Nearly half of 33 countries expected to face extremely high water stress by 2040 are in the Middle East, where surface water is limited and demand is high, said experts who ranked 167 nations.

Thirteen Middle Eastern countries plus the Palestinian Territories are projected to face extremely high water stress in 25 years' time, and eight fell in the global Top 10: Bahrain, Kuwait, the Palestinian Territories, Qatar, United Arab Emirates, Saudi Arabia and Oman.

Researchers from the World Resources Institute (WRI) - who compiled the first index measuring competition for and depletion of surface water, such as lakes and rivers, each decade from 2010 to 2040 - said the Middle East is already probably the least water-secure region in the world.

It draws heavily on groundwater and desalinated sea water, and faces "exceptional water-related challenges for the foreseeable future", they wrote in their findings.

Betsy Otto, director of the WRI's Global Water Programme, said it was important for governments to understand the potential risks they face in terms of the water needed to run their economies, including rising demand as populations grow and the still uncertain impacts of climate change.

"The good news ... is countries can take actions to reduce that stress and the risk associated with how they manage water resources," Otto told the Thomson Reuters Foundation, citing Singapore as an example of a state that uses innovative methods.

One measure likely to become more common in the Middle East and elsewhere is water reuse systems that recycle waste water.

"It doesn't make a lot of sense to treat water to a potable standard, allow it to be used by households and then essentially throw it away," Otto said.

Some Middle Eastern countries already rely on desalination, a technique to remove salt from sea and ground water. These and other highly water-stressed nations may also need to move away from producing their own food because agriculture gobbles water, Otto noted.

Saudi Arabia, for example, has said its people will depend entirely on grain imports by 2016, the WRI researchers said.

While political turmoil may be the top concern in the Middle East today, drought and water shortages in Syria likely contributed to the social unrest that stoked its civil war, the WRI experts said, as some 1.5 million people - mainly farmers and herders - moved to urban areas unable to provide enough jobs and services.

Water has also played a significant role in the decades-old conflict between the Palestinian Territories and Israel, they noted.

"It's unlikely that water becomes the cause of the conflict, but it can become an accelerator or multiplier of those conflicts," Otto said.

The analysis singled out four countries whose level of water stress is due to rise particularly sharply between 2010 and 2040 - Chile, Estonia, Namibia and Botswana - putting new pressure on their businesses, farms and communities.

It also warned that national-level rankings mask large differences within countries. The United States, for example, is ranked 49 for 2010 and 47 for 2040, but California is currently grappling with a crippling drought.

That smoothing effect could help explain why some West African and Central American countries that regularly suffer drought-related food crises in rural areas have a low water-stress ranking.

Otto said some of those countries may also see their future water supply from precipitation increase due to climate change, and that agriculture may be rain-fed rather than relying on irrigation, avoiding direct demand on surface waters.

In southern Africa and other parts of the world where water supply is projected to fall as demand grows, policy makers should act to prevent water stress worsening, Otto said.

"We need to understand the relationship between available supply and demand for that water, and we need to take steps to use the water we have more efficiently and more effectively," she said.