The Water Cycle
Evaporation
• Water on Earth's
surface enters the
atmosphere through
evaporation. Heat
energy from the Sun
breaks the bonds that
hold water molecules
together. The free
molecules spread out
and become a gas.
Evaporation
• The Sun is the driving
force behind the
water cycle, which
begins with
evaporation. Nearly
90% of moisture in
the atmosphere
evaporates from
oceans, seas, lakes,
and rivers.
Evaporation
• On a global scale, the
amount of evaporated
water equals the amount
of precipitation. This
keeps the water cycle in
balance. Although global
evaporation equals global
precipitation, it is not
equally distributed over
the earth. Evaporation is
greater than precipitation
over oceans. The
opposite is true over land,
where precipitation is
greater than evaporation.
Evaporation
• Evaporation requires
energy and water. It takes
about 600 calories of heat
to evaporate one gram of
water. The warmer the
climate is, the more heat
energy is available for
evaporation. Most of the
world's evaporation
happens in tropical areas
near the equator.
Infiltration and Runoff
• Earth's surface acts like a
giant sponge as
precipitation seeps into
soil and rock.
• This process is called
infiltration.
• Infiltrated water
replenishes groundwater,
which can be deep
underground.
• The amount of infiltration
depends on several
factors:
Infiltration and Runoff
• Precipitation intensity and
duration—more water
infiltrates when
precipitation happens
over an extended period.
• Soil properties—sandy
soils absorb more water
than clay soils.
• Soil saturation—dry soil
absorbs more water than
wet soil.
• Slope of the land—flat
areas absorb more water
than slopes.
Infiltration and Runoff
• Water that doesn't infiltrate flows over the
ground as surface runoff. If you've ever seen a
stream of water flowing downhill during a
rainstorm, you have seen surface runoff.
• Rivers and streams also carry surface runoff.
• About 1/3 of runoff water returns to the ocean.
• The rest reenters the atmosphere through
evaporation and transpiration.
• Eventually, this water will condense and fall
back to the surface as precipitation.