How Water Cycles Around
the World
the World
Water Comprised simply of two atoms of
hydrogen and one of oxygen, water is a remarkable substance in many ways. Water
covers nearly three-fourths of the surface of our planet, and it is present in
the Earth’s atmosphere and in its crust. It also comprises a large part of all
plant and animal matter. It is the only natural resource that exists naturally
in three forms: liquid, solid (snow, ice) and gas (clouds). Unlike most mineral
resources, it is renewable — it exists in an endless cycle, moving between its
gaseous, liquid and solid forms. This “hydrologic cycle” comprises nature’s
method of replenishing, redistributing and purifying the world’s natural water
resources.
hydrogen and one of oxygen, water is a remarkable substance in many ways. Water
covers nearly three-fourths of the surface of our planet, and it is present in
the Earth’s atmosphere and in its crust. It also comprises a large part of all
plant and animal matter. It is the only natural resource that exists naturally
in three forms: liquid, solid (snow, ice) and gas (clouds). Unlike most mineral
resources, it is renewable — it exists in an endless cycle, moving between its
gaseous, liquid and solid forms. This “hydrologic cycle” comprises nature’s
method of replenishing, redistributing and purifying the world’s natural water
resources.
To understand how water moves in our world,
attention must be directed to the hydrologic cycle. The hydrologic cycle is
essentially a water continuum, representing the different paths through which
water circulates and is transformed in the natural environment (Fig. 1). Being
a cycle, it has no specific beginning or ending. Rather, liquid water from the Earth’s
surface, particularly the oceans, is evaporated into a gaseous form and enters
the atmosphere as water vapor (clouds). The atmospheric moisture is eventually
returned to the Earth’s surface in the form of rain or snow. It is estimated
that approximately 100, 000 cubic kilometers (about 20% of the total global
annual precipitation) falls onto the land surface of the continents. The liquid
fresh water moves over the land surface on its journey back to the ocean.
During its overland journey, it creates rivers, lakes, wetlands and/or
groundwater aquifers. Further, a portion
(so-called endorheic water-bodies) has no direct access to the oceans. As
discussed in a later section, examples of the latter include the Caspian Sea in
Eastern Europe, the Aral Sea in south-central Asia, and Pyramid Lake in the
western United States.
attention must be directed to the hydrologic cycle. The hydrologic cycle is
essentially a water continuum, representing the different paths through which
water circulates and is transformed in the natural environment (Fig. 1). Being
a cycle, it has no specific beginning or ending. Rather, liquid water from the Earth’s
surface, particularly the oceans, is evaporated into a gaseous form and enters
the atmosphere as water vapor (clouds). The atmospheric moisture is eventually
returned to the Earth’s surface in the form of rain or snow. It is estimated
that approximately 100, 000 cubic kilometers (about 20% of the total global
annual precipitation) falls onto the land surface of the continents. The liquid
fresh water moves over the land surface on its journey back to the ocean.
During its overland journey, it creates rivers, lakes, wetlands and/or
groundwater aquifers. Further, a portion
(so-called endorheic water-bodies) has no direct access to the oceans. As
discussed in a later section, examples of the latter include the Caspian Sea in
Eastern Europe, the Aral Sea in south-central Asia, and Pyramid Lake in the
western United States.
It
is estimated that approximately 42,000 cubic kilometres of precipitation flows
back to the oceans through the world’s rivers each year. Some of the
precipitation will step down into the Earth’s surface and become groundwater.
Some of it will be taken up by plants and subsequently released in gaseous form
back into the atmosphere via a process called transpiration. A substantial
quantity of water is returned to the atmosphere in this manner, thereby
short-circuiting the full hydrologic cycle. It is estimated, for example, that
a hectare of corn can transpire about 30-40 cubic metres of water back into the
atmosphere each day. Nevertheless, because of their enormous surface area, the
most important source of water in the atmosphere is evaporation from the
oceans, which comprises nearly 90% of the total global evaporation. Indeed, it
is the fact that more water evaporates from the oceans than is directly
precipitated back, thus creating the driving force for the hydrologic cycle.
is estimated that approximately 42,000 cubic kilometres of precipitation flows
back to the oceans through the world’s rivers each year. Some of the
precipitation will step down into the Earth’s surface and become groundwater.
