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South Florida's hydrologic systems

Change in water balance in southeast Florida

History of the Study
Regional System
Hydrologic Systems
- Importance of Water
- Aquifers
> Water Balance
- Hydrologic Changes
- Quantity Problems
- Resource Limits
- Quality
Final Word
PDF version
After several decades of urban and agricultural growth - with the attendant environmental changes - southeast Florida's residents have come to recognize that man and his environment are inseparable and that only through prudent, long-range land-use planning and water management can he attain an acceptable balance between development and environmental preservation. Until such plans are formulated and implemented, it is important that planning and water-management agencies use caution in their decisions involving growth that may effect further environmental changes, particularly those decisions affecting water resources, because of the reliance of the environment and urban growth upon southeast Florida's finite water supply.

The surface-water systems and interconnected aquifers of southeast Florida contain large quantities of freshwater in storage, but a major part should be considered to be dead storage-freshwater that occurs below sea level that ordinarily is not available for use. The remainder is usable freshwater stored above sea level in lakes, sloughs, canals, surface reservoirs (conservation areas), and shallow aquifers. The usable storage annually is increased by 1,270 to 1,524 mm (50 to 60 in) of rainfall and is reduced by evapotranspiration, outflow to the ocean, and pumping. In southeast Florida, usable storage available in the system at the end of the dry season is increasingly important because it is this storage that must sustain future increasing demands and retard seawater intrusion. This quantity frequently is governed by the amount of rainfall late in the rainy season because much of the early season rainfall is discharged through canals to the ocean to reduce the possibility of flooding in urban areas. The schematic diagrams in figure 23 furnish an insight to the water balance in southeast Florida. They indicate the progressive relative changes in the disposition of the annual rainfall brought about by step-by-step modifications of the hydrologic system, such as drainage and flood control, water management, and urban and agricultural expansion.

Before drainage was begun, south Florida was characterized by vast inland wetlands of long-period inundation and coastal ground-water levels that were perennially higher than those recorded in 1974. The preurbanization condition from the standpoint of the water balance is simulated in figure 23. Usable storage was maximum, and most of the annual loss of freshwater was by evapotranspiration and by continuous ground-water outflow along the coast that provided a narrow nearshore brackish-water habitat for biologic productivity. The major surface flow to the ocean was by slow southward sheet flow primarily through the Everglades to Florida Bay and the Gulf of Mexico and, during heavy rainfall, through the transverse glades that dissected the coastal ridge. Seawater intrusion was limited to the near-coastal zone only because of the generally high water levels. During years of deficient rainfall, parts of the interior wetlands became dry, fires occurred, and seawater intruded some of the coastal lowland areas. Burned peat beds are reminders of past droughts.

schematic diagram showing changes in the water balance in southeast Florida through 1974

FIGURE 23. Schematic diagram showing changes in the water balance in southeast Florida through 1974. [larger image]

Sixty years of man's encroachment upon the south Florida environment caused irreversible changes in and heavy stress on the hydrologic systems, particularly in the southeast. By the mid-I940's, large coastal and interior wetland areas were drained by canals to reclaim land for urbanization and agriculture. Before 1946 the Everglades would virtually go dry before the end of the dry seasons, freshwater outflow of canals would cease, and, during prolonged drought, seawater would move inland along canal channels and infiltrate into and contaminate adjacent parts of the Biscayne aquifer, the only source of fresh ground water. At the end of the dry season of 1945, seawater intrusion had affected large segments of the aquifer along coastal Dade County, and several of Miami's municipal supply wells yielded salty water. Water levels in south Dade County and in what is now the eastern part of Everglades National Park were as low as 0.6 m (2 ft) below sea level (Parker and others, 1955). The water balance for the 1945 condition is shown as the uncontrolled-drainage condition in figure 23.

The irreversible effects of the decline of interior water levels included the drying out and aeration of Everglades peat and the resulting transition from the formation of peat to the destruction of peat (Stephens, 1969). Since drainage began in the early 1900's until 1953, nearly 1.8 m (6 ft) of peat and muck soil were lost by oxidation, compaction, wind erosion, and burning in the upper Everglades near Lake Okeechobee. An additional 0.3 m (1 ft) was lost from 1954 to 1968. Stephens (1969) estimated that if depletion continues at its current rate, by 1990 much of the organic soil in the upper Everglades will be too shallow to support economically viable agricultural production.

Uncontrolled drainage in southeast Florida was halted in 1946 by the installation of control structures (barriers) near the outlets of most drainage canals. These structures mitigated the recurring problems of seawater intrusion and excessively low water levels. During the rainy season the controls are opened to release water for flood prevention in the urban and agricultural areas, and during dry seasons they are closed to prevent overdrainage and to retard seawater intrusion. Canal flows have been controlled since 1946, and regional water management in southeast Florida was begun with storage of water in Conservation Area 3 after 1962. Water control in the 1950's and management in the 1960's reduced canal outflows from the Everglades (Leach and others, 1972) but increased losses from the coastal ridge area where flood prevention was necessary in the rapidly expanding urban areas.

The sum effects on the water balance over the years caused by land reclamation, urbanization, increased water demand, water control, and water management in southeast Florida through 1973 are shown as controlled-drainage and water-management condition in figure 23. The most notable difference between the condition of uncontrolled drainage and that of controlled drainage and water management was the increase in usable storage under the controlled and managed situation. Although water use increased many fold, the increase in usable storage was a direct result of the regulation of canal flows to the ocean. Such flow regulation and water-management practices by the FCD enabled flows to the Everglades National Park to be maintained at rates deemed by park officials to be the minimum to reproduce an approximation of historic discharge there and to be adequate to control water levels in the growing urban areas to prevent flooding.

The total daily flow to the ocean of the major canals of the lower east coast ranges from 28 m3/s (1,000 ft3/s) (2.5 million m3 or 650 million gallons per day) during an extremely dry year, to more than 193 m3/ s (6,800 ft3/s) (16.6 million ma or 4.4 billion gallons per day) during a wet year. For an average year, the daily flow is about 72 m3/s (2,550 ft3/s) (6.4 million m3 or 1.7 billion gallons per day). These flows to the ocean are considered necessary by the FCD to prevent flooding in urban and agricultural areas. They represent also the quantities of water that are potentially salvageable to satisfy future demands.

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