Figure 1 Modern south Florida system with NSM grid and NSM rivers hightlighted. (Click on image for full-sized version.)

The south Florida ecosystem has been greatly altered during the last 100 years (fig 2). Drainage of the south Florida watershed began in the early 1880's, and by the early 1990's about 50 percent of the historic Everglades had been drained. In response to flooding and to provide water for a variety of human uses, a complex water-management system that includes levees, well fields, pumps, canals, and control structures was constructed throughout south Florida. Today, the south Florida ecosystem includes urban areas near the coast where nearly 4 million people live, intensively developed agricultural areas in the northern Everglades, and rangelands and wetlands throughout the region.

The South Florida Water Management Model (SFWMM) was developed by the South Florida Water Management District (SFWMD) in the late 1970's and early 1980's to simulate the hydrology and the highly managed water system in an approximately 7,600 mi² (square mile) area of south Florida. The effects of water-management activities, including ground-water pumping and canal control-structure operations, on time-varying ground- and surface-water conditions and on canal flows are simulated for selected land-use and water-management scenarios. The SFWMM is currently used by the SFWMD to evaluate feasible water-supply alternatives for projected land use and water demand in south Florida during the next 10 - 20 years.

A multimillion dollar, interagency effort is underway to restore significant portions of the south Florida ecosystem and to enhance the quantity, quality, and timing of freshwater flows to the remaining Everglades.

Figure 2 Comparison of south Florida in 1900 and today. (Click on image for full-sized version.)

A key component of this restoration effort involves returning hydropatterns (primarily frequency, duration, depth, and spatial extent of water inundation) at selected key locations in the Everglades to those which might have occurred in the natural system before human-induced changes altered the landscape and hydrology, or pre-drainage conditions. The Natural System Model (NSM) was developed to simulate the pre-drainage condition hydrology of south Florida (see explanation above). The NSM has been proposed as the "best available tool" for estimating hydropattern targets for restoration efforts.

The South Florida Ecosystem Program is an intergovernmental effort to re-establish and maintain the ecosystem of south Florida. One element of the restoration effort is the development of a firm scientific basis for making management decisions. The U.S. Geological Survey (USGS) is one of the agencies that provides this needed scientific information through the USGS South Florida Ecosystem Program.

The accuracy and validity of the NSM cannot be tested using traditional modeling approaches because hydrologic data from the pre-drainage south Florida ecosystem do not exist for comparison with model results. Moreover, accurate, detailed information on historic vegetative and topographic conditions required for NSM operation is largely unavailable, leading to additional uncertainty in model output. Calibrated model parameters from the ground-water and canal flow dominated SFWMM are transferred directly to the NSM in which most of the water is transported by overland flow, but the full effects of these assumptions are unknown.

The performance of the NSM primarily has been evaluated by using three approaches. First, because the fundamental algorithms used in the NSM are the same as those used in the SFWMM, and because the SFWMM appears to perform adequately, it has been assumed that the NSM properly simulates the important hydrologic processes. Second, a series of tests were performed by the SFWMD to identify (1) the sensitivity of NSM output to changes in selected model parameters and (2) geographic areas in which the simulated hydrology is most sensitive to changes in model parameters. Third, results from the NSM have been compared with available but somewhat limited historic information on soils, vegetation, and estimated historic hydroperiods.

The NSM has been proposed as the best available tool for setting hydropattern targets for use in efforts to restore portions of the Everglades. Restoration costs may exceed one billion dollars, and decisions made using NSM results could have important and direct implications for the entire south Florida region.

A study is being conducted to determine if the NSM can provide a reasonable simulation of south Florida hydrology for pre-drainage condi-tions, or the natural system, using recent climatic data. The absence of measured hydrologic, topographic, and vegetation data from the natural system for model construction and testing requires the application of novel procedures to determine if NSM results are "reasonable". Only selected components and features of the model are being reviewed because of the limited resources and time available for the review. The review is focusing on issues identified during discussions with the SFWMD, U.S. Army Corps of Engineers, National Park Service, and Florida Department of Environmental Protection. The study is being conducted by the USGS in cooperation with the U.S. Army Corps of Engineers, Jacksonville, Florida District.