Some of it will be taken up by plants and subsequently released in gaseous form
back into the atmosphere via a process called transpiration. A substantial
quantity of water is returned to the atmosphere in this manner, thereby
short-circuiting the full hydrologic cycle. It is estimated, for example, that
a hectare of corn can transpire about 30-40 cubic metres of water back into the
atmosphere each day. Nevertheless, because of their enormous surface area, the
most important source of water in the atmosphere is evaporation from the
oceans, which comprises nearly 90% of the total global evaporation. Indeed, it
is the fact that more water evaporates from the oceans than is directly
precipitated back, thus creating the driving force for the hydrologic cycle.
A substantial input of heat energy is
required to melt ice into liquid water, and to boil it into water vapor. A
substantial uptake of heat energy also is required to freeze it. These
properties give water a considerable capacity to resist freezing or boiling in
response to temperature changes. Another unusual property is that water is most
dense at 4¾C, when still in liquid form. Because ice (solid water at 0¾C) is
less dense than liquid water at 4¾C, it will float on the top of a water-body,
with the liquid water below it (Photo 2). Otherwise, lakes would freeze from
the bottom up, freezing all life contained within it. Further, water can
dissolve materials in its flow over the land surface and carry the materials as
it moves. If the water volume and velocity are sufficiently large, flowing water
also can pick up and carry materials that it cannot readily dissolve. A small
mountain stream, for example, will not be able to carry large rocks along the
stream channel, while a large, swiftly- flowing river can readily carry rocks,
soil and other materials in its flow. A third factor is that humans can control
the movement of water, including such measures as pumping it upstream against
the force of gravity, pooling or storing it in different locations, and even
moving it over long distances.
required to melt ice into liquid water, and to boil it into water vapor. A
substantial uptake of heat energy also is required to freeze it. These
properties give water a considerable capacity to resist freezing or boiling in
response to temperature changes. Another unusual property is that water is most
dense at 4¾C, when still in liquid form. Because ice (solid water at 0¾C) is
less dense than liquid water at 4¾C, it will float on the top of a water-body,
with the liquid water below it (Photo 2). Otherwise, lakes would freeze from
the bottom up, freezing all life contained within it. Further, water can
dissolve materials in its flow over the land surface and carry the materials as
it moves. If the water volume and velocity are sufficiently large, flowing water
also can pick up and carry materials that it cannot readily dissolve. A small
mountain stream, for example, will not be able to carry large rocks along the
stream channel, while a large, swiftly- flowing river can readily carry rocks,
soil and other materials in its flow. A third factor is that humans can control
the movement of water, including such measures as pumping it upstream against
the force of gravity, pooling or storing it in different locations, and even
moving it over long distances.
Except for that portion evaporated or
transpired directly back into the atmosphere, most of the water reaching the
land surface in the form of rain or snow (plus the dissolved and particulate
matter it accumulates during the course of its overland journey) will eventually
find its way back to the oceans via transport in streams, rivers, lakes,
reservoirs, wetlands and groundwater aquifers,
to begin its cycle anew.
transpired directly back into the atmosphere, most of the water reaching the
land surface in the form of rain or snow (plus the dissolved and particulate
matter it accumulates during the course of its overland journey) will eventually
find its way back to the oceans via transport in streams, rivers, lakes,
reservoirs, wetlands and groundwater aquifers,
to begin its cycle anew.
Although it is impossible to get an
exact figure, it is estimated that the oceans contain approximately 1.3-1.4
billion cubic kilometres of water, comprising about 97% of all the water on
Earth. Of this global total, the volume of fresh water is estimated to be about
35 million cubic kilometres. It is estimated that about 75% of the world’s
freshwater volume is locked up in frozen form in polar ice caps and glaciers,
14% is located deep underground beyond easy human reach, 11% is in groundwater
at depths accessible to human use, 0.35% is in lakes, 0.03% is in rivers, 0.06%
exists as soil moisture, and 0.035% is in the atmosphere (Fig. 2). To
illustrate the relative volumes, if all the water on Earth could be put into a
gallon jug (about 3.85 liters), the quantity readily available for human use
would be equal to about one tablespoon.
exact figure, it is estimated that the oceans contain approximately 1.3-1.4
billion cubic kilometres of water, comprising about 97% of all the water on
Earth. Of this global total, the volume of fresh water is estimated to be about
35 million cubic kilometres. It is estimated that about 75% of the world’s
freshwater volume is locked up in frozen form in polar ice caps and glaciers,
14% is located deep underground beyond easy human reach, 11% is in groundwater
at depths accessible to human use, 0.35% is in lakes, 0.03% is in rivers, 0.06%
exists as soil moisture, and 0.035% is in the atmosphere (Fig. 2). To
illustrate the relative volumes, if all the water on Earth could be put into a
gallon jug (about 3.85 liters), the quantity readily available for human use
would be equal to about one tablespoon.
