Cape Town is in the unenviable situation of being the first major city in the modern era to face the threat of running out of drinking water.

However, the plight of the drought-hit South African city is just one extreme example of a problem that experts have long been warning about - water scarcity.

Despite covering about 70% of the Earth's surface, water, especially drinking water, is not as plentiful as one might think. Only 3% of it is fresh.

Over one billion people lack access to water and another 2.7 billion find it scarce for at least one month of the year. A 2014 survey of the world's 500 largest cities estimates that one in four are in a situation of "water stress"

According to UN-endorsed projections, global demand for fresh water will exceed supply by 40% in 2030, thanks to a combination of climate change, human action and population growth.

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It shouldn't be a surprise, then, that Cape Town is just the tip of the iceberg. Here are the other 11 cities most likely to run out of water.

1. São Paulo

Brazil's financial capital and one of the 10 most populated cities in the world went through a similar ordeal to Cape Town in 2015, when the main reservoir fell below 4% capacity.

At the height of the crisis, the city of over 21.7 million inhabitants had less than 20 days of water supply and police had to escort water trucks to stop looting.

It is thought a drought that affected south-eastern Brazil between 2014 and 2017 was to blame, but a UN mission to São Paulo was critical of the state authorities "lack of proper planning and investments".

The water crisis was deemed "finished" in 2016, but in January 2017 the main reserves were 15% below expected for the period - putting the city's future water supply once again in doubt.