Major city may have to evacuate as water is running out

Iran's capital is counting down to "day zero" - the day the water runs out and the taps run dry.

Reservoirs that supply Tehran's 15 million residents are almost empty. The Karaj dam, which supplies a quarter of the city's drinking water, is just 8% full.

Water rationing has begun in some areas, with the flow from taps reduced or even stopped altogether overnight. President Masoud Pezeshkian has urged people to use water sparingly - or the city, or at least parts of it, may even have to evacuate.

So what's going on? Rain should start falling in the autumn after Iran's hot dry summer. But according to the country's National Weather Forecasting Centre, this has been the driest September to November period in half a century, with rainfall 89% below the long-term average.

The combination of low rainfall and high heat has lasted for more than five years, leaving the country parched. But the weather - and the shadow of climate change - aren't the only factors in Tehran's water crisis.

According to the Centre for Strategic and International Studies, the population of the metropolitan area of the city has almost doubled from 4.9 million in 1979 to 9.7 million today. But water consumption has risen even faster, quadrupling from 346 million cubic metres in 1976 to 1.2 billion cubic metres now.

Increasing wealth has allowed more people to buy washing machines and dishwashers. To supplement supplies from reservoirs, Tehran has had to turn to natural aquifers underground, which provide between 30% and 60% of its tap water in recent years.

But that puts the city in direct competition with farmers who draw on the water to irrigate crops. Levels are falling by 101 million cubic metres a year around Tehran, according to analysis in the journal Science Advances.

That's water that has accumulated from many decades of rain - and will take at least as long to replenish. Read more from Sky News:Could a volcanic eruption have spread the Black Death?The words you have mispronounced all year - and how to say them Professor Kaveh Madani, the former deputy head of Iran's environment department and now director of the United Nations University Institute for Water, Environment and Health, said chronic mismanagement of natural resources has led to what he calls water bankruptcy.

He told Sky News: "These things were not created overnight. "They're the product of decades of bad management, lack of foresight, overreliance and false confidence in how much infrastructure and engineering projects can do in a country that is relatively water short." Government ministers blame the water shortage on climate change, water leaks from pipes and the 12-day war with Israel.

Whatever the reason, it underlines the threat of water scarcity to global cities. Tehran is not alone.

Cape Town in South Africa narrowly avoided taps running dry eight years ago after a city-wide effort to save water. Even London, known for its rain, is at risk of drought.

Supplies haven't kept up with population growth and booming demand. As Tehran has found, droughts that are being made more likely and more severe with climate change can expose the fragility of water supply..