Unit 4: Water and its management
4.1 The distribution of water on Earth
More than 75% of Earth is covered in water, although
only 3% of this water is fresh and potentially useful for
humans. Less than 1% of all freshwater is readily
available for people to use. Half of the available
freshwater is found in just six countries: Brazil, Russia,
Canada, Indonesia, China and Colombia.
4.2 The water cycle
The change in the state of water will lead to a change in where it is. With the warmth of the
Sun, water evaporates and becomes vapour. This vapour rises to the sky, condenses and
turns into clouds. Under certain conditions those clouds will fall as rain or snow
(precipitation). That water may then enter the soil and be taken up by plants, and then
released in the process of transpiration. Other possibilities are that it will flow along the
ground in a process called surface run-off, be stopped from reaching the ground
(interception), or enter the ground by infiltration and then become part of groundwater
flow if it flows through rocks, or through flow if it flows through soil.
4.3 Why humans need water
Domestic purpose
Used for drinking and cooking (3%), in MEDCs about 50% of domestic water is used for
washing and flushing the toilet, and the other 20% is used for washing clothes. It can also be
used for watering the garden and lost in leaks.
Industrial purpose
Used for cooling in the production of electricity and used as a solvent for a wide range of
substances.
Agricultural purpose
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, Used for irrigation. Plants need water to transport minerals and food, to keep their cells rigid,
and to combine with CO2 to make food in photosynthesis. Domestic animals also require a
reliable source of water.
4.4 The main sources of freshwater for human use
Fresh water is visible on the Earth’s surface as lakes, rivers and swamps →
surface water
There is also water in the ground → groundwater. Water can also be stored in the
spaces of porous rocks, this is called an aquifer. Water from the sea can also be
transformed into potable water by desalination.
Water from rivers
Can be taken out by simply dipping a bucket into it. A reservoir may be created
behind a dam or by the side of the river, which forms a large store of water.
Usually this water is not safe, but it can be treated. Another type of reservoir is
the service reservoir, in which treated water is stored for use → eg: water towers,
cisterns.
Water from the ground
Water that infiltrates the ground may accumulate in porous rocks, this is called an aquifer.
The most common way in which water is obtained is to sink wells into them. A well can be
dug by hand or bored into rock by machinery. If the water is not under pressure, it has to be
raised to the top of the well, this can be done by lowering a bucket on a rope or with a hand
operated pump. If the water is under pressure, the aquifer is referred to as an artesian
aquifer. This water will rise to the surface without the need of a pump.
Water from the sea
There are two processes to make salt water potable.
The first method of desalination is distillation, in which the water is boiled and released as
vapour, leaving the salt behind. The vapour is condensed as liquid water and then it can be
used. This method is about 10-30% efficient, it produces large amounts of waste, salt water
(brine) which needs to be disposed of. This can be a source of pollution, because energy is
needed and the production of energy may cause pollution. Distillation may be no more costly
unless there are alternative local sources.
The second desalination method is reverse osmosis. Salt water is pumped at high pressure
through very fine membranes to remove salt. Reverse osmosis is about 30-50% efficient.
Brine is also a byproduct which needs to be disposed of. Reverse osmosis also requires
energy but less than distillation.
4.5 Availability of safe drinking water around the world
Physical water scarcity: there isn’t enough water for human needs, maybe because of low
rainfall or high evaporation levels.
Economic water scarcity: there is enough water available but there’s no money to
extract/treat enough of it for human needs.
Even if water is available, it may not be safe to drink. These are two ways to ensure that
water is potable:
- sanitation systems, which ensure that dirty water doesn’t mix with potable water
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4.1 The distribution of water on Earth
More than 75% of Earth is covered in water, although
only 3% of this water is fresh and potentially useful for
humans. Less than 1% of all freshwater is readily
available for people to use. Half of the available
freshwater is found in just six countries: Brazil, Russia,
Canada, Indonesia, China and Colombia.
4.2 The water cycle
The change in the state of water will lead to a change in where it is. With the warmth of the
Sun, water evaporates and becomes vapour. This vapour rises to the sky, condenses and
turns into clouds. Under certain conditions those clouds will fall as rain or snow
(precipitation). That water may then enter the soil and be taken up by plants, and then
released in the process of transpiration. Other possibilities are that it will flow along the
ground in a process called surface run-off, be stopped from reaching the ground
(interception), or enter the ground by infiltration and then become part of groundwater
flow if it flows through rocks, or through flow if it flows through soil.
4.3 Why humans need water
Domestic purpose
Used for drinking and cooking (3%), in MEDCs about 50% of domestic water is used for
washing and flushing the toilet, and the other 20% is used for washing clothes. It can also be
used for watering the garden and lost in leaks.
Industrial purpose
Used for cooling in the production of electricity and used as a solvent for a wide range of
substances.
Agricultural purpose
Página 1 | 7
, Used for irrigation. Plants need water to transport minerals and food, to keep their cells rigid,
and to combine with CO2 to make food in photosynthesis. Domestic animals also require a
reliable source of water.
4.4 The main sources of freshwater for human use
Fresh water is visible on the Earth’s surface as lakes, rivers and swamps →
surface water
There is also water in the ground → groundwater. Water can also be stored in the
spaces of porous rocks, this is called an aquifer. Water from the sea can also be
transformed into potable water by desalination.
Water from rivers
Can be taken out by simply dipping a bucket into it. A reservoir may be created
behind a dam or by the side of the river, which forms a large store of water.
Usually this water is not safe, but it can be treated. Another type of reservoir is
the service reservoir, in which treated water is stored for use → eg: water towers,
cisterns.
Water from the ground
Water that infiltrates the ground may accumulate in porous rocks, this is called an aquifer.
The most common way in which water is obtained is to sink wells into them. A well can be
dug by hand or bored into rock by machinery. If the water is not under pressure, it has to be
raised to the top of the well, this can be done by lowering a bucket on a rope or with a hand
operated pump. If the water is under pressure, the aquifer is referred to as an artesian
aquifer. This water will rise to the surface without the need of a pump.
Water from the sea
There are two processes to make salt water potable.
The first method of desalination is distillation, in which the water is boiled and released as
vapour, leaving the salt behind. The vapour is condensed as liquid water and then it can be
used. This method is about 10-30% efficient, it produces large amounts of waste, salt water
(brine) which needs to be disposed of. This can be a source of pollution, because energy is
needed and the production of energy may cause pollution. Distillation may be no more costly
unless there are alternative local sources.
The second desalination method is reverse osmosis. Salt water is pumped at high pressure
through very fine membranes to remove salt. Reverse osmosis is about 30-50% efficient.
Brine is also a byproduct which needs to be disposed of. Reverse osmosis also requires
energy but less than distillation.
4.5 Availability of safe drinking water around the world
Physical water scarcity: there isn’t enough water for human needs, maybe because of low
rainfall or high evaporation levels.
Economic water scarcity: there is enough water available but there’s no money to
extract/treat enough of it for human needs.
Even if water is available, it may not be safe to drink. These are two ways to ensure that
water is potable:
- sanitation systems, which ensure that dirty water doesn’t mix with potable water
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