Cape Town, one of the biggest cities in South Africa and a famed tourist attraction, is warning its residents that they will soon have to queue for water.

The drought-stricken city announced on Thursday that it will begin marking 200 collection points where its 3.7 million residents will be required to queue for a rationed supply of water on "Day Zero" – currently forecast to be April 21.

If it happens, Cape Town would become the first major city in the world to shut down entirely the supply of running water in all of its homes.

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"We have reached a point of no return," Cape Town Mayor Patricia de Lille told a press briefing on Thursday.

"Day Zero is now very likely," she said. "Despite our urging for months, 60 per cent of Capetonians are callously using more than 87 litres per day. It is quite unbelievable that a majority of people do not seem to care and are sending all of us headlong towards Day Zero."

As part of the latest emergency measures, city officials next week will begin announcing the 200 collection points where residents will be required to queue after Day Zero, she said.

The city has suffered two consecutive years of severe drought. By some scientific calculations, the severity of the drought is so rare that it would normally happen only once in a millennium.

Cape Town, on the Atlantic Ocean coast, traditionally has rainy winters. In precolonial times, the Cape peninsula was known in a local language as "the place where the clouds gather."

Climate change is believed to be one of the factors causing Cape Town to have a growing number of dry seasons. Last year's drought was one of the worst in several decades.

Climate researchers at the University of Cape Town have estimated that low-rainfall years have become twice as frequent in the city over the past century. Climate-change models have predicted that Cape Town will become increasingly dry.

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Under the plans for Day Zero, each resident of Cape Town would be allowed only 25 litres of water per day, which must be collected from neighbourhood taps. Each supply point would be shared by about 20,000 people per day. The city is consulting police and army officials to decide how to maintain law and order at the collection points.

Only vital services, such as hospitals and clinics, would be allowed to continue their existing water supply after Day Zero. Most schools and many businesses would have to close.

To avoid this disaster, the city is now asking residents to limit themselves to 50 litres per day. But voluntary appeals haven't worked in the past. When the city earlier pleaded with them to limit themselves to 87 litres per day, only 39 per cent of residents complied. (Canada, by contrast, is one of the world's biggest per-capita water users. Canadians used an average of 251 litres per day in 2011.)

The mayor, Ms. de Lille, announced on Thursday that a punitive tariff would be imposed on excessive water users in a last-ditch effort to avoid Day Zero. City council will vote on the measure on Friday. It would exponentially increase the water tariff for anyone using more than 6,000 litres of water per month. The city is also planning to install water-management devices on the supplies of the heaviest users.

The expected date of Day Zero has been moved forward several times recently as the drought persisted. The city has asked residents to reduce their water use in a variety of ways, including a two-minute limit on showers, using a cup instead of running taps for brushing teeth and shaving, and flushing toilets only when necessary.

For more than a year, Capetonians have been banned from using water for swimming pools, watering gardens or washing cars. But the measures haven't solved the problem, and the restrictions have been increasingly tightened.

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The city relies on nearby dams for much of its water, but the water level in the dams has now fallen to just 28 per cent of their capacity, Ms. de Lille said. When the level falls to 13.5 per cent, the city will declare Day Zero.

Officials have been trying to reduce the city's water consumption to 500 million litres per day, less than half of normal use.

For a while, the effort seemed to be working. By early this month, water use had fallen to 578 million litres per day. But this week it rebounded to 618 million litres. The new goal is 450 million litres, the city announced.

"For each day that Cape Town uses more than 500 million litres, the city moves closer to Day Zero," Ms. de Lille said.

"The only way Cape Town can avoid Day Zero is if every single resident saves water."

The Cape Town government, run by the Democratic Alliance, the main opposition party in national politics, has been criticized for its ineffective response to the water crisis. It had recently announced a drought levy on all water users, but on Thursday it scrapped the plan after widespread complaints. There was a "massive outcry," the mayor said.

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As part of its long-term response to the crisis, the city is building desalination plants to obtain drinking water from the Atlantic Ocean. But a large-scale use of desalination plants would be too expensive for the city to afford, experts say.

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