Many e stimates of the global water resources have been made over the years using a variety of methods. Some key facts are below.

• The total volume of water on Earth is ~1.4 billion km3.
• The volume of freshwater resources is ~35 million km3, or about 2.5% of the total volume.
  Of these freshwater resources, ~24 million km3 or 68.9% is in the form of ice and permanent snow cover in mountainous regions, the Antarctic and Arctic regions.
• Some 8 million km3 or 30.8% is stored underground in the form of groundwater (shallow and deep groundwater basins up to 2  000 metres, soil moisture, swamp water and permafrost). This constitutes about 97% of all the freshwater that is potentially available for human use.
• Freshwater lakes and rivers contain an estimated 105 000 km3 or ~0.3% of the world's freshwater.
• The total usable freshwater supply for ecosystems and humans is ~200 000 km3 of water, which is < 1% of all freshwater resources, and only 0.01% of all the water on Earth.
  

A World of Salt
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The graphic above shows the total volume of saltwater and freshwater, and provides a rough breakdown of the storage of freshwater.

Glaciers and icecaps cover about 10% of the world's landmass. These are concentrated in Greenland and Antarctica and contain ~70% of the world's freshwater. Unfortunately, most of these resources are located far from human habitation and are not readily accessible for human use.

According to the United States Geological Survey (USGS), 96% of the world's frozen freshwater is at the South and North poles, with the remaining 4% spread over 550 000 km2 of glaciers and mountainous icecaps measuring about 180 000 km3.

Groundwater is by far the most abundant and readily available source of freshwater, followed by lakes, reservoirs, rivers and wetlands:

• Groundwater represents over 90% of the world's readily available freshwater resource. About 1.5 billion people depend upon groundwater for their drinking water supply.
• The amount of groundwater withdrawn annually is roughly estimated at ~600-700 km3, representing about 20% of global water withdrawals.
• A comprehensive picture of the quantity of groundwater withdrawn and consumed annually around the world does not exist.

Freshwater Lakes are mainly located at high altitudes, with nearly 50% of the world's lakes in Canada alone. Many lakes, especially those in arid regions, become salty through evaporation, which concentrates the inflowing salts. The Caspian Sea, the Dead Sea, and the Great Salt Lake are among the world's major salt lakes.

Reservoirs are artificial lakes, produced by constructing physical barriers across flowing rivers, which allow the water to pool and be used for various purposes. The volume of water stored in reservoirs worldwide is estimated at 4 286 km3.

Wetlands include swamps, bogs, marshes, mires, lagoons and floodplains. The 10 largest wetlands in the world by area are: West Siberian Lowlands (780 000-1 000 000 km2), Amazon River (800 000 km2), Hudson Bay Lowlands (200 000-320 000 km2), Pantanal (140 000-200 000 km2), Upper Nile River (50 000-90 000 km2), Chari-Logone River (90 000 km2), Hudson Bay Lowlands in the South Pacific (69 000 km2), Congo River (40 000-80 000 km2), Upper Mackenzie River (60 000 km2), and North America prairie potholes (40 000 km2).

Global Freshwater Resources
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The graphic above shows the quantity and distribution of gloabl freshwater resources by global region and location.

Major River Basins of the World
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The graphic above shows the loaction of the worlds major river basins.

Because much of the world's surface water is far from concentrations of human settlements, not all of it is readily usable.

• It is estimated that the freshwater available for human consumption varies between 12 500 km3 and 14 000 km3 each year.
• Many countries in Africa, the Middle East, Western Asia, and some Eastern European countries have lower than average quantities of freshwater resources available to their populations.
• Due to rapid population growth, the potential water availability of Earth's population decreased from 12 900 m3 per capita per year in 1970 to 9 000 m3 in 1990, and to less than 7 000 m3 in 2000.
• In densely populated parts of Asia, Africa and Central and Southern Europe, current per capita water availability is between 1 200 m3 and 5 000 m3 per year.
• The global availability of freshwater is projected to drop to 5 100 m3 per capita per year by 2025.
• This amount would be enough to meet individual human needs if it were distributed equally among the world's population. It is estimated that 3 billion people will be in the water scarcity category of 1 700 m3 per capita per year by 2025 (UNEP, 2002).

The uneven distribution of freshwater creates major problems of access and availability. For example:

• Asia and the Middle East are estimated to have 60% of the world's population (~3 674 000 000 people in 2000), but only 36% of its river runoff - much of which is confined to the short monsoon season.
• South America, by contrast, has an estimated 6% of the global population (~342 000 000 people in 2000) and 26% of its runoff.

Availability of Freshwater in 2000
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The graphic above shows the availability of freshwater in 2000 based on average river flows and groundwater discharge.

The World's Surface Water
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The graphic above shows precipitation by global region and the percentage of evaporation versus runoff.

Water is transported in different forms within the hydrological cycle or 'water cycle'. It is estimated that each year about 502 800 km3 of water evaporates over the oceans and seas, 90% of which (458 000 km3) returns directly to the oceans through precipitation, while the remainder (44 800 km3) falls over land.

With evapo-transpiration totalling about 74 200 km3, the total volume in the terrestrial hydrological cycle is about 119 000 km3. About 35% of this, or 44 800 km3, is returned to the oceans as run-off from rivers, groundwater and glaciers. A considerable portion of river flow and groundwater percolation never reaches the ocean, having evaporated in internal runoff areas or inland basins lacking an outlet to the ocean. However, some groundwater that bypasses the river systems reaches the oceans. Annually the hydrological cycle circulates nearly 577 000 km3 of water.