Atmospheric deposition is the primary
means by which water is distributed over the Earth’s surface. Ironically, it is
estimated that there is a sufficient quantity of fresh water to supply all
present and foreseeable human water needs if the water were distributed evenly
around the world. Unfortunately, however, although nature is bountiful in
supplying fresh water, it also displays a confounding fickleness in not
distributing the water equitably around the world. This observation applies to
both location and timing. Some areas receive large quantities of precipitation
each year. In contrast, semi-arid and arid regions are characterized by a
limited or scarce water supply (Photo 3). The world’s water resources are also
unequally distributed in regard to the water flows in different regions. The
Parama River (Photo 4) is the second largest river in South America after the
Amazon River. The latter carries 16% of the world’s water runoff (Photo 5). In
contrast, arid and semi-arid regions only account for about 2% of the total
global runoff, even though they comprise about 40% of the Earth’s land surface.
means by which water is distributed over the Earth’s surface. Ironically, it is
estimated that there is a sufficient quantity of fresh water to supply all
present and foreseeable human water needs if the water were distributed evenly
around the world. Unfortunately, however, although nature is bountiful in
supplying fresh water, it also displays a confounding fickleness in not
distributing the water equitably around the world. This observation applies to
both location and timing. Some areas receive large quantities of precipitation
each year. In contrast, semi-arid and arid regions are characterized by a
limited or scarce water supply (Photo 3). The world’s water resources are also
unequally distributed in regard to the water flows in different regions. The
Parama River (Photo 4) is the second largest river in South America after the
Amazon River. The latter carries 16% of the world’s water runoff (Photo 5). In
contrast, arid and semi-arid regions only account for about 2% of the total
global runoff, even though they comprise about 40% of the Earth’s land surface.
Nature
is also inconsistent with regard to the timing of the precipitation. Some
regions receive the bulk of their annual precipitation at one time of the year,
while the primary human water needs may occur at a different time during the
year. This is a major impetus for the construction of artificial lakes or
impoundments, thereby allowing humans to store water for use at a time of their
choosing (Photo 6).
is also inconsistent with regard to the timing of the precipitation. Some
regions receive the bulk of their annual precipitation at one time of the year,
while the primary human water needs may occur at a different time during the
year. This is a major impetus for the construction of artificial lakes or
impoundments, thereby allowing humans to store water for use at a time of their
choosing (Photo 6).
Whatever its distribution and
timing, adequate supplies of fresh water are an obvious and fundamental
requirement for socioeconomic development. This is readily evident in the
limited economic development that characterizes most semi-arid and arid regions
in the absence of major human intervention to overcome the water deficit.
timing, adequate supplies of fresh water are an obvious and fundamental
requirement for socioeconomic development. This is readily evident in the
limited economic development that characterizes most semi-arid and arid regions
in the absence of major human intervention to overcome the water deficit.
Rivers are a very most
important component of the hydrologic cycle, being a primary determinant of
human settlement patterns and economic development. They also represent the
major portion of the world’s water withdrawals and water consumption. Water
runoff over the land surface is the primary interface between human activities
and their impacts on water supplies. Thus, human actions in a watershed are the primary factor determining the quantity and
quality of water available for human water uses, as well as for maintaining
natural ecosystems. One can even characterize human water use as a
type of “anthropogenic hydrologic cycle”, in that humans typically
extract water, use and degrade it, discharge it, extract and treat it when
needed again, and then re-use it in a continuing cycle. Unfortunately, the
cycle can become increasingly expensive and difficult to maintain as humans
continue to pollute and/or otherwise abuse their water resources.
important component of the hydrologic cycle, being a primary determinant of
human settlement patterns and economic development. They also represent the
major portion of the world’s water withdrawals and water consumption. Water
runoff over the land surface is the primary interface between human activities
and their impacts on water supplies. Thus, human actions in a watershed are the primary factor determining the quantity and
quality of water available for human water uses, as well as for maintaining
natural ecosystems. One can even characterize human water use as a
type of “anthropogenic hydrologic cycle”, in that humans typically
extract water, use and degrade it, discharge it, extract and treat it when
needed again, and then re-use it in a continuing cycle. Unfortunately, the
cycle can become increasingly expensive and difficult to maintain as humans
continue to pollute and/or otherwise abuse their water resources.
The water cycle
The
most important thing is to understand the states in which water occurs.
most important thing is to understand the states in which water occurs.
