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The water cycle, or hydrologic cycle,
describes the continuous motion of water on Earth. Since water
first appeared, it has been constant in quantity and continuously
in motion.
The cycle is driven by the sun. The sun's heat evaporates
water from the surface of oceans, land and living things,
such as plants. Clouds are formed as the water vapour cools
and condenses. When clouds become heavy with moisture, the
water falls back to the Earth as precipitation, such as rain
or snow. In this way, on average, every water molecule passes
through the cycle every 10 to 15 days. However, some water
molecules can remain in the ocean for up to 1,500 years.
The fresh water supply should be continually recycled and
purified through this water cycle. The cycle operates well
as long as people don't:
- pollute water faster than it is
replenished
- overload the cycle with either
degradable or non-biodegradable waste
- withdraw more water from the underground
deposits faster than it can be replenished
Surface Water/Groundwater
The fresh water humans and animals use comes from two sources:
surface water and groundwater. Precipitation that doesn't
seep into the ground or evaporate into the atmosphere is called
surface water. This water is located on the Earth's surface
in streams, lakes, wetlands and reservoirs.
The precipitation that seeps into the ground and fills the
pores in the rocks and soil is called groundwater. There is
40 times as much water underground as there is in the world's
streams, lakes and rivers. This water eventually returns to
the surface via underground springs and streams.
Porous, water-saturated layers of
rock that produce large amounts of water are called aquifers.
Most aquifers are recharged with water naturally through precipitation,
which flows slowly downward through the rock and soil.
Pollution
Human activity can affect the water source at many points.
Water is vulnerable to pollution because it is a solvent and
can therefore break down and absorb many materials. Under
normal circumstances, any body of water has the ability to
absorb, break down and recycle any introduced materials, but
when it can no longer do so, all additional substances become
pollutants.
Water pollution involves the release of foreign substances
into the water system. These accumulate and interfere with
the functioning of plants and animals in the aquatic ecosystem.
Around the world, factories and industries use water for cooling
and manufacturing processes. This water sometimes includes
toxins that can be absorbed by water vapour in clouds, then
returned to the Earth as acid rain. Fertilizers used by farms
can also seep into the soil and into waterways, causing contamination.
Natural Contaminants
In many countries of the South, people rely on rivers and
streams as their water source. This water can also be contaminated
with disease-causing germs and bacteria. People use the water
for many things, such as drinking, washing clothes and taking
baths. As a result of this, bacteria can enter the water and
cause illness that can be fatal. In rural communities in the
normal circumstances, any body of water has the ability to
absorb, break down and recycle any introduced materials, but
when it can no longer do so, all additional substances become
pollutants.
Some illnesses are spread by water insects. Malaria, sleeping
sickness, river blindness and yellow fever are all spread
by certain types of flies and mosquitoes that breed in or
bite near water.
Drought
Droughts occur when an area receives below-average amounts
of rain over a long period of time. They can occur anywhere,
but usually arid and semi-arid climates are more prone to
drought. Areas which faced water shortages in this decade
include northern Africa, northern China, large parts of the
Middle East and parts of the western United States. In Canada,
southern Saskatchewan and the interior valleys of British
Columbia experience frequent droughts.
Too much water can also be a problem. Prolonged rains anywhere
can cause streams and lakes to overflow and flood surrounding
land areas. In India, 90 percent of annual precipitation falls
between June and September. This causes flooding, often destroying
homes and crops. Closer to home, both Manitoba and Quebec
have experienced flooding in recent years.
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