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

Environmental Setting-- The Natural System
Watersheds and Coastal Waters

Environmental Setting-- The Altered System
Drainage and Development
Public Lands
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

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 Natural System

Average annual rainfall
Figure 6. Average annual rainfall in south Florida, 1951-80. (Water Resources Atlas of Florida, 1984.) Click on image to open larger picture (33.5k).


Annual average rainfall in south Florida ranges from about 40 to 65 in. (fig. 6). The east coast usually receives the greatest amount of rainfall, whereas the Florida Keys and the areas near Lake Okeechobee and Charlotte Harbor usually receive the least. More than half the rain falls in the wet season from June through September (fig. 7) and is associated with thundershowers, squalls, and tropical cyclones. Afternoon thundershowers are frequent over land, where moisture-laden air from sea breezes and wetlands warms, and rises, and the moisture condenses to form clouds (Pardue and others, 1992). The wet season often has a bimodal rainfall pattern with two maxima, one in early and one in late summer (Thomas, 1974; Duever and others, 1994). Rainfall during the remainder of the year is usually the result of large frontal systems and is broadly distributed rather than localized. April and May typically have the lowest rainfall. Annual and seasonal rainfalls, however, vary from year to year, as shown in figure 8 and the table below.
Average monthly rainfall and pan evaporation
Figure 7. Average monthly rainfall (South Florida Water Management District, 1993) for the lower east coast (1915-1985) and average pan evaporation (Duever and others, 1994) at selected locations in south Florida in 1988. (above) Click on image to open larger picture (5.7k).

Rainfall above and below average
Figure 8. Rainfall above and below the average annual rainfall for 20 stations in south Florida, 1895-1990. (Data from the National Climatic Center.) (above) Click on image to open larger picture (10.9k).

Mean, maximum, and minimum inches of rainfall for the lower east coast of Florida, 1915-85 (South Florida Water Management District, 1993)
Period Mean Maximum Minimum
Annual 51.9 77.5 36.7
Wet season 34.5 53.5 23.4
Dry season 17.4 30.9 7.3

Duever and others, (1994) analyzed severe droughts at several stations from 1910 through 1980 and reported that, whereas some droughts were fairly widespread, others were more localized, even over distances of only 30 mi. The variability in rainfall is often characterized by multiyear wet and dry cycles (fig. 8). These cycles are apparent in the average annual rainfall in the South Florida Water Management District (SFWMD) network where average annual rainfall decreased by 1.3 in/yr from 1900 through 1920, increased by 1.3 in/yr from 1921 through 1970, and decreased by 3 in/yr from 1971 through 1991 (Robert Hammeric, South Florida Water Management District, oral commun., 1994). Analysis of the longest period of rainfall record in south Florida (Fort Myers) indicates, however, that the trend for total annual rainfall and variability in mean annual rainfall for 10-year intervals, has not changed at that location since the late 1800's (Duever and others, 1994).

Tropical cyclones (hurricanes and tropical storms) produce the most severe weather conditions in south Florida. The high tides and heavy rains associated with these storms can produce coastal and inland flooding, and strong winds can cause extensive damage. Rainfall often exceeds 5 in. Tropical cyclones have repeatedly passed through the region, most frequently in late summer or early fall. Between 1871-81, 138 tropical cyclones, passed near or over the region (Neumann and others, 1981; Duever and others, 1994); some evidence indicates that hurricane strikes have declined during this span (Robert Hammeric, South Florida Water Management District, oral communication, 1994). These storms varied greatly in size, amount of rainfall, and windspeed and, thus, in their effects on the region. Generally, cyclones have had their greatest effects near the coast and on coral reefs where storm surges have eroded and buried natural communities (Tabb and Jones, 1962; Ball and others, 1967). Noncoastal areas are primarily affected by heavy rains that cause flooding and by strong winds that damage plant communities (Craighead and Gilbert, 1962; Alexander, 1967; Loope and others, 1994). Despite the immediate damage, natural communities in south Florida have evolved with these storms and have adapted to them (Pimm and others, 1994).

Photo of farmer watering fields

Evapotranspiration in south Florida has been estimated to be from 70 to 90 percent of the rainfall in undisturbed wetlands (Kenner, 1966; Dohrenword, 1977). Evaporation from open water is greatest in late spring when temperatures and windspeeds are high and relative humidity is low, and is least in winter when temperatures, windspeeds, and humidity are low (fig. 7). Evapotranspiration is greatest during the summer wet season when water is available for surface evaporation and vegetative transpiration (Duever and others, 1994).

The climate in south Florida is subtropical and humid. Average temperatures are in the mid-70's °F annually, ranging from about 60 °F in midwinter to about 80 °F in summer (Florida Department of National Resources, 1974). Temperatures in coastal areas are moderated by the Gulf of Mexico and the Atlantic Ocean (fig. 9). The summer heat is tempered by sea breezes near the coast and by frequent afternoon and evening thundershowers. The southern one-third of central Florida and the lower two-thirds of the coastline have nearly freeze- free climates (fig. 10). Although freezes do occur, their pattern and severity is erratic from year to year (Duever and others, 1994). The low frequency of freezes has allowed a number of tropical species to colonize and survive in the area.

Average annual temperature
Figure 9. Average annual temperature in south Florida. (Thomas, 1974.) (left) Click on image to open larger picture (19.2k).
Numbers of hours temperatures fell to 32 degrees F
Figure 10. Total numbers of hours where temperatures fell to 32 degrees Fahrenheit or lower between November 1937 and March 1967. (Winsberg, 1990.) (right) Click on image to open larger picture (19.9k).

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