Part 1: Water facts sheet
What are the facts?
Water is found in three states: liquid, solid and gas. There are many forms that these take on
our Earth:
solid: eg snow, ice, hail
liquid: eg rain, freshwater, seawater, droplets in clouds
gas: eg water vapour.
The Earth’s water budget
The way water moves between the Earth’s surface and the atmosphere and back again is called
the water cycle.
The water cycle is said to be a closed system – like an aquarium – because the amount of
water in our world stays the same; it just cycles between the land, the water and the atmosphere
over and over again.
Water can also leave the cycle in the upper layers of the atmosphere. The sun’s rays (radiation)
split water molecules into two gases – hydrogen and oxygen – that move away into outer space.
The water that is added to the cycle (gained) and the water that is lost from the cycle are about
equal.
The total supply of water in the world has been estimated at about 14 million cubic kilometres –
enough to cover the whole of the Earth’s surface to a depth of 2.5 kilometres.
The water is distributed in different ways.
Source
Atmosphere
Surface water (streams, ponds, lakes, rivers)
Groundwater
Glaciers and ice caps
Oceans and seas
Amount of water
0.001%
0.022%
0.397%
2.38%
97.2%
The amount of water in the atmosphere in any one place varies between 0 per cent and 4per
cent.
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What goes up must come down
The processes involved in the water cycle have to do with the way in which water particles
(molecules) behave when water is in each of its states – as a solid (ice), as a liquid (water) or as
a gas (vapour).
Here is a simple explanation of some of the processes involved.
Evaporation: Heat energy makes liquid water particles move faster. They eventually move so
fast that some break away from the surface of the liquid. These particles enter the atmosphere
as vapour. This process of changing from liquid to vapour is called evaporation.
Transpiration: Water enters the roots of plants. Some is lost from openings in the surfaces of
leaves. Water evaporates from the surface of leaves into the atmosphere. This is the process of
transpiration. So some liquid water from the soil ends up as water vapour in the atmosphere by
passing through plants.
Condensation: The temperature of air becomes lower the higher it is in the atmosphere.
Therefore, when water vapour rises it becomes cooler. The particles lose energy to the
surroundings and slow down and begin to stick together, forming tiny droplets. Very large
numbers of these tiny droplets form clouds. Tiny ‘dust’ particles in the air can help this process
because the vapour can collect around them. It is sometimes possible to ‘make’ rain by using
aircraft to ‘seed’ clouds with tiny particles. The process of vapour changing to a liquid is called
condensation.
Precipitation: When the tiny droplets collect together as a cloud they can become heavy
enough to fall as rain. If the droplets freeze around small ice particles, hail forms. In stormy
conditions the hail stones can get bigger. If the winds in the atmosphere cause the tiny
hailstones to move up and down in the atmosphere, they gather an extra layer of ice for every
loop.
In very cold conditions, the vapour can change directly to ice, and snowflakes form.
What happens to the water?
When various types of precipitation reach the Earth, different things can happen depending on
where the rain, hail or snow lands.
Water can soak into the ground in the process called infiltration. It can be stored as
groundwater and might eventually reach the oceans by flowing underground. If it soaks down far
enough it could leave the water cycle temporarily. It may later be returned to the water cycle by
volcanic activity.
Depending on soil and surface conditions, water could run off the surface into rivers and lakes.
It could land directly in the oceans and waterways. Nearly 40 per cent of all precipitation that
falls on to the Earth flows back across the land, ending up in the sea.
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Human impact on the water cycle
Humans have affected how much water soaks into the ground, how it moves across the land
and what it carries with it. These actions can affect the way water is cycled. What happens in
one place can affect what water is available in another place. There is much controversy, for
example, over the number of dams on the Murray and Snowy rivers, and the huge dam being
built in China; what the result of having these dams will be on the life in the rivers; and who will
get to use the dammed water.
Damming rivers, irrigating, and replacing natural vegetation with crops are just a few ways
humans affect the environment and interfere with the cycling of water. Global warming has an
effect too.
Concentrating populations in large urban centres affects the cycling of water. Run-off is greater
and faster in cities because of the surfaces – roads and cement do not absorb water like the soil
they replaced. Huge demands are placed on water supplies. There is the problem of disposing
of large volumes of watery wastes. Where does the water come from and what happens to the
wastes? We have to provide for the future and maintain the water cycle.
Living things need water to survive. Our food crops need it too. Industries use massive
quantities. Our way of life makes us expect water to be there at the turn of a tap. Perhaps it is
only in times of drought that we think about where it comes from. One of the problems we have
is that water is not distributed evenly.We have to think of ways of using our water wisely. We
need to recycle water efficiently.
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