Earth's waters are 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.
Hydrologic
Cycle Interactive
See
the water cycle in action.
Watershed
Interactive
Study the parts of the watershed
process.
The same water throughout history has been transferred
from the oceans into Earth's atmosphere and then dropped upon land where
it eventually moves 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
which then falls to the Earth. About 70 percent of this water evaporates
back into the atmosphere. The water that doesn't evaporate remains on
the surface and runs off into ponds, lakes and rivers or seeps into the
soil to become 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, about
five gallons of every 100,000 gallons of water on Earth 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.
The
Hydrologic Cycle