Download The Water Cycle Reading

The Water Cycle
Why do you think the Earth is often called
the “water planet”? Perhaps an astronaut
suggested this name. From space, an
astronaut can see that there is much more
water on land on planet Earth. Oceans cover
nearly 71% of the Earth’s surface.
Water on Earth
Figure 1 shows how Earth’s water is
distributed. Most of the Earth’s water—
roughly 97 percent-- is salt water that is
found in the oceans. Only 3 percent is fresh
water. Of that 3 %, about three quarters is
found in the huge masses of ice near the
North and South Poles. A fraction more is
found in the atmosphere. Most water in the
atmosphere is invisible water vapor, the
gaseous form of water. Less than 1 percent
of the water on Earth is fresh water that is
available for humans to use.
Oceans
To explore where Earth’s
water is found, you can take an imaginary
boat trip around the world. Your journey
starts in Miami, Florida. From here, you can
sail completely around the world without
ever going ashore. Although people have
given names to regions of the ocean, these
regions are all connected forming a single
world ocean.
Ice How can you get back to Miami?
You could sail all the way around South
America. But watch out for icebergs!
These floating chunks of ice are made of
fresh water. Icebergs in the southern Pacific
and Atlantic oceans have broken off the
massive sheets of ice that cover most of the
Antarctica. You would also find icebergs in
the Arctic Ocean around the North pole.
Rivers and Lakes.
To see examples
of fresh water in rivers and lakes, you’ll
have to make a side trip inland. Sail north
past Nova Scotia, Canada, to the beginning
of the St. Lawrence Seaway. Navigate
through the series of locks along the St.
Lawrence River. Suddenly the river widens
and you enter Lake Ontario, one of North
America’s five Great Lakes. Together, the
Great Lakes contain nearly 20 percent of all
the water in the world’s freshwater lakes.
Below Earth’s Surface When rain or
snow falls, some of the water soaks into the
ground. This water trickles down through
spaces between the particles of soil and
rock. Eventually the water reaches a layer
that it cannot move through. Water that fills
the cracks ad spaces in underground soil and
rock layers is called groundwater. Far
more fresh water is located underground
than in all Earth’s rivers and lakes.
Figure 1.
The Water Cycle
Water is naturally recycled through the
water cycle. The water cycle is the
continuous process by which water moves
through the living and nonliving parts of the
environment. In the water cycle, water
moves from bodies of water, land and
living things on Earth’s surface to the
atmosphere and back to Earth’s surface.
The sun is the source of energy that drives
the water cycle.
Water Evaporates
Water moves
continuously through the water cycle. The
cycle has no real beginning or end. You can
follow a water molecule through one
complete cycle in figure 2.
Think about a molecule of water floating
near the surface of an ocean. The sun is
shining and the air is warm. Soon the
molecule has absorbed enough heat energy
to change state. It evaporates and becomes
water vapor. Evaporation is the process by
which molecules at the surface of a liquid
absorb enough energy to change to the
gaseous state. Although the water comes
from the salty ocean, it becomes fresh
through the process of evaporation. The salt
remains in the ocean.
Large amounts of water evaporate
constantly from the surfaces of oceans and
large lakes. In addition, small amounts
evaporate from the soil, puddles, and even
from your skin. A significant amount of
water is given off by plants. Plants draw
water from the soil through their roots.
Eventually the water is given off through the
leaves as water vapor in a process called
transpiration.
Figure 2
Clouds Form Once a water molecule
has found its way into the atmosphere, warm
air carries it upward. Higher up, the air
tends to become much colder. Cold air
holds less water vapor than warm air. Some
of the water vapor cools and condenses into
liquid water. Condensed droplets of water
clump together around tiny dust particles in
the air, forming clouds. This process is
called condensation.
Water Falls as Precipitation
As
more water vapor condenses, the water
droplets in a cloud eventually become so
heavy that they fall back to Earth. Water
that falls to Earth as rain, snow, hail, or sleet
is called precipitation. Most water
molecules probably spend only about 10
days in the atmosphere before falling back
to Earth. Most precipitation falls directly
into the oceans. Water in the ocean may
stay there for many years before
evaporating, thus continuing the cycle.
When precipitation falls on land,
some of the water evaporates again
immediately. Some called runoff runs over
the surface of the land into rivers and lakes.
From there it may eventually evaporate or
flow back into the ocean. Some water
trickles down into the ground and forms
groundwater. Ground water may move
underground until it reaches a river, lake, or
ocean. Once groundwater reaches the
surface, it can continue through the cycle by
evaporating again.
Precipitation is the source of all fresh
water on and below Earth’s surface. The
water cycle renews the usable supply of
fresh water on Earth. For millions of years,
the total amount of water on Earth has
remained fairly constant. In the world as a
whole, the rates of evaporation ad
precipitation are balanced.
Water Used in the Home
Task
Showering for 5 minutes
Brushing teeth with water
running
Washing hands
Flushing traditional toilet
Flushing “low-flow” toilet
Washing one load of laundry
Running dishwasher
Washing dishes by hand
Water
Used
(liters)
95
10
7.5
23
6
151
19
114
Figure 3
How Water is Used
The water people use at home is just a small
percentage of all the water used in the
United States. In addition to household
purposes, people use water for agriculture,
industry, transportation and recreation.
Agriculture
A constant supply of
fresh water is essential to grow food.
However, some parts of the United States
don’t receive enough regular rainfall for
agriculture. In such areas, farmland must be
irrigated. Irrigation is the process of
supplying water to areas of land to make
them suitable for growing crops. In the
United States, more water is used for
irrigating farmland than for any other single
use.
Industry Think about the objects in a
typical school locker. Did you know that
water is needed to produce all these objects?
Even though water is not part of the final
products, it plays a role in the industrial
processes that created them.
Industries use water in many other
ways. For example, power plants and steel
mills both need huge volumes of water to
cool down hot machinery. Water that is
used for cooling can often be recycled, or
used again for another purpose.
Transportation and Recreation
Oceans and rivers have served as natural
highways for transporting people and goods
since ancient times. You can probably think
of a lot of ways water is used for recreation.
And if you ski, snowboard, or skate, you are
enjoying water in its frozen state.
Water and Living Things
Here’s a
riddle for you: What do you and an apple
have in common? You both consist mostly
of water! In fact, water is a large part of
every living thing. Water makes up nearly
two thirds of your body. That water is
necessary to keep you body functioning.
Water is essential for living things to
grow, reproduce, and carry out other
important processes. Plants use water plus
carbon dioxide and energy from the sun to
make food in a process called
photosynthesis. Another way living things
use water is as a home. An organism’s
habitat is the place where it lives and that
provides the things it needs to survive. Both
fresh and salt water provide habitats for
many different types of living things.
Cornell Notes Topics for The Water Cycle Reading Packet pp. 1-­‐4 Note: Draw a line to divide the page into two columns. Label your notes with a title, date, your name. Leave 4 lines between each note item to fill in with class notes later. Number your notes. 1. How much of the Earth’s water is salt water and how much of the Earth’s surface does it cover? 2. Where is most of the Earth’s fresh water found? 3. Make a type of graphic organizer called a concept map showing the distribution of water on the Earth. Use the pie chart graphic organizer in figure 1 to get your information. 4. What percent of the Earth’s water is usable by humans? 5. What are three different forms that fresh water takes? 6. What causes or drives the water cycle? 7. evaporation 8. condensation 9. precipitation 10. Does the amount of water on the Earth change from year to year? 11. Runoff 12. groundwater 13. List an agricultural use, an industrial use, and a household use of water that you relied on today. 13. How are plants a part of the water cycle?