From disappearing lakes and dwindling rivers to military threats over shared resources, water is a cause for deep concern in many parts of the world. Click on the map to read about some of the world's water hotspots.
Ninety-five percent of the United States' fresh water is underground. One crucial source is a huge underground reservoir, the 800-mile Ogallala aquifer which stretches from Texas to South Dakota and provides an estimated third of all US irrigation water.
The aquifer was formed over millions of years, but has since been cut off from its original natural sources and is being steadily depleted. In some areas its level is dropping by three to five feet (90 - 150cm) a year. Estimates for its remaining lifespan vary in different areas, ranging from 60 to 250 years.
Many farmers in the Texan High Plains, which rely particularly on the underground source, are now turning away from irrigated agriculture as they become aware of the hazards of over-pumping.
Mexico City is sinking because of the amount of water being pumped out from beneath its foundations.
One of the largest and most populous cities in the world, it was once a lush land of lakes.
The city draws 80% of its water from aquifers below it, and has sunk an estimated nine metres into the soft, drained lake bed since the 1900s.
It already buys in a third of its water from surrounding areas, and an estimated million people are dependent on water trucks.
Although work is being done on its rusting pipe system, 27% of the city's water is still wasted through leaks.
The battle to provide water for Spain's parched southern coast has generated major controversy in recent years.
A 4.2 billion euro plan to divert water from the River Ebro to supply the area around Valencia, Almeria and Murcia was abandoned by the incoming Socialist government in 2004.
Tens of thousands had protested against the project, which was criticised by environmentalists concerned that it would encourage misuse of water and that the Ebro's fragile delta would suffer.
Work had already begun and developers were planning new tourist developments and golf courses when the project was scrapped.
The new government plans to build several desalination plants instead to provide water for the near-desert region.
Lake Chad, once a huge lake straddling the borders of Chad, Niger, Nigeria and Cameroon, has shrunk by 95% since the mid 1960s.
The region's climate has changed during that time, with the monsoon rains which previously replenished the lake now greatly reduced.
Local weather changes, rather than global warming, are blamed, but human activities such as overgrazing and crop irrigation are thought to have made the situation worse.
Nine million farmers, fishermen, and herders in the region now face water shortages, crop failure, livestock deaths, collapsed fisheries, soil salinity and increasing poverty.
There are plans to divert water from a tributary to the Congo to replenish the lake, and also to establish better management of the remaining water.
Oil has recently been found in the Chadian sector of the lake, raising hopes of a longer-term solution to the region's economic problems.
The Nile is vitally important to the survival of 160 million people in 10 countries who share the basin in which it flows.
To Egypt in particular, the river is a matter of life and death as the country has almost no other source of water.
A 1929 treaty between Britain and Egypt said no work would be done on the river that would reduce the volume of water reaching Egypt.
But tensions have been rising as neighbouring countries question the treaty - Tanzania, for example, is building a pipeline to extract drinking water and Ethiopia is planning to use the water for irrigation.
Cairo has said in the past that it was ready to use force to protect its access to the 7,000km-long river. Talks took place in 2004, but an agreement is yet to emerge.
With 5% of the world's population trying to survive on 1% of its water, there is strong competition for water in the Middle East.
A series of dry years - together with population growth - has recently increased the pressure. Both Israel and Jordan rely on the River Jordan - but Israel controls it and has cut supplies during times of scarcity.
The level of the Sea of Galilee has dropped in recent years, sparking fears that Israel's main reservoir will become salinated.
The Palestinians - whose water supply is also controlled by Israel - say supplies are intermittent and expensive, and that the underground aquifer which they share with Israel has become depleted and damaged through overuse. Israeli settlers in the West Bank use several times more water than their Palestinian neighbours.
To help ease the crisis, Israel has agreed to buy water from Turkey and is investigating building desalination plants.
Drainage and irrigation schemes carried out by the government of Saddam Hussein in southern Iraq have led to the loss of an estimated 90% of one of the world's most significant wetlands.
A vast network of canals has diverted water from the 20,000 square kilometres of marsh land between the Tigris and Euphrates, in places leaving nothing but salty, crusted earth behind.
