4.3
The Water Cycle
Sometimes quickly, sometimes slowly, water is always on the
move, forever changing state and location. This movement is
not restricted to Earth’s surface, for water sinks deep below
ground level and rises high into the atmosphere. It is this
movement of water, known as the water cycle and illustrated in
Figure 1, that is responsible for much of our weather, that keeps
rivers and lakes full, and that allows water to purify itself and
sustain many forms of life.
c Eventually, so much water
gathers in the clouds that
the air currents can no longer
keep it aloft. It falls to Earth
as precipitation: rain, hail,
sleet, or snow.
b As the air rises, it cools, and cannot
hold as much water vapour. The
cooled water vapour in the air
becomes liquid again, a process called
condensation. Tiny drops collect
around dust particles, forming clouds
or fog.
c
b
d
a Heat energy from the sun
causes water on Earth’s
surface to change to
water vapour, a process
called evaporation.
Salts, pollutants, and
impurities are left behind
as the pure water rises
into the air as water
vapour. Water also
evaporates from the
soil, animals, and plants.
Water evaporating
from plants is called
transpiration.
Figure 1
The water cycle
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Unit 4
a
e
g
h
f
k
Understanding Concepts
1. What is the water cycle?
d Snow falling in the mountains or polar regions may
remain frozen for years. Gradually, layers accumulate
and their pressure turns the bottom layers of snow
to ice, forming a glacier. Some snow and ice at the
surface can also change directly back into water
vapour. This is called sublimation.
e Water flowing along the surface of the ground is
called runoff. This water gathers in rivers, lakes,
and oceans. Lakes or ponds form wherever a
basin (a natural depression) allows water to gather.
2. Name and describe any four
changes of state that occur in
the water cycle.
Making Connections
3. How has the water cycle helped
determine population patterns
in North America? Can you give
examples of where people have
ignored the water cycle’s effects
in determining where to live?
Exploring
f
Estuaries are areas where fresh water from
rivers and salt water from the oceans mix to
form moderately salty or brackish water.
g Marshes are low-lying, treeless areas of
soft, wet ground that is usually covered
by water for at least part of the year.
Marshes may contain either fresh or salt
water and are characterized by the
grasses, cattails, and other plants that
grow there.
h Swamps, like marshes,
are areas of low-lying,
wet land that at times is
covered by water.
Swamps also can be
fresh or salt water but,
unlike marshes, contain
many trees and shrubs.
i
i
j
j
You sometimes find
deserts on one side
of mountains. Air loses
much of its moisture
as it blows over the
mountains, therefore
little precipitation falls
on the far (leeward) side.
Ground water is water that has
soaked into the earth, where it
passes through gravel, sand, soil,
and porous rock on its way back
to rivers, lakes, and oceans.
k Water that returns to the ocean is not the same pure
water that evaporated. Even precipitation that falls directly
into the oceans may contain trace amounts of acid or
other chemicals from reactions that take place in the
atmosphere. Because of water’s ability to dissolve many
substances in both liquid and vapour form, pollutants,
chemicals, and dissolved minerals and salts can be carried
by ground and surface water into lakes and oceans.
SKILLS HANDBOOK: 4A Research Skills 8C Multimedia Presentations
4. Unfortunately, pollutants can
enter the water cycle at any stage.
4A Using the Internet, newspapers,
and other resources, research one
type of pollution. Describe and
illustrate how it can enter the
water cycle. Propose some ways
to remedy the problem.
5. Imagine that you are a droplet of
water on the calm surface of a
lake. It is a warm summer’s day.
You feel the sun beating down on
you, warming you, giving you
energy. You begin to move, faster,
then faster still. The warmer it
gets, the more energy you have.
You move more freely. Suddenly,
you break free from the water’s
hold. Now you are floating ....
Create a presentation of the
8C
water cycle in an interesting way
for young children—through a
story (as in the example above),
visually, dramatically, or musically.
Reflecting
6. What does the word “cycle”
mean? Apply this meaning to
the water cycle.
How might you use your knowledge
of the water cycle and the changes
of state that take place to help you
design a water purifier?
Water Systems
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