Fresh water (or freshwater) is naturally occurring water on Earth's
surface in ice sheets, ice caps, glaciers, icebergs, bogs, ponds,
lakes, rivers and streams, and underground as groundwater. Fresh water
is generally characterized by having low concentrations of dissolved
salts and other total dissolved solids. The term specifically excludes
seawater and brackish water although it does include mineral-rich
waters such as chalybeate springs.
Fresh water is not the same as potable water (or drinking water): Much
of the earth's surface fresh water and groundwater is unsuitable for
drinking without some form of treatment. This is because fresh water
can be polluted by human activities or it may be contaminated due to
naturally occurring processes.
The term "sweet water" is used to describe fresh water in contrast to
4 Numerical definition
5 Aquatic organisms
Fresh water as a resource
6.2 Limited resource
Fresh water withdrawal
6.4 Causes of limited fresh water
Fresh water in the future
6.6 Choices in the use of fresh water
7 See also
9 External links
Rivers, lakes, and marshlands, such as (from top) South America's
Amazon River, Russia's
Lake Baikal, and the
Everglades in the United
States, are types of freshwater systems.
Fresh water habitats are divided into lentic systems, which are the
stillwaters including ponds, lakes, swamps and mires; lotic, or
running-water systems; and groundwater which flows in rocks and
aquifers. There is, in addition, a zone which bridges between
groundwater and lotic systems, which is the hyporheic zone, which
underlies many larger rivers and can contain substantially more water
than is seen in the open channel. It may also be in direct contact
with the underlying underground water.
The majority of fresh water on
Earth is in ice caps.
The source of almost all fresh water is precipitation from the
atmosphere, in the form of mist, rain and snow.
Fresh water falling as
mist, rain or snow contains materials dissolved from the atmosphere
and material from the sea and land over which the rain bearing clouds
have traveled. In industrialized areas rain is typically acidic
because of dissolved oxides of sulfur and nitrogen formed from burning
of fossil fuels in cars, factories, trains and aircraft and from the
atmospheric emissions of industry. In some cases this acid rain
results in pollution of lakes and rivers.
In coastal areas fresh water may contain significant concentrations of
salts derived from the sea if windy conditions have lifted drops of
seawater into the rain-bearing clouds. This can give rise to elevated
concentrations of sodium, chloride, magnesium and sulfate as well as
many other compounds in smaller concentrations.
In desert areas, or areas with impoverished or dusty soils,
rain-bearing winds can pick up sand and dust and this can be deposited
elsewhere in precipitation and causing the freshwater flow to be
measurably contaminated both by insoluble solids but also by the
soluble components of those soils. Significant quantities of iron may
be transported in this way including the well-documented transfer of
iron-rich rainfall falling in Brazil derived from sand-storms in the
Sahara in north Africa.
Water distribution on Earth
Visualisation of the distribution (by volume) of water on Earth. Each
tiny cube (such as the one representing biological water) corresponds
to approximately 1000 cubic km of water, with a mass of approximately
1 trillion tonnes (200000 times that of the
Great Pyramid of Giza
Great Pyramid of Giza or 5
times that of
Lake Kariba, arguably the heaviest man-made object). The
entire block comprises 1 million tiny cubes.
Water is a critical issue for the survival of all living organisms.
Some can use salt water but many organisms including the great
majority of higher plants and most mammals must have access to fresh
water to live. Some terrestrial mammals, especially desert rodents
appear to survive without drinking but they do generate water through
the metabolism of cereal seeds and they also have mechanisms to
conserve water to the maximum degree.
Out of all the water on Earth, saline water in oceans, seas and saline
groundwater make up about 97% of it. Only 2.5–2.75% is fresh water,
including 1.75–2% frozen in glaciers, ice and snow, 0.5–0.75% as
fresh groundwater and soil moisture, and less than 0.01% of it as
surface water in lakes, swamps and rivers. Freshwater lakes
contain about 87% of this fresh surface water, including 29% in the
African Great Lakes, 22% in
Lake Baikal in Russia, 21% in the North
American Great Lakes, and 14% in other lakes. Swamps have most of the
balance with only a small amount in rivers, most notably the Amazon
River. The atmosphere contains 0.04% water. In areas with no fresh
water on the ground surface, fresh water derived from precipitation
may, because of its lower density, overlie saline ground water in
lenses or layers. Most of the world's fresh water is frozen in ice
sheets. Many areas suffer from lack of distribution of fresh water,
such as deserts.
Fresh water can be defined as water with less than 500 parts per
million (ppm) of dissolved salts.
Part of a series on
Fresh water (< 0.05%)
Brackish water (0.05–3%)
Saline water (3–5%)
Brine (> 5%)
Bodies of water
Salt lake • Hypersaline lake • Salt
Brine pool • Bodies by salinity
Other sources give higher upper salinity limits for fresh water, e.g.
1000 ppm or 3000 ppm.
Fresh water creates a hypotonic environment for aquatic organisms.
This is problematic for some organisms with pervious skins or with
gill membranes, whose cell membranes may burst if excess water is not
excreted. Some protists accomplish this using contractile vacuoles,
while freshwater fish excrete excess water via the kidney. Although
most aquatic organisms have a limited ability to regulate their
osmotic balance and therefore can only live within a narrow range of
salinity, diadromous fish have the ability to migrate between fresh
water and saline water bodies. During these migrations they undergo
changes to adapt to the surroundings of the changed salinities; these
processes are hormonally controlled. The eel (Anguilla anguilla) uses
the hormone prolactin, while in salmon (Salmo salar) the hormone
cortisol plays a key role during this process.
Many sea birds have special glands at the base of the bill through
which excess salt is excreted. Similarly the marine iguanas on the
Galápagos Islands excrete excess salt through a nasal gland and they
sneeze out a very salty excretion.
Freshwater molluscs include freshwater snails and freshwater bivalves.
Freshwater crustaceans include freshwater crabs, and others.
Fresh water as a resource
Water fountain found in a small Swiss village. They are used as a
drinking water source for people and cattle. Almost every Alpine
village has such a water source.
An important concern for hydrological ecosystems is securing minimum
streamflow, especially preserving and restoring instream water
Fresh water is an important natural resource
necessary for the survival of all ecosystems. The use of water by
humans for activities such as irrigation and industrial applications
can have adverse impacts on down-stream ecosystems. Chemical
contamination of fresh water can also seriously damage eco-systems.
Pollution from human activity, including oil spills and also presents
a problem for freshwater resources. The largest petroleum spill that
has ever occurred in fresh water was caused by a Royal Dutch Shell
tank ship in Magdalena, Argentina, on 15 January 1999, polluting the
environment, drinkable water, plants and animals.
Fresh and unpolluted water accounts for 0.003% of total water
Changing landscape for the use of agriculture has a great effect on
the flow of fresh water. Changes in landscape by the removal of trees
and soils changes the flow of fresh water in the local environment and
also affects the cycle of fresh water. As a result, more fresh water
is stored in the soil which benefits agriculture. However, since
agriculture is the human activity that consumes the most fresh
water, this can put a severe strain on local freshwater resources
resulting in the destruction of local ecosystems.
In Australia, over-abstraction of fresh water for intensive irrigation
activities has caused 33% of the land area to be at risk of
salination. With regards to agriculture, the
World Bank targets
food production and water management as an increasingly global issue
that will foster debate.
Fresh water is a renewable and variable, but finite natural resource.
Fresh water can only be replenished through the process of the water
cycle, in which water from seas, lakes, forests, land, rivers, and
reservoirs evaporates, forms clouds, and returns as precipitation.
Locally however, if more fresh water is consumed through human
activities than is naturally restored, this may result in reduced
fresh water availability from surface and underground sources and can
cause serious damage to surrounding and associated environments.
Fresh water withdrawal
Fresh water withdrawal is the quantity of water removed from available
sources for use in any purpose, excluding evaporation losses. Water
drawn off is not necessarily entirely consumed and some portion may be
returned for further use downstream.
Causes of limited fresh water
The increase in the world population and the increase in per capita
water use puts increasing strains on the finite resources avialability
of clean fresh water The
World Bank adds that the response by
freshwater ecosystems to a changing climate can be described in terms
of three interrelated components: water quality, water quantity or
volume, and water timing. A change in one often leads to shifts in the
others as well.
Water pollution and subsequent eutrophication also
reduces the availability of fresh water.
Fresh water in the future
Many areas of the world are already experiencing stress on water
availability. Due to the accelerated pace of population growth and an
increase in the amount of water a single person uses, it is expected
that this situation will continue to get worse. A shortage of water in
the future would be detrimental to the human population as it would
affect everything from sanitation, to overall health and the
production of grain.
Choices in the use of fresh water
With one in eight people in the world not having access to safe
water it is important to use this resource in a prudent manner.
Making the best use of water on a local basis probably provides the
best solution. Local communities need to plan their use of fresh water
and should be made aware of how certain crops and animals use water.
As a guide the following tables provide some indicators.
Table 1 Recommended basic water requirements for human needs (per
Minimum, litres / day
Range / day
Cooking and Kitchen
Water Requirements of different classes of livestock 
Average / day
Range / day
76 L (20 US gal)
57 to 95 L (15 to 25 US gal)
57 L (15 US gal)
8 to 76 L (2 to 20 US gal)
38 L (10 US gal)
23 to 53 L (6 to 14 US gal)
38 L (10 US gal)
30 to 53 L (8 to 14 US gal)
8 L (2 US gal)
8 to 11 L (2 to 3 US gal)
Table 3 Approximate values of seasonal crop water needs 
Crop water needs mm / total growing period
African Great Lakes
List of countries by freshwater withdrawal
List of countries by total renewable water resources
Properties of water
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