Here
on Earth, you’ll find water in three states: solid (ice), liquid (water) and
gas (water vapor). Our planet is the only one in our solar system where water
exists in all three of these states. We’re very lucky, because ours is the only
planet to have so much water on its surface and in its atmosphere.
on Earth, you’ll find water in three states: solid (ice), liquid (water) and
gas (water vapor). Our planet is the only one in our solar system where water
exists in all three of these states. We’re very lucky, because ours is the only
planet to have so much water on its surface and in its atmosphere.
So why do we talk about a
water cycle?
water cycle?
It’s because water
evaporates, condenses and precipitates back to earth continually in an endless
cycle. The atmosphere is like a giant cover above our planet. The water simply
cannot escape, so it’s perfectly true to say that our planet has never lost so
much as drop of water.
evaporates, condenses and precipitates back to earth continually in an endless
cycle. The atmosphere is like a giant cover above our planet. The water simply
cannot escape, so it’s perfectly true to say that our planet has never lost so
much as drop of water.
Evaporation: Water evaporates
everywhere: from puddles, swimming pools, lakes and rivers…. but also from
trees and other plants by a process called evapotranspiration.
everywhere: from puddles, swimming pools, lakes and rivers…. but also from
trees and other plants by a process called evapotranspiration.
Condensation: When the quantity of water
in the atmosphere becomes too great, it condenses to form clouds. These clouds
then produce precipitation in the form of rain, snow or hail. It is generally
accepted that the atmosphere contains 13,000 cubic kilometers
(approximately 3,120 cubic miles) of water, and that water vapor remains in the
atmosphere for around eight days on average.
in the atmosphere becomes too great, it condenses to form clouds. These clouds
then produce precipitation in the form of rain, snow or hail. It is generally
accepted that the atmosphere contains 13,000 cubic kilometers
(approximately 3,120 cubic miles) of water, and that water vapor remains in the
atmosphere for around eight days on average.
Precipitation: When water returns to the
ground, it may be absorbed by plants, and may also run off into rivers, streams
and lakes.
It may also percolate very slowly through the soil and collect at a deeper
level below the surface. This is known as the groundwater table.
ground, it may be absorbed by plants, and may also run off into rivers, streams
and lakes.
It may also percolate very slowly through the soil and collect at a deeper
level below the surface. This is known as the groundwater table.
It’s a story with no end!
Because, as you will have worked out, the cycle then begins all over again.
Because, as you will have worked out, the cycle then begins all over again.
You can now see that water
travels on an endless journey. On its way, it collects things from the
substances it passes though; things like organic matter and mineral salts. It
can also pick up bacteria.
travels on an endless journey. On its way, it collects things from the
substances it passes though; things like organic matter and mineral salts. It
can also pick up bacteria.
All these reasons explain why
you’ve almost certainly heard someone say something like: “This water has
a taste to it”. And it’s true! Water isn’t the same everywhere! There is
no such thing as totally pure, tasteless drinking water. Water brings plenty of
souvenirs back from its travels! Water is good at hiding its secrets from us.
We can’t simply limit its definition to two atoms of hydrogen and one of
oxygen.
you’ve almost certainly heard someone say something like: “This water has
a taste to it”. And it’s true! Water isn’t the same everywhere! There is
no such thing as totally pure, tasteless drinking water. Water brings plenty of
souvenirs back from its travels! Water is good at hiding its secrets from us.
We can’t simply limit its definition to two atoms of hydrogen and one of
oxygen.
The water we drink and wash
with contains a lot of naturally-occurring elements as well.
with contains a lot of naturally-occurring elements as well.
Did
you know that the water you drank this morning might have been the same water a
dinosaur drank millions of years ago? Or it may have been the same water that
supported Columbus’ ships on the sea. There is the same amount of water on
Earth today as there has always been. The water keeps moving around in an
endless cycle called The
Water Cycle.
you know that the water you drank this morning might have been the same water a
dinosaur drank millions of years ago? Or it may have been the same water that
supported Columbus’ ships on the sea. There is the same amount of water on
Earth today as there has always been. The water keeps moving around in an
endless cycle called The
Water Cycle.
Water
itself is the only substance that exists in liquid, gas and solid form – the
keys to the water cycle. Here’s how the cycle works: Water evaporates from
oceans, rivers and lakes (water in its liquid form) and rises into the
atmosphere (water in its gas form) where it condenses to form clouds.
Precipitation then falls to the earth in the form of rain (water in its liquid)
or snow (water in its solid form) where it flows into oceans, rivers and lakes
and the process begins again.
itself is the only substance that exists in liquid, gas and solid form – the
keys to the water cycle. Here’s how the cycle works: Water evaporates from
oceans, rivers and lakes (water in its liquid form) and rises into the
atmosphere (water in its gas form) where it condenses to form clouds.
