The Water Cycle
Chapter 11-1
The Water Cycle
Water on the Earth
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H2O
Almost all the water
on Earth is salt
water (97%).
Of the three
percent that is
fresh water, 76% is
frozen in the ice
caps at the poles.
Groundwater
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Most of the fresh
water that is not
frozen is
underground.
The water that
falls as
precipitation sinks
into the ground
(permeable) and
eventually reaches
a layer it cannot
pass through
(impermeable).
The Water Cycle
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Continuous process
by which water
moves through the
living and nonliving
parts of the
environment.
The water cycle is
driven by the sun.
Evaporation
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The process when the
sun heats up the
water in rivers, lakes,
and oceans and turns
it into vapor or steam.
The water vapor goes
into the atmosphere.
Most of the water
vapor that evaporates
into the air comes
from the oceans. The
salt does not get
enough energy to
become a gas, so it
stays behind.
So rain is always
fresh water!
Clouds

Water vapor cools as
it rises in the air.
As it reaches the
dew point, the water
vapor will condense
around dust
particles and form
clouds.
Precipitation
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Condensed water
vapor that gets
heavy enough to fall
to the ground as
rain, snow, sleet, or
hail.
Most water
molecules only spend
about ten days in
the air before
falling back as
precipitation.
Surface Runoff

Much of the
precipitation runs
off the surface and
flows downhill into
streams. These flow
into larger streams,
then rivers, and
eventually flow back
into the ocean.
Infiltration
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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.
Groundwater

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
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One final process is important in
t
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.
The greatest living movers of
water are plants.
Earth’s Water Supply
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The total amount of
water on the Earth has
not changed much since
early in its history.
The same water is cycled
over and over.
The water you drink at
lunch was probably drunk
by a dinosaur millions of
years ago!