Hydrologic Cycle

Categories: Biogeochemical cycles; ecology

The hydrologic cycle is a continuous system through which water circulates through vegetation, in the atmosphere, in the ground, on land, and in surface water such as rivers and oceans.

The sun and the force of gravity provide the energy to drive the cycle that provides clean, pure water at the earth's surface. The total amount of water on earth is an estimated 1.36 billion cubic kilometers. Of this water, 97.2 percent is found in the earth's oceans. The ice caps and glaciers contain 2.15 percent of the earth's water. The remainder, 0.65 percent, is divided among rivers (0.0001 percent), freshwater and saline lakes (0.017 percent), groundwater (0.61 percent), soil moisture (0.005

percent), the atmosphere (0.001 percent), and the biosphere and groundwater below 4,000 meters (0.0169 percent). While the percentages of water appear to be small for these water reservoirs, the total volume of water contained in each is immense.


Evaporation is the process whereby a liquid or solid is changed to a gas. Heat causes water molecules to become increasingly energized and to move more rapidly, weakening the chemical force that binds them together. Eventually, as the temperature increases, water molecules move from the ocean's surface into the overlying air. The rate of evaporation is influenced by radiation, temperature, humidity, and wind velocity.

Each year about 320,000 cubic kilometers of water evaporate from these oceans. It is estimated that an additional 60,000 cubic kilometers of water evaporate from rivers, streams, and lakes or are transpired by plants each year. A total of about 380,000 cubic kilometers of water is evapotranspired from the earth's surface every year.

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