The manner in which the physics of the natural system and the numerical approximation of these processes are represented in the NSM is being evaluated. Evaluations are being made by reviewing the scientific literature, reviewing the model results, conducting test simulations for selected special cases, and comparing NSM results with an independent flow model. Specific issues include

1. Numerical solution of the differential equations describing overland, ground-water, and channel flows;
2. Representation of the modeled region by the 2-mile by 2-mile grid;
3. Appropriate temporal and spatial resolution of input and boundary data for vegetation, rainfall, and evapotranspiration; and
4. Evapotranspiration calculations.

The equations describing the ground- water component of the system appear to be complete and to be numerically solved in a rigorous manner. However, the ground-water boundary condition was not con-tinuous at the Atlantic Ocean bound-ary. Revisions were made by the SFWMD staff to adjust this ground-water boundary condition.

Some discrepancies in the compu-tation of overland flow were identified, including representation of the flow resistance, the use of flux limiters required to maintain numerical stability, and possible computer coding errors. The SFWMD staff made revisions to the NSM 4.3 to address these issues. The revised NSM 4.3 (called NSM 4.4) resulted in decreases in annual average ponding depth relative to those produced by NSM 4.3, changes in flow patterns near Lake Okeechobee, but little change in the annual average hydroperiod (number of days of inundated conditions at a site). However, annual average ponding depths simulated by NSM 4.4 were essentially the same as annual average ponding depths simulated by an earlier version of the NSM (NSM 4.1).

Channel-flow algorithms were evaluated. Although some inconsistencies in the computation of river flows were identified, these inconsistencies had little effect on simulated results. This is because the pre-drainage flows are dominated by overland-flow processes.

One potentially important issue not addressed in this study, because of limited time, concerns model boundary conditions. Boundary conditions generally are required for the operation of hydrologic models, and can be considered as the model input needed to allow the simulation of conditions within the modeled area to be performed.

Boundary conditions for the NSM are both time-varying and steady, and include conditions at the lateral boundaries of the model (river inflows and outflows, water level in Florida Bay and the Atlantic Ocean, and marine ground-water flow conditions), at the surface (water or soil) exposed to the atmosphere (precipitation, evapotranspiration, and wind), at the ground surface (flow resistance and soil infiltration parameters), and at the bottom of the aquifer (no flow). In some cases, the NSM boundary conditions represent a simplification of the physical system; for example ocean tides are represented by a monthly mean tide level. In other cases, the NSM boundary conditions very nearly represent the physical system. Daily rainfall is used as model input.

By definition, boundary conditions affect simulated results throughout the modeled region. However, the effects of the boundary condition simplifications and the sensitivity of model results to changes in the boundary conditions are not well established for the NSM. Some specific issues which could be considered are:

• Are the model boundaries at the appropriate locations?
• Are the estimates of river inflows to Lake Okeechobee and outflows from the lake reasonable approximations of the natural system?
• What is the effect of a variable mean sea level on model results?
• What are the effects of a daily fluctuating tide on model results?
• What is the effect of more refined topographic resolution on simulated results and, in particular, on simulated depth of flow?
• How sensitive are model results to changes in any of the boundary conditions?

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For more information contact:

Eric Swain
U.S. Geological Survey
9100 N.W. 36th St.
Federal Reserve Bank Bldg., Room 107
Miami, FL 33178
(305) 717-5825
email:edswain@usgs.gov

For more information about the South Florida Ecosystem Program, contact:

Aaron Higer
U.S. Geological Survey
3301 Gun Club Road
West Palm Beach, Florida 33406
(407)687-6560
email: ahiger@usgs.gov

Related information:

SOFIA Project: Review and Evaluation of a Model for Simulating the Natural Hydrology of South Florida