Freshwater is basic to life: it supports plants and animals in
almost all ecosystems, even those in deserts. Aquatic
ecosystems are perhaps the most obvious evidence of the importance
of freshwater: lakes, ponds, rivers, streams and wetlands support
a wide variety of species and provide important ecological functions.
Freshwater is also a basic component of human society. Not only
does freshwater support resources such as agriculture, fish and
forests upon which society depends for food, clothing and shelter,
but freshwater also provides a medium for transportation and energy
production, governs settlement patterns, provides a major recreational
resource for society, and inspires artistic and cultural expression.
Water in the World
Figure 1 below demonstrates just how little of the world's total
water supply is fresh water, and the tiny amount of the freshwater
that is not in ice nor underground. ("Underground water"
is defined as water at or beneath the water table; water found as
soil moisture is considered to be surface water).
[D] Click for larger version, 21 KB Figure 1. The World's Water Supply
Canada is one of the most fortunate nations when it comes to available
freshwater: Canada has only 0.5% of the world's population, but
its landmass contains approximately 9% of the world's renewable
water supply (i.e. water replenished by precipitation on a short-term
basis). Tables 1 and 2 below show the earth’s salt water and
freshwater stocks.
Table 1. Major Stocks of Salt Water on Earth (thousand cubic kilometres)
Major Stocks of Salt Water on Earth (thousand cubic kilometres)
Oceans |
1
338 000 |
96.54 |
Saline/brackish
groundwater |
12 870 |
0.93 |
Saltwater
Lakes |
85 |
0.006 |
Total
salt water stocks |
1
350 955 |
97.48 |
Total
water on earth(1000 kilometres cubed) |
1
385 984 |
100.00 |
|
Source. Adapted from: Peter H. Gleik, The
World's Water 2000-2001. Washington, DC: Island Press, 2000.
Table 2. Major Stocks of Fresh Water on Earth (thousand cubic
kilometres)
Major Stocks of Fresh Water on Earth (thousand cubic kilometres)
Glaciers,
permanent snowcover |
24
064 |
1.74 |
68.7 |
Fresh
groundwater |
10 530 |
0.76 |
30.06 |
Ground
ice, permafrost |
300 |
0.022 |
0.86 |
Total
frozen and underground freshwater stocks |
34
894 |
2.52
|
|
Freshwater
lakes |
91 |
0.007 |
0.26 |
Soil moisture |
16.5 |
0.001 |
0.05 |
Atmospheric
water vapor |
12.9 |
0.001 |
0.04 |
Marshes,
wetlands* |
11.5 |
0.001 |
0.03 |
Rivers |
2.12 |
0.0002 |
0.006 |
Incorporated
in biota |
1.12 |
0.0001 |
0.003 |
Total
not frozen or underground freshwater stocks |
135 |
0.010 |
100.00 |
Total
freshwater stocks |
35
029 |
2.53 |
|
Total
water on earth(1000 kilometres cubed) |
1
385 984 |
100.00 |
|
|
Source. Adapted from: Peter H. Gleik, The
World's Water 2000-2001. Washington, DC: Island Press, 2000.
* Marshes, wetlands and water incorporated in biota are often mixed
salt and fresh water.
Note: Totals may not add due to rounding.
The Hydrologic Cycle
Water is unique among substances found in the earth's environment
by occurring in all three states of matter: solid, liquid and gas.
Both salt and fresh water occur as a liquid, but all forms of solid
and gaseous water are freshwater. As a solid, water occurs commonly
as glaciers, icefields and snow. As a liquid, freshwater occurs
as surface water, soil moisture and groundwater (water in the ground,
but found at the water table and below). Finally, as a gas, water
occurs as water vapour. The water that drops to earth as precipitation
can be in any of these forms, as rain, snow and fog demonstrate.
Each of these expressions of freshwater has a special place in
ecology, and in uses by humans. The hydrologic
cycle shows how types and locations of water link to one another.
[D] Click for larger version, 48 KB Figure 2. The Hydrologic Cycle
The hydrologic cycle (see figure 2 above) is an
endless circulation of water from one state and place to another
- from the atmosphere to the earth and its return to the atmosphere
through condensation, precipitation, evaporation and transpiration.
- Evaporation / transpiration: On average, as
much as 40% of precipitation in Canada is either evaporated or
transpired. Transpiration is the process by which plants lose
water to the atmosphere, generally through their leaves.
- Percolation into the ground: Water moves downward
through cracks and pores in the soil and rocks to the water table.
Water can move back up by capillary action or it can move either
vertically or horizontally under the earth's surface until it
re-enters a surface water system.
- Surface runoff: Water runs overland into nearby
streams and lakes: the steeper the land and the less porous the
soil, the greater the runoff. Overland flow is runoff before the
water reaches a normal river or drainage channel. Overland flow
is particularly visible in urban areas, due to the high proportion
of area covered by asphalt and concrete. Ultimately all runoff
goes into rivers and eventually into the ocean.
Although the hydrologic cycle balances what goes up with what comes
down, one phase of the cycle is "frozen" in colder regions
during the winter season. During the Canadian winter, most of the
precipitation is simply stored as snow or ice on the ground. Later,
during the spring melt, huge quantities of water are released quickly.
This results in the familiar heavy spring runoff, and often results
in localized flooding. Later sections of this issue discuss flooding
problems.
Water, Ecosystems and Sustainable Development
In nature nothing exists alone. Living things, including humans,
relate to each other and are supported by their non-living environment.
These complex relationships are called ecosystems.
Water is not only vital to life, but as it is in continuous interaction
with the surrounding air and land and living things, it is also
a vital component of healthy functioning of any ecosystem.
Understanding ecosystems is key to achieving a sustainable environment
and economy. Sustainable
development is a management philosophy whereby the use of resources
and the environment today does not damage prospects for their use
by future generations. To make the development truly sustainable,
social, economic and environmental aspects must be integrated. Canada's
water resources must be developed in harmony with natural ecosystems
so that neither the water resource nor the plant and animal life
dependent on it are depleted or destroyed for short-term gain and
at the expense of their potential benefits for future generations
of people. Long-term economic growth depends on a healthy environment.
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