HYDROLOGIC AND METEOROLOGIC FACTORS AFFECTING GROUNDWATER AND SURFACE-WATER LEVELS AT TREE ISLANDS IN THE EVERGLADES OF SOUTHERN FLORIDA

Hydrologic and meteorological data were continuously monitored at three tree islands (3AN1, 3AS3, and 3BS1) from 2000 through 2008. Tree islands are low elongated hills that are slightly elevated (<5 ft) above the flat, low-lying terrain of the Florida Everglades. The numerous tree islands provide unique habitats for plants and animals; therefore, successful restoration of the Everglades ecosystem is dependent upon acquiring a better understanding of their geologic and hydrologic systems. These data, collected as part of a cooperative study, provide an in-depth understanding of the factors affecting the hydrology of tree islands.

Four pairs of monitor wells were installed at each of the three tree islands. Each well pair consisted of a deep well (25 – 49 ft deep) and a shallow piezometer (4 – 15 ft deep). Surface-water stage was monitored using a stilling well installed at each island.

Water levels in the wells fluctuated seasonally, with annual minimums generally occurring in May at the end of the dry season, and annual maximums occurring in October at the end of the wet season. Cumulative rainfall during the wet season ranged from 17 - 36 inches, whereas cumulative rainfall during the dry season was 10 – 25 inches. Increases in groundwater levels in wells were positively correlated to individual rainfalls and to the total sum of precipitation that occurred during each wet season. Daily declines in water levels during the dry season coincided with peak mid-day values of photosynthetic radiation. These declines occurred even when the water levels in wells were below land surface, indicating uptake of water by vegetation is important. A depression formed in the water table at 3AN1 in May 2007, probably as a result of evapotranspiration from the trees covering the island.

Vertical groundwater flow was upward 70 – 90% of the time in the marshes surrounding 3AN1 and 3AS3. Vertical groundwater movement underlying each of the two islands was mixed, indicating the islands served alternately as points of recharge and discharge, depending upon the season.

Vertical groundwater flow was downward 56 – 76% of the time at 3BS1, possibly because of withdrawals from nearby urban areas in Miami-Dade County. Additional study is needed to further define the local groundwater flows cells underlying each island.