Water is life. All living organisms are predominantly
made of water: human beings about 60 %, fish about 80 %, plants between 80 % and 90 %. Water is necessary for all chemical
reactions that occur in living cells and is also the medium through which information is exchanged between cells. The sustainability
of human development depends on the hydrological cycle, since water is essential for food production and all living ecosystems.
Freshwater Resources
Seventy percent of the earth's surface is covered
by water. Ninety-seven of this water is contained in oceans, hence salty and unsuitable for drinking or irrigation. Of
the remaining 3 % of freshwater, only 0,3 % is found in rivers and lakes, the rest being frozen.
Water is a renewable resource, made continuously
available through solar energy, which enables it to evaporate from oceans and land, redistributing it around the
world. This water runs off in rivers and refills our aquifers. On an annual basis, rainfall exceeds evaporation
on continents by 44 000 km3. This amount of water returns to the oceans as river and groundwater
runoff. This is called the Water Cycle.
Freshwater Use
If this runoff were evenly distributed in space
and time, freshwater resources would be greatly sufficient to provide water for all. A quick calculation shows that water
available for human consumption represents 15 000 litres per person and per day. However, this figure does not reflect
reality, because freshwater resources are not evenly distributed. Rainfall and runoff are apportioned in both space and time
in an irregular manner. Some regions receive enormous quantities of water, others receive almost none. Many regions
get nearly 100% of their precipitation during a brief rainy season.
Therefore, renewable water potentially available
for human consumption is evaluated at 10 000 to 12 000 km3 per year. Out of this quantity, only 30 % was
withdrawn in 2000, or around 4 000 km3, and 15 %, or around 2 000 km3, was consumed (i.e.
evaporated). At the global level, the water situation is not so alarming, but due to uneven distribution, some
countries face water scarcity (see Water Crisis).
What are the various uses of freshwater?
Traditionally, three main sectors of freshwater
use are distinguished. The domestic sector includes household and municipal uses and water used for commercial establishments
and public services. Industrial use includes water withdrawn for industry, an increasing part of it being used to cool power
plants. The agricultural sector includes water for irrigation and livestock.
A differentiation must be made between
water availability, water withdrawals and water consumption. "Available water" is the quantity of renewable water
resources available for human use. "Water withdrawals" refers to water diverted from streams or rivers and pumped from
groundwater aquifers for human use, but not necessarily consumed. Part of the withdrawn water is returned
after use and is subsequently reused or restored to the environment. The quantity that is not reused or left in nature represents
"consumed water", namely water that is evaporated or incorporated into products and organisms, so that it becomes
temporally unavailable to the other users.
|
Sector |
Water Withdrawals |
Water Consumption |
|
Agriculture |
66 % |
93 % |
|
Industry |
20 % |
4 % |
|
Domestic use |
10 % |
3 % |
|
Evaporation from reservoirs |
4 % |
|
Source: Shiklomanov, 1999
courtesy world water council
