The circulation of water, in any form, from the surface of the earth to the atmosphere and back again is called the hydrologic cycle. A comprehensive study of the water resources of any area must, therefore, include data on the climate of the area.

The humid subtropical climate of southeastern Florida is characterized by relatively high temperatures, alternating semi-annual wet and dry seasons, and usually light but persistent winds.

The recurrence of drought in an area having relatively large rainfall such as southeastern Florida indicates that the agencies that remove water are especially effective. Two of the most important of the agencies associated with climate are evaporation and transpiration, or "evapotranspiration". Evaporation losses from permanent water areas are believed to average between about 40 and 45 inches per year. Over land areas indirect methods must be used to determine losses by evapotranspiration; necessarily, these values are not precise.

Because of their importance in the occurrence and movement of both surface and ground waters, detailed studies were made of the geology and geomorphology of southern Florida.

As a result of widespread crustal movements, southern Florida emerged from the sea in late Pliocene time and probably was slightly tilted to the west. At the beginning of the Pleistocene the continent emerged still farther as a result of the lowering of sea level attending the first widespread glaciation. During this epoch, southern Florida may have stood several hundred feet above sea level.

During the interglacial ages the sea repeatedly flooded southern Florida. The marine members of the Fort Thompson formation in the Lake Okeechobee-Everglades depression and the Caloosahatchee River Valley apparently are the deposits of these interglacial invasions by the sea. The fresh-water marls, sands, and organic deposits of the Fort Thompson formation appear to have accumulated during glacial ages when sea level was low and the area was a land surface partly occupied by fresh-water lakes and marshes. Elsewhere in southern Florida the deposits are mainly marine limestones and sandy terrace deposits.

The Pliocene surface upon which these Pleistocene sediments were deposited was highest to the north and west of the present Everglades and Kissimmee River basin, and it sloped gently to the south, southeast, and east. On this slightly sloping floor, alternately submerged and emerged, the later materials were built; these materials modified by wind, rain, and surface and ground waters, have largely determined the present topographic and ecologic character of southern Florida.

The most important aquifer in southern Florida, and the one in which most of the wells are developed, is the Biscayne aquifer. It is composed of parts of the Tamiami formation (Miocene), Caloosahatchee marl (Pliocene), Fort Thompson formation, Anastasia formation, Key Largo limestone, Miami oolite, and Pamlico sand (Pleistocene). In some parts of southern Florida, the Pamlico sand and the Anastasia formation are not a part of the Biscayne aquifer; however, they are utilized in the development of small water supplies. Most of the Caloosahatchee marl and the Fort Thompson formation in the Lake Okeechobee area is of very low permeability. In the northern Everglades their less permeable parts contain highly mineralized waters, which appear to have been trapped since the invasions by the Pleistocene seas. These waters have been modified by dilution with fresh ground water and by chemical reactions with surrounding materials.

Sea-level fluctuations, starting at the close of the Pliocene with the highest levels and progressing toward the Recent with successively lower levels, have built a series of nearly flat marine terrances abutting against one another much like a series of broad stearsteps. Erosion and solution have defaced and, in places, have obliterated the original surficial forms of these old sea bottoms, shores, and shoreline features, but their remnants today are widespread and, in some places, are easily recognizable.