The World's Water Cycle
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The graphic above shows the world's water cycle in terms of global precipitation, evaporation, evapotransportation and runoff.

Freshwater use by continents is partly based on several socio-economic development factors, including population, physiography, and climatic characteristics.

• Annual global freshwater withdrawal has grown from 3 790 km3 (of which consumption accounted for 2 070 km3 or 61%) in 1995, to ~4 430 km3 (of which consumption accounted for 2 304 km3 or 52%) in 2000.
• In 2000, about 57% of the world's freshwater withdrawal, and 70% of its consumption, took place in Asia, where the world's major irrigated lands are located.
• In the future, annual global water withdrawal is expected to grow by about 10-12% every 10 years, reaching approximately 5 240 km3 (or an increase of 1.38 times since 1995) by 2025. Water consumption is expected to grow at a slower rate of 1.33 times.
• In the coming decades, the most intensive growth of water withdrawal is expected to occur in Africa and South America (increasing by 1.5-1.6 times), while the smallest growth will take place in Europe and North America (1.2 times).

The agricultural sector is by far the biggest user of freshwater:

In the industrial sector, the biggest share of freshwater is stored in reservoirs and dams for electrical power generation and irrigation. However, the volume of water evaporated from reservoirs is estimated to exceed the combined freshwater needs of industry and domestic consumption. This greatly contributes to water losses around the world, especially in the hot tropical regions.

Industrial uses account for about 20% of global freshwater withdrawals. Of this, 57-69% is used for hydropower and nuclear power generation, 30-40% for industrial processes, and 0.5-3% for thermal power generation.

Domestic water use is related to the quantity of water available to populations in cities and towns.

Freshwater Withdrawl by Sector in 2000
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The graphic above shows the worldwide percentage slit of water withdrawl by sector (Agriculture, Industry and Deomestic)

Global Freshwater Withdrawl by Dominant Usage
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The graphic above shows the dominant sector in terms of freshwater usage by country

Although the absolute quantities of freshwater on Earth have remained approximately the same, the uneven distribution of water and human settlement continues to create growing problems of freshwater availability and accessibility.

According to Population Action International, based upon the UN Medium Population Projections of 1998, more than 2.8 billion people in 48 countries will face water stress or scarcity conditions by 2025. Of these countries, 40 are in West Asia, North Africa or Sub-Saharan Africa. Over the next two decades, population increases and growing demands are projected to push all the West Asian countries into water scarcity conditions. By 2050, the number of countries facing water stress or scarcity could rise to 54, with their combined population being 4 billion people - about 40% of the projected global population of 9.4 billion.

Some 460 million people - more than 8% of the world's population - live in countries using so much of their freshwater resources that they can be considered highly water stressed. A further 25% of the population lives in countries approaching a position of serious water stress. Water scarcity occurs when the amount of water withdrawn from lakes, rivers or groundwater is so great that water supplies are no longer adequate to satisfy all human or ecosystem requirements, resulting in increased competition between water users and demands.

World's Freshwater Supplies
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The graphic above shows the annual renewable supplies per capita per river basin.

Freshwater Stress
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The graphic above shows the 2025 forecast for counries in relation to stress on their freshwater supplies

Freshwater biodiversity: Although freshwater ecosystems such as rivers, lakes and wetlands occupy less than 2% of the Earth's total land surface, they provide a wide range of habitats for a significant proportion of the world's plant and animal species.

Although many are yet to be discovered, the number of freshwater species worldwide is estimated at between 9 000 and 25 000 (Cosgrove and Rijsberman, 2000). This number is rapidly decreasing due to human interference.

Physical alteration, habitat degradation, excessive water withdrawal and pollution have contributed directly or indirectly to the decline in freshwater species. Other factors that reduce freshwater biodiversity include the incursion of non-native species and the mismanagement of inland fisheries. Today, an estimated 20% of the world's freshwater fish are vulnerable, endangered or extinct.

The harvest of freshwater fish is likely to increase either through capture fisheries or aquaculture (otherwise known as 'fish farming'). In many developing countries, freshwater fish provide a significant contribution to the diets of local communities.

• The introduction of the non-native Nile Perch to Africa's Lake Victoria in 1954, combined with pollution loading and increased water turbidity resulting from agriculture and industrial development, has greatly reduced indigenous fish populations. Kenya, for example, reported only 0.5% of its commercial fish catch as Nile Perch in 1976. Five years later, the proportion was 68%. Lake Victoria, the second largest lake in the world, has lost an estimated 200 different endemic cichlid species found nowhere else, while the remaining 150 are endangered. Two-thirds of the freshwater species introduced into the tropics worldwide have become established.
• In Africa and Asia, fish provide 21% and 28% of all animal protein, respectively (Revenga et al., 1998). The figures are more significant in landlocked countries, where data on the fish caught are often not formally recorded, and their importance is not fully known.
• In 1999, the reported fish production from inland waters totalled 28 million tonnes, with contributions of 8.2 and 19.8 million tonnes from capture fisheries and aquaculture, respectively. With major under-reporting from subsistence fisheries, these figures could be twice as high (FAO, 2000).

The over-exploitation and mismanagement of fisheries, particularly when combined with other manmade stresses, can lead to the collapse of regional fish faunas. In many countries, aquaculture is rapidly increasing in response to declining natural fisheries, often exacerbating the degradation of inland and coastal ecosystems through habitat alteration, pollution and the introduction of alien species.

Fish Diversity in Freshwater Systems
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The graphic above shows diversity of fish species in freshwater systems globally.