Turkish dams upstream are also thought to have reduced the water flow and contributed to the wetlands' fate.
Most of the Marsh Arabs fled, facing both political persecution under Saddam Hussein's regime and the loss of the freshwater which sustained their way of life.
Since the US-led invasion of Iraq, local people have attempted to restore water flow, but there are reports that this has led to disease as much of the water is contaminated.
A UN project to restore the area was announced in July 2004.
Water-rich by Middle-Eastern standards, Turkey has in recent years undertaken an ambitious project to sell water from its Manavgat river across the region.
It is still vulnerable to shortages, however - just a few weeks after Turkey agreed to sell water to Israel, officials were warning of a water crisis.
Turkey has spent billions of dollars in the past decades building dams to increase its water reserves and boost its hydroelectric capabilities.
Two particular projects the Ilisu and Yusefeli dams, have faced delays after several Western companies withdrew funding following bad publicity over human rights concerns.
Another project, a system of 22 dams on the Tigris and Euphrates rivers, has provoked criticism from downstream neighbours Iraq and Syria.
The Aral Sea in Central Asia was once the world's fourth biggest inland sea, and one of the world's most fertile regions. But economic mismanagement has turned the area into a toxic desert.
The two rivers feeding the sea, the Amu Darya and the Syr Darya, were diverted in a Soviet scheme to grow cotton. Between 1962 and 1994, the level of the Aral Sea fell by 16 metres.
The surrounding region now has one of the highest infant mortality rates in the world, and anaemia and cancers caused by chemicals blowing off the dried sea bed are common.
China is undertaking two huge projects to tackle flooding in the south and drought in the north.
The Three Gorges Dam under construction on the Yangtze River aims to control flood waters and generate power.
The dam will provide 10% of the country's electricity when finished. More than 600,000 have been moved to make way for a reservoir longer than Lake Michigan behind the $25bn dam.
In the north, all three rivers feeding China's Northern Plain are severely polluted, damaging health and limiting irrigation.
The lower reaches of the Yellow River, which feeds China's most important farming region, run dry for at least 200 days every year.
In the north China plain, 30 cubic kilometres more water is being pumped to the surface each year by farmers than is replaced by the rain.
As groundwater is used to produce 40% of the country's grain, experts warn that water shortages could make the country dependent on grain imports.
To counter this, work has begun on China's biggest ever construction project - a massive scheme to channel billions of cubic metres of water from the Yangtze to the replenish the dwindling Yellow River.
The River Ganges
The most sacred Hindu river, the Ganges, is suffering from depletion, pollution and has been the source of a long-running dispute between India and Bangladesh.
The Gangotri glacier at the head of the River Ganges is retreating at a rate of 30 metres per year - experts blame climate change.
Deforestation in the Himalayas has caused subsoil streams flowing into the river to dry up.
Downstream, India controls the flow to Bangladesh with the Farakka Barrage, 10km on the Indian side of the border.
Until the late 1990s, India used the barrage to divert the river to Calcutta to stop the city's port drying up during the dry season.
This denied Bangladeshi farmers water and silt, and left the Sundarban wetlands and mangrove forests at the river's delta seriously threatened. The two countries have now signed an agreement to share the water more equally.
Water quality, however, remains a huge problem, with high levels of arsenic and untreated sewage in the river water.
Australia is the continent with the least rainfall, apart from Antarctica.
Its two largest rivers, the Murray and the Darling, have been extensively dammed for power and irrigation, reducing flows to the sea by three-quarters - but providing three million people and 40% of Australia's farms with water.
Salt rising to the surface as the lower reaches of the Murray dried out has destroyed prime agricultural land. Wetlands have shrunk, species numbers have dropped and the Australian National Trust has declared the whole river an "endangered area".
In the east, the Snowy River was dammed and diverted to the Murray basin decades ago to water the country's dry interior. But the ecological impact on the depleted river was so great that some flow was restored in 2002.
Water extraction from the Murray river was capped in 1995 and programmes to repair some of the destruction are now under way.