Rainfall is highly variable both spatially and temporally. Coastal rainfall is generally 8-10 inches per year greater than interior rainfall. Seasonally, more than 70% of the annual rainfall occurs during the wet season months of May through October.

Drainage and development lowered evapotranspiration (ET) in the EAA and urbanized LEC. During the wet season rainfall exceeds ET, and large volumes of runoff are discharged to the Atlantic Ocean. During the dry season ET often exceeds rainfall, and large quantities of irrigation supplies are needed for sustaining agriculture, urban landscape and golf courses.

(Numbers correspond to maps below. Click on map or graphs for full-sized version.)

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To facilitate development of the lower east coast, drainage canals were constructed to lower the water table and drain surface water from the Everglades to the Atlantic Ocean. To protect the developing areas from Everglades flood water, an extensive east-coast protective levee system was constructed.

A result of this drainage and impoundment was to shift the steepest groundwater gradients westward from the coastal ridge to the eastern boundary of the Everglades.

The combination of this high gradient with the highly transmissive surficial aquifer produces very high groundwater outflows from the Everglades to the coast. This flow is helpful during dry periods when local rainfall is not sufficient to recharge the aquifer. But during wet periods, groundwater flows mostly to tide.

Transmissivity Values for the Surficial Aquifer

The map above to the right illlustrates the transmissivity of the surficial aquifer.  The Biscayne aquifer formation is more than 100 feet thick at the Atlantic coast;  it gradually reduces in thickness toward the west-northwest and thins out at the western side of WCA -3A.  The aquifer is characterized by limestone rock riddled with solution holes and is one of the most permeable aquifers in the world. (Click on images above for a full-sized version.)

Current and Future Demand Estimates

Water Demands Met, or to be Met, by Lake Okeechobee, the WCAs, and New Storage Areas

The human population of Florida's lower east coast (Palm Beach, Broward & Miami-Dade Counties) is expected to grow to 7 million by 2050 (40% increase from the year 2000). This large growth is expected to further stress the surficial aquifer system. But if local resources are properly developed, the demand on Lake Okeechobee and the WCAs is not expected to increase significantly. Although future agriculture water needs are estimated to not change significantly, the needs of the environment (Everglades, estuaries and bays) continue to become better defined, and are estimated to increase by more than 300%.

Modifications to the system are needed not only to meet the region's growing needs; but they are also needed as much or more to reverse the unintended ecological damage caused by drainage and impoundment.