Water for the 21st Century:
The Hydrologic Cycle
The Hydrologic Cycle is the endless circulation of all the world's water, over and over, ever since Creation in the beginning of geologic time.
The Hydrologic Cycle has neither beginning nor end. It is a natural machine, a constantly running distillation and pumping system.
The Sun supplies heat energy, and this together with the force of gravity keeps the water moving. It moves from the Earth to the atmosphere as evaporation and transpiration (from plants and animals); then from the atmosphere to the Earth as condensation and precipitation (rain or snow). It continues on Earth as streamflow and ground-water movement.
As a cycle, this water system has neither beginning nor end, but from man's point of view the oceans are the major source, the atmosphere is the deliverer, and the land is the user. In this system no water is lost or gained, but the amount of water available to the user may fluctuate because of variations at the source, or more usually, in the delivering agent.
In the geologic past, large alterations in the cyclic roles of the atmosphere and the oceans have produced deserts and ice ages across entire continents. Even now, small alterations of the local patterns of the Hydrologic Cycle produce floods and droughts.
Once fallen, water may run swiftly to the sea in rivers, or may be held in a lake for a hundred years, in a glacier for thousands of years, or in the ground for 10,000 years or more. Or it may evaporate immediately. Regardless of how long the water may be delayed, it is eventually released to enter the cycle once more.
Of the 102,000 cubic miles of water that passes into the atmosphere annually
78,000 cubic miles fall directly back into the oceans
9,000 cubic miles, including some "ground water" is carried by streams to the ocean, and
15,000 cubic miles of water remaining maintains life processes, principally as soil moisture which provides water necessary to vegetation. This water continues in the Hydrologic Cycle as it reaches the atmosphere again by evapotranspiration.
More than 2 million cubic miles of fresh water is stored in the Earth, about half within a half mile of the surface. This is more than 35 times the amount held on the surface in lakes, rivers, and inland seas, but in turn, is relatively small compared to the 7 million miles stored in glaciers and icecapes. The 317 million cubic miles of water held by the oceans constitute 97.3 percent of the Earth's supply.
AAUW - Martha Black - 1/3/88
(Information extracted from U.S. Geological Survey pamphlet.)