Water Cycle
Water is constantly being cycled between the atmosphere, the ocean and land. This cycling is a very important process that helps sustain life on Earth.
As the water evaporates, vapors
rise and condense into clouds. The clouds move over the land, and precipitation falls in
the form of rain, ice or snow. The water fills streams and rivers, and eventually flows
back into the oceans where evaporation starts the process anew.
Water's state (solid, liquid or gas) is determined mostly by temperature. Although water continuously changes states from solid to liquid to gas, the amount of water on Earth remains constant. There is as much water now as there was hundreds of millions of years ago.
There are six
important processes that make up the water cycle. These are:
Evaporation
Evaporation is the process where a liquid,
in this case water, changes from its liquid state to a gaseous state. Liquid water becomes
water vapor. Although lower air pressure helps promote evaporation, temperature is the
primary factor. For example, all of the water in a pot left on a table will eventually
evaporate. It may take several weeks. But, if that same pot of water is put on a stove and
brought to a boiling temperature, the water will evaporate more quickly.
During
the water cycle some of the water in the oceans and freshwater bodies, such as lakes and
rivers, is warmed by the sun and evaporates. During the process of evaporation, impurities
in the water are left behind. As a result, the water that goes into the atmosphere is
cleaner than it was on Earth.
Condensation
Condensation is the opposite of
evaporation. Condensation occurs when a gas is changed into a liquid. Condensation occurs
when the temperature of the vapor decreases.
When the water droplets formed from condensation are very small,
they remain suspended in the atmosphere. These millions of droplets of suspended water
form clouds in the sky or fog at ground level. Water condenses into droplets only when
there are small dust particles present around which the droplet can form.
Precipitation
When the temperature and atmospheric
pressure are right, the small droplets of water in clouds form larger droplets and
precipitation occurs. The raindrops fall to Earth.
As a result of evaporation, condensation and precipitation, water
travels from the surface of the Earth goes into the atmosphere, and returns to Earth
again.
Surface
Runoff
Much of the water that returns to Earth as
precipitation runs off the surface of the land, and flows down hill into streams, rivers,
ponds and lakes. Small streams flow into larger streams, then into rivers, and eventually
the water flows into the ocean.
Surface runoff is an important part of the water cycle because,
through surface runoff, much of the water returns again to the oceans, where a great deal
of evaporation occurs.
Infiltration
Infiltration is an important process where
rain water soaks into the ground, through the soil and underlying rock layers. Some of
this water ultimately returns to the surface at springs or in low spots downhill. Some of
the water remains underground and is called groundwater.
As the water infiltrates through the soil and rock layers, many
of the impurities in the water are filtered out. This filtering process helps clean the
water.
Transpiration
One final process is important in the water
cycle. As plants absorb water from the soil, the water moves from the roots through the
stems to the leaves. Once the water reaches the leaves, some of it evaporates from the
leaves, adding to the amount of water vapor in the air. This process of evaporation
through plant leaves is called transpiration. In large forests, an enormous amount of
water will transpire through leaves.