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The movement and storage of water in south Florida is represented in a schematic diagram in figure 31, and the average annual flows from 1980 through 1989 in the area are summarized in figure 32. The Kissimmee River annually discharged about 0.8 million acre-ft to Lake Okeechobee. In turn, the lake annually discharged about 0.4, 0.2, and 0.5 million acre-ft south into the EAA, east into the St. Lucie Canal, and west into the Caloosahatchee Canal, respectively. The average annual discharge to coastal waters was about 1.3, 0.3, and 1.4 million acre-ft, respectively, from the Caloosahatchee and St. Lucie Canals, and into the Atlantic Ocean from 11 canals south of the St. Lucie Canal. The average discharge under the Tamiami Trail to the southern Everglades and the Big Cypress Swamp was about 1.2 million acre-ft.
The movement and storage of water in southeastern Florida from 1980 through 1989 was evaluated by the South Florida Water Management District (1993) by using a water-budget approach. Budgets were developed for the region and for subbasins in the region, which includes Lake Okeechobee, the three water conservation areas, the Everglades Agricultural Area, the eastern part of Everglades National Park, and the developed Atlantic Coastal Ridge (fig. 33). Hydrologic components used in developing the water budgets were those estimated directly from measurement data, which included rainfall, canal flows, and consumptive pumpage, and those estimated by using a computer model, the South Florida Water Management Model (SFWMM), which includes evapotranspiration, overland flow, ground-water flow, levee seepage, and both surface- and ground- water storage changes. There are varying degrees of uncertainty in the measured and the model estimates. Model results probably overestimate the coastal outflows and underestimate the evapotranspiration in developed areas (South Florida Water Management District, 1993). Although the model is capable of simulating a 25-year period, lack of contiguous-structure- flow and canal-flow data in the developed area of the lower eastern coast prevented the development of water budgets for longer than the 10-year period (South Florida Water Management District, 1993, p. C-8). The 1980 through 1989 period includes typical hydroperiods although the entire period is considered to be somewhat dry compared with the long-term average (South Florida Water Management District, 1993).
A summary of the water budget (table 3) shows a negative change in storage, which indicates more outflow than inflow. Most of this change (-338,000 acre- ft/yr) occurs in the Lake Okeechobee part of the model and would amount to a lowering of the lake level by about 5 ft during the 1980s. According to the South Florida Water Management District (1993), the negative change in storage is merely the result of including a low-storage drought year (1989) as the last year in the model simulation. The report indicates that, if a longer time period had been used for the simulation, the change in annual storage would have more nearly approached zero (South Florida Water Management District, 1993, p. 11-10).
Most of the surface-water outflow from the modeled area goes into the Atlantic Ocean (fig. 35). An annual average of 3.3 million acre-ft is estimated to have been discharged to the Atlantic from 1980 through 1989, whereas the combined discharges to the Shark River and Taylor Sloughs and C-111 averaged 813,100 acre-ft. (South Florida Water Management District, 1993; Light and Dineen, 1994). Annual measured discharges to the Atlantic Ocean (1980-89) at the 12 canals shown in figure 32, which represents a substantial but incomplete estimate of outflow to the ocean, was about 1.8 million acre-ft or about 1.5 million acre-ft less than the model estimate. Although coastal outflows to the Atlantic Ocean may be overestimated by the model because of uncertainties in the budget (South Florida Water Management District, 1993), the magnitude of the outflows suggest that capture or redirection of these flows represents a potential for additional water supply for Everglades restoration (Light and Dineen, 1994).
Models, like the SFWMM, have proven to be extremely useful management tools in the short term (periods generally less than a few years). Great care must be taken, however, when interpreting the results of simulated decadal-scale processes from models that are designed for short-term management objectives. Oreske and others (1994) observed that verification and validation of numerical models of natural systems is impossible. They concluded that models are most useful when used to challenge existing assumptions, rather than to validate them.
U.S. Department of the Interior, U.S. Geological Survey
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Last updated: 04 September, 2013 @ 02:03 PM(KP)