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A Region Under Stress-- Home
A Region Under Stress-- Introduction

Environmental Setting-- The Natural System
Physiography
Climate
Geology
Hydrology
Watersheds and Coastal Waters

Environmental Setting-- The Altered System
Drainage and Development
Public Lands
Agriculture
Urbanization
Water Use
Water Budget

Water and Environmental Stress
Loss of Wetlands and Wetland Functions
Soil Subsidence
Degradation of Water Quality
Mercury Contamination
Effects on Estuaries, Bays, and Coral Reefs

Summary and Research Needs
References

Related Links

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U.S. Department of the Interior
U.S. Geological Survey
Circular 1134

The South Florida Environment - A Region Under Stress

Environmental Setting--
The Altered System


Figure showing water storage and movement
Figure 31. Water storage and movement in south Florida (arrows indicate dominate flow direction). (Modified from South Florida Water Management District, 1993.) Click on image to open larger picture (23.3k).

Water Budget

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).

Map showing average annual discharge from major canals
Figure 32. Average annual discharge from major canals in south Florida (left). Click on image to open larger picture (30.2k).
Map showing major water budget subbasins
Figure 33. Major water budget subbasins of southeastern Florida. (South Florida Water Management District, 1993.) (right) Click on image to open larger picture (27.5k).

Figure comparing estimated average annual inflows and outflows
Figure 34. Comparison of estimated average annual inflows and outflows (as percentages) for the modeled region in south Florida. (South Florida Water Management District, 1993.) Click on image to open larger picture (18.1k).

Table 3. Preliminary estimates of natural water budget, in 1,000 acre-feet, for the lower east coast area, south Florida, 1980-89

[area, 5,814 square miles. From South Florida Water Management District, 1993]

Component Average annual
(Jan-Dec)
Wet season
(June-Oct)
Dry season
(Nov-May)
1980-81
(June-May)
1988-89
(June-May)
Rainfall 15,398 9,336 6,050 12,237 13,694
Evapotranspiration 11,729 5,668 6,024 11,359 11,285
Net groundwater outflow 298 150 151 274 289
Structure/tributary outflow 1,794 904 934 473 1,113
Structure/canal outflow 3,956 1,953 2,032 2,857 3,015
Wellfield pumpage 723 301 424 663 847
Net overland outflow 905 583 326 753 1,212
Changes in storage -338 +1,632 -1,937 -3,526 -1,971

The average annual budget summary for the modeled area shows that rainfall dominates inflow and that evapotranspiration dominates outflow (fig. 34). Rainfall directly accounts for 89 percent of the total inflow, and river and stream inflows indirectly account for another 11 percent. Ground water contributes less than 1 percent of the total inflow, and evapotranspiration accounts for 66 percent of the total outflow. Canal discharge to tidewater is the next largest outflow (22 percent). Overland flow, which is primarily to the Shark River and Taylor Sloughs, and pumpage for consumptive use contribute about 6 and 4 percent, respectively, to the average total outflow.

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.


Figure showing total discharge in billion gallons
The word, swamp, as we understand it, has no application whatever to the Everglades ... it is a country of pure water ... [that] is moving in one direction or another depending on the natural topography of the country ... the air is wholesome, pure, free from disease ... near the coast and the mangroves the mosquitoes thrive; but deep in the Everglades, in the winter time at least you can sleep comfortably without a net. No stagnant pools exist for the larvae to thrive in.

Hugh L. Willoughby, 1898

Figure 35. Total discharge to Shark River Slough, Taylor Slough, C-111, and the Atlantic Ocean, 1980-89. (Light and Dineen, 1994.) Click on image to open larger picture (5.7k).


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