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 Watch
the water cycle in action. Press the play button to begin. |
The Hydrologic Cycle
The Earth's water is
continually in motion in a process known as the hydrologic cycle. The
process consists of water entering the atmosphere through either evaporation
or transpiration and returning to the Earth's surface through condensation
and precipitation.
The waters of our long history are the waters of today.
The same water has been transferred time and time again from the
oceans into Earth's atmosphere and then dropped upon land and ultimately,
moved back to the seas. It evaporates from
oceans, streams, lakes, rivers and other forms of surface water and rises
into the sky as water vapor.
Water vapor comes together in the form of precipitation.
About 70 percent of this water evaporates back into the atmosphere. About
10 percent of the total amount remains on the surface and runs off into
ponds, lakes and rivers. The remaining 20 percent seeps into the soil
and becomes groundwater.
The sun supplies the energy to keep the water moving from
the Earth to the atmosphere and back to Earth. At any given time, only
about five gallons of every 100,000 gallons of water is in motion. Viewed
as a cycle of nature, this process has neither a beginning nor an end.
No water is actually gained or lost, but the amount of water available
to any user may fluctuate and changes in water quality.
In our geologic past, changes in this cycle produced ice
ages across continents and created vast deserts. Today, even small changes
in local weather cycle patterns produce floods or droughts that affect
many people. When turning on the faucet for a glass of water, a person
is tapping into the Earth's hydrologic cycle.
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