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At a Glance Cape Town, home to more than 4 million, is in the midst of the worst drought to hit South Africa in more than 100 years.

City officials say they will "turn off the tap" in April when dam levels are expected to reach 13.5 percent of capacity.

Officials in Cape Town have set April 22 as "Day Zero", the day the city will cut off water to residents unless drastic measures are taken to decrease usage.

The city that is home to 4 million is in the midst of the worst drought to hit South Africa in more than 100 years and is rapidly running out of water. The city said Tuesday on Facebook that it will "turn off the tap" when dam reserves reach 13.5 percent. The date at which dam reserves are expected to reach that milestone keeps getting pushed forward.

The situation is dire. Dams supplying the city with usable water dropped this week to 29.7 percent, the city of Cape Town posted to Facebook on Wednesday. Only 19.7 percent of the water is usable. Several times a day, the city encourages residents via social media to conserve water.

The South African city is a tourist mecca, and tourists are being reminded of the gravity of the situation when they arrive, the New York Times reports.

"A single flush uses 5 days of drinking water," a sign greets tourists. "Our taps will run dry if we don't act now."

To avoid Day Zero, the city has reduced water pressure and banned residents from watering gardens and washing cars. Most public swimming pools are closed, and the city resorted in February to shaming the top 100 water consumers by releasing their identities to the public.

In July, Executive Mayor Patricia de Lille asked residents to limit their water use to no more than 23 gallons a day, which is roughly equivalent to the amount of water needed to wash a single load of laundry in a newer model washing machine, according to the United States Geological Survey. A single toilet flush can use up to three gallons of water.

On Monday, De Lille released a statement noting that 54 percent of residents had heeded the city's request to keep consumption under 23 gallons per day.

"This compares well to only 37 percent of Cape Town’s residents saving water during the first weeks of December 2017," she added. "But consumption remains too high with half of Cape Town’s residents still not keeping to the consumption limit of 87 liters of water per person per day."

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De Lille says she hopes it won't come down to Day Zero, but the city is already planning for that eventuality. Should the city be forced to turn off the taps, 200 water stations guarded by police and the military will be set up to ration out roughly 6.6 gallons (25 liters) of water per day per resident.

Cape Town isn't the only city dealing with water issues in a warming world.

The World Wildlife Fund estimates two-thirds of the world may face water shortages by 2025 as droughts become more frequent because of global warming.