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South Florida's hydrologic systems

Water-resource limit 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
Low water levels and the resultant inland advance of saltwater in the Biscayne aquifer in parts of south Dade County during the prolonged droughts of 1970-71 and 1973-74 were direct signals of impending water shortages within the FCD system and tended to confirm predictions by the US. Army Corps of Engineers (1968) that water deficits would occur about 1976. The deficits are most pronounced and occur earliest each year in Dade County, particularly south Dade County, because that is the terminus of the hydrologic system of the FCD and the most difficult area to replenish from the water-storage areas. These shortages result not only in low (below-sea-level) water levels and seawater intrusion but also in stress or change in biologic communities.

The capacity for storing water within the already developed areas of coastal southeast Florida cannot be increased appreciably because of the commitment to agricultural and urban flood protection by the water-management agencies. During much of the rainy season and for varying periods afterward, primary canals are permitted to discharge freshwater to minimize the possibility of flooding in urban areas and in nearshore and inland agricultural areas. Therefore the parts of the system where development has taken place can no longer return to the high water levels that occurred before development, and these parts must rely increasingly upon ground-water inflow and releases from undeveloped inland areas and water-storage areas for water-supply replenishment to satisfy projected demands. As a consequence, water shortages occur during years of subnormal rainfall because of the deficiency of water stored in the conservation areas and Lake Okeechobee. As urban demands increase, the shortages will become increasingly acute, even during years of normal rainfall, unless a reduction in flows to the ocean can be effected by more stringent water-management practices.

The prime objective of land-use planning is to establish an optimum rate and extent of urban and agricultural growth to minimize environmental degradation. Because of the heavy reliance of the ecosystems of south Florida upon the water resources, any additional permanent diversion of freshwater from the interior wetlands could further disrupt existing plant and animal communities there. These diversions could be effected by extending drainage works inland from their 1974 limits or by steadily increasing pumping and the consumptive use of water. Diversions by either method would increase the eastward losses from the water conservation areas and would reduce the extent and annual period of inundation there and in Everglades National Park, which receives much water from the conservation areas.

If growth continues with no implementation of water-management practices beyond those now in operation, water levels will continue to decline gradually throughout southeast Florida, attended by a corresponding incremental increase in the number of coastal areas affected by seawater intrusion, which can result in other environmental disruptions. These hydrologic conditions are shown in the sketch for the year 2000 (fig. 25) in which the usable storage has virtually been depleted. Since many of the large municipalities are served by wells near the coast, some wells will inevitably be affected by seawater intrusion, and new wells will have to be drilled inland. A large-scale shift of pumping from the coast to the interior would accelerate the diversion of water from the wetlands.

schematic diagram showing water balance in southeast Florida in year 2000
FIGURE 25. Schematic diagram showing water balance in southeast Florida in year 2000. [larger image]

If, however, ordinances were enacted aimed toward protecting the remainder of the natural fresh-water environment by prohibiting further water diversions from the wetlands (and if growth continues), then water supplies to satisfy future demands would have to come from the salvage, through implementation of more stringent water-management practices, of much of the storm water presently flowing to the sea during rainy seasons. These quantities of water, such as those that were excess in 1974, would then become the potential supplies to satisfy future demands of southeast Florida.

The quantity of freshwater discharged through the primary canal system of southeast Florida varies widely from year to year. As indicated, the total average outflow rate to the ocean of 14 major canals ranges from about 30 m3s (1,000 ft3/s) to more than 192 m3/s (6,800 ft3/s). The average outflow rate for an average rainfall year is 72 m3/s (2,550 ft3/s). In considering the freshwater flow to the ocean as potential future water supplies, it would not be feasible to salvage all the discharge. Part must be discharged for flood protection in the urban coastal areas, but this part contains most of the pollution from urban runoff. Part of the excess rain that falls inland from the densely urbanized coast, however, can be salvaged by backpumping it into the conservation areas. The quantity backpumped might be 50 percent or more of the present seaward flow to the ocean. During the driest years this would be at least 1.2 million m3/day (325 million gallons/day) that would be available for use during the dry season. The water balance for southeast Florida under such water-management practices as backpumping and raising the level of Lake Okeechobee is shown in figure 25 (year 2000).

Implementation of plans for backpumping and increasing storage in Lake Okeechobee (by raising levels) not only would make more freshwater available, but it would also make the operational system more flexible. An increase in storage capability of Lake Okeechobee would minimize the need for frequent release of freshwater to the ocean through the St. Lucie Canal and the Caloosahatchee River, as is presently required for protection against hurricane damage, and would thereby enhance the possibility of storing surplus water needed to alleviate the effects of prolonged drought. Backpumping facilities and practices and the ability to move water through the management system would tend to compensate for nonuniform rainfall patterns in southeast Florida and to make possible the movement of water from areas of surplus to areas of shortage. Some of the environmental implications of backpumping are discussed further under the section on "Water Quality."

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