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 The
water cycle is the continuous movement of water in and around the Earth.
As previously mentioned, water never really goes away -- it just changes
form. The sun drives the entire water cycle and is responsible for its
two major components: condensation and evaporation. When the sun heats
the surface of water, it evaporates and ends up in the atmosphere as
water vapor. It cools and rises, becoming clouds, which eventually
condense into water droplets. Depending on the temperature of the
atmosphere and other conditions, the water precipitates as rain, sleet,
hail or snow.
Some of this precipitation is captured by tree canopies and evaporates
again into the atmosphere. The precipitation that hits the ground
becomes runoff, which can accumulate and freeze into snow caps or
glaciers. It can also infiltrate the ground and accumulate, eventually
storing in aquifers. An aquifer is a large deposit of groundwater that
can be extracted and used. This runoff also comes from snowmelt, which
occurs when the sun and climate changes melt snow and ice. Finally, some
of this runoff makes it way back into lakes and oceans, where it is
again evaporated by the sun. You can learn more about the water cycle in
How the Earth Works.
Water that falls to the ground and stays in the soil ends up evaporating
and retiring to the atmosphere. But groundwater, which is the major
source of our drinking water, can accumulate in aquifers over thousands
of years. Unconfined aquifers have the water table, or the surface where
water pressure equals atmospheric pressure, as their upper boundaries.
Confined aquifers often lie below unconfined aquifers and have a layer
of rock or other materials as their upper boundaries.
In the United States, the oldest groundwater, known as fossil water, is
contained in the Ogallala Aquifer. Lying below about 175,000 square
miles (450,000 square kilometers) of eight states in the Great Plains,
the Ogallala Aquifer stores about 2,900 million acre-feet (3,600 million
kilometers cubed) of water [source: High Plains/Ogallala Aquifer]. The
Ogallala Aquifer was formed between 2 and 6 million years ago, when the
Rocky Mountain chain was forming. Because the climate of the Great
Plains is arid, water in the aquifer is being used faster than it can be
recharged. That's why some scientists refer to using fossil water
aquifers as water mining.
Groundwater may also exist on other planets. Images from the Mars Global
Surveyor spacecraft show what looked like gullies carved out by rivers
of water on the surface of the planet. According to NASA, the water is
probably 300 to 1,300 feet (100 to 400 meters) below the surface. Europa,
one of Jupiter's moons, may also have subsurface water. As our need for
water outweighs the Earth's supply, scientists wonder if we may one day
mine for water on the other planets and moons in our solar system.
Water has a lot of unique and amazing properties that make it so
important to life. They're why we're constantly looking for better ways
to obtain and conserve it.
Source: http://science.howstuffworks.com |
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