Precipitation then falls to the earth in the form of rain (water in its liquid)
or snow (water in its solid form) where it flows into oceans, rivers and lakes
and the process begins again.
The Earth’s Water
Supply
Of all the water on Earth, only a small
amount is available for us to use. It’s true! 97.2% of the Earth’s water supply
is salt water.
Only 2.8% is fresh water! Of the total supply of water on Earth,
amount is available for us to use. It’s true! 97.2% of the Earth’s water supply
is salt water.
Only 2.8% is fresh water! Of the total supply of water on Earth,
§
0.6% is groundwater (we can use some of
this water)
0.6% is groundwater (we can use some of
this water)
§
0.01% is in lakes and streams (we can
use some of this water)
0.01% is in lakes and streams (we can
use some of this water)
§
2.2% is in glaciers and icecaps
2.2% is in glaciers and icecaps
§
0.001% is water vapor
0.001% is water vapor
What’s an acre-foot?
In
the water world, water is commonly measured in acre-feet. But what is an
acre-foot? One acre-foot of water can fill one acre of land, approximately the
size of a football field, 1 foot deep. An acre-foot contains 325,900 gallons of
water, and can supply the annual indoor and outdoor needs of one to two urban
households.
the water world, water is commonly measured in acre-feet. But what is an
acre-foot? One acre-foot of water can fill one acre of land, approximately the
size of a football field, 1 foot deep. An acre-foot contains 325,900 gallons of
water, and can supply the annual indoor and outdoor needs of one to two urban
households.
How much water does it
take to
§
Brush your teeth? – 2 to 5 gallons
Brush your teeth? – 2 to 5 gallons
§
Wash the car? – 50 gallons
Wash the car? – 50 gallons
§
Use the dishwasher? – 8 to 15 gallons
Use the dishwasher? – 8 to 15 gallons
§
Flush the toilet? – 1.5 to 4 gallons
(each flush)
Flush the toilet? – 1.5 to 4 gallons
(each flush)
§
Take a shower or bath? – 17 to 24
gallons
Take a shower or bath? – 17 to 24
gallons
§
Run the washing machine? – 35 to 50
gallons (each load)
Run the washing machine? – 35 to 50
gallons (each load)
It’s important that we all work to save
water. About half the water we use each year is used outdoors – watering the
garden and lawn, filling the swimming pool and washing the car. Ways to reduce
your water use outside include using a shut-off faucet when washing the car and
landscaping with plants that use less water.
water. About half the water we use each year is used outdoors – watering the
garden and lawn, filling the swimming pool and washing the car. Ways to reduce
your water use outside include using a shut-off faucet when washing the car and
landscaping with plants that use less water.
Indoors, most of the water a family
uses is in the bathroom. Saving water is important. In the bathroom, the
easiest way to save water is to shut off the faucet while you brush your teeth
or take shorter showers and not using the toilet to flush trash. Installing
low-flush toilets and low-flow showerheads can also help save lots of water. An
ultra-low-flush toilet uses just 1.5 gallons per flush compared to 4 gallons
per flush for a traditional toilet. In the kitchen make sure you wash only full
loads in the dishwasher. And if you need a new machine, ask your parents to
take a look at some of the water-efficient models that can reduce water usage
to 6 gallons per load. New washing machines also offer significant water
savings, using up to 40 percent less water per load than older machines, and
they can save energy too!
uses is in the bathroom. Saving water is important. In the bathroom, the
easiest way to save water is to shut off the faucet while you brush your teeth
or take shorter showers and not using the toilet to flush trash. Installing
low-flush toilets and low-flow showerheads can also help save lots of water. An
ultra-low-flush toilet uses just 1.5 gallons per flush compared to 4 gallons
per flush for a traditional toilet. In the kitchen make sure you wash only full
loads in the dishwasher. And if you need a new machine, ask your parents to
take a look at some of the water-efficient models that can reduce water usage
to 6 gallons per load. New washing machines also offer significant water
savings, using up to 40 percent less water per load than older machines, and
they can save energy too!
Check with your local water supplier to
see if they will conduct a water audit of your home to see where and how you
can save water. Also, check to see if they offer rebates on the purchase of
water-efficient appliances – some do.
see if they will conduct a water audit of your home to see where and how you
can save water. Also, check to see if they offer rebates on the purchase of
water-efficient appliances – some do.
