reestablishing a more natural volume, timing, and spatial distribution of flows to these estuaries. Calibration of present-day models with measured ground and surface water flow rates at selected points will improve the accuracy of both the present-day model and its natural system model correlary.

Even before the above improvements are made, the Natural System Model can be used to guide efforts to restore more natural volumes and timing of freshwater flow to South Florida estuaries, including Florida Bay. An appendix to the Restoration Objectives Document briefly explains this process.

It is very reasonable to assume the Natural System Model will be available and suitable for the restoration project because the model is operational and efforts to extend its geographic extent and address problematic variables identified in previous reviews (Gunderson and Holling 1991) are already underway. To assure this, we recommend acceleration of the improvement projects, as listed below, to insure inclusion of their output in early reconnaissance planning: