Fresh water (or freshwater) is any naturally occurring water
and brackish water
. Fresh water is generally characterized by having low concentration
s of dissolved salts
and other total dissolved solids
. Though the term specifically excludes seawater and brackish water, it does include mineral-rich waters such as chalybeate
Fresh water may include water in ice sheet
s, ice cap
s, and even underground water called groundwater
Water is critical to the survival of all living organisms
. Some organisms can thrive on salt water, but the great majority of higher plants and most mammal
s need fresh water to live.
Fresh water is not always potable water
, that is, water safe to drink. Much of the earth
's fresh water (on the surface and groundwater) is to a substantial degree unsuitable for human consumption without some treatment. Fresh water can easily become polluted by human activities
or due to naturally occurring processes, such as erosion.
Fresh water can be defined as water with less than 500 parts per million
(ppm) of dissolved salt
Other sources give higher upper salinity limits for fresh water, e.g. 1000 ppm or 3000 ppm.
[[File:Earth water distribution.svg|thumb|upright=1.3|Visualisation of the distribution (by volume) of water on Earth. Each tiny cube (such as the one representing biological water) corresponds to approximately 1400 cubic km of water, with a mass of approximately 1.4 trillion tonnes (235000 times that of the [[Great Pyramid of Giza]] or 8 times that of [[Lake Kariba]], arguably the heaviest man-made object). The entire block comprises 1 million tiny cubes.]]
Fresh water habitats are classified as either lentic systems
, which are the stillwaters including pond
s, lakes, swamp
s and mire
which are running-water systems; or groundwater
s which flow in rocks and aquifer
s. 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 cap
The source of almost all fresh water is precipitation
from the atmosphere
, in the form of mist
. 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
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
as well as many other compounds in smaller concentrations.
areas, or areas with impoverished or dusty soils, rain-bearing winds can pick up sand
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
Saline water in ocean
s and saline groundwater
make up about 97% of all the water on Earth
. Only 2.5–2.75% is fresh water, including 1.75–2% frozen in glacier
and snow, 0.5–0.75% as fresh groundwater and soil
moisture, and less than 0.01% of it as surface water
s and river
s. 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 sheet
s. Many areas suffer from lack of distribution of fresh water, such as deserts.
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 mammal
s must have access to fresh water to live. Some terrestrial mammals, especially desert rodent
s, appear to survive without drinking, but they do generate water through the metabolism
seeds, and they also have mechanisms to conserve water to the maximum degree.
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 protist
s accomplish this using contractile vacuole
s, 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
'') uses the hormone prolactin
, while in salmon
(''Salmo salar'') the hormone cortisol
plays a key role during this process.
Many sea bird
s have special glands at the base of the bill through which excess salt is excreted. Similarly the marine iguana
s on the Galápagos Islands
excrete excess salt through a nasal gland and they sneeze out a very salty excretion.
s include freshwater snails
and freshwater bivalve
s. Freshwater crustaceans include freshwater crab
s and crayfish
Unfortunately freshwater biodiversity faces many threats. The World Wide Fund for Nature's Living Planet Index noted an 83% decline in the populations of freshwater vertebrates between 1970 and 2014. These declines continue to outpace contemporaneous declines in marine or terrestrial systems. The causes of these declines are varied but are related to what Reid et al. call the "dirty dozen". The dirty dozen are:
# A rapidly changing climate
# Online wildlife trade and invasive species
# Infectious disease
# Toxic algae blooms
# Hydropower damming and fragmenting of half the world's rivers
# Emerging contaminants, such as hormones
# Engineered nanomaterials
# Microplastic pollution
# Light and noise interference
# Saltier coastal freshwaters due to sea level rise
# Calcium concentrations falling below the needs of some freshwater organisms
# The additive—and possibly synergistic—effects of these threats
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 and unpolluted water accounts for 0.003% of total water available globally.
The increase in the world population and the increase in per capita water use puts increasing strains on the finite resources availability 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.
Many areas of the world are already experiencing stress on water availability (or water scarcity). 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.
An important concern for hydrological ecosystems is securing minimum streamflow
, especially preserving and restoring instream water allocations
. Fresh water is an important natural resource necessary for the survival of all ecosystem
s. The use of water by humans for activities such as irrigation and industrial applications can have adverse impacts on down-stream ecosystems.
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.
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. Chemical contamination of fresh water can also seriously damage eco-systems.
Uses of water include agricultural
Water used for agriculture is called "agricultural water" or farm water
The World Bank's work and publications on water resourcesFresh Water National Geographic