For the first time, a major coastal city may run out of water — and 'Day Zero' is looming

The supply of water in Cape Town, South Africa, is on track to reach critical levels by May.
Officials are urging locals and travelers not to flush the toilets unless it's absolutely necessary.
The city is hoping it won't have to resort to turning off the taps.

The water-conservation maxim "if it's brown, flush it down; if it's yellow, let it mellow" is becoming something of a rallying cry for city officials in Cape Town, South Africa.

The city's water supply is on track to reach lows that will prompt strict mandatory rationing before May.

Even travelers to the city, which is home to nearly 4 million people, are being asked to conserve. The New York Times reports that safari-, beach-, and vineyard-bound tourists arriving at the city's international airport are being reminded to be cautious about their flushes.

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

The city has been grappling with its worst drought in over 100 years, fueled by months of dry weather. In October, Executive Mayor Patricia de Lille asked her fellow Capetonians to limit their water use to no more than 87 liters a day. In February the city released a list of its top 100 water consumers, The Huffington Post (now HuffPost) reported, hoping to shame people into using less.

It's tough to know exactly how much water we each pull from the tap every day, but a single toilet flush can drain about 9 liters (more than 2 gallons), and a five-minute shower uses about 47 liters (12.5 gallons). Even a typical pot of coffee will set you back another liter or more, so it's easy to see how quickly water use can add up from a quick morning routine.

To help people track how much of the municipal resource they're using every day, Cape Town has created an online water-consumption calculator. Some residents have installed their own tanks to collect water for gardening and are using buckets to conserve shower water.

The city is also trying to find new water sources, since dams that supply the city are hovering at just about 30% of their storage capacity. But the efforts may not be enough to keep residents hydrated. The coastal city says it will be forced to turn off the taps when the dams fall to 13.5% capacity. At that point, they'll switch to a system of daily water allotments that people will have to collect at checkpoints around town, according to The Times.

Cape Town isn't the only city in water trouble. The World Wildlife Fund estimates that by 2025 two-thirds of the world will be dealing with water shortages as droughts become more frequent in a warming world.

Scientists warn that cities across Asia could also get hit with big water problems by 2050 if drastic measures aren't taken there. In addition to climate extremes like drought and environmental stressors, population growth is creating challenges around freshwater consumption. As more people in densely populated countries like China and India move out of poverty, water resources are further strained.

The Cape Town drought means rainwater suppliers and well-diggers are "doing a roaring trade," Bloomberg reports, as people who can afford to pay line up to stay supplied.

It remains possible that Capetonians will never reach "Day Zero," when the city projects it will turn off the taps. But for now, there's just a single chance of rain in the Cape Town forecast for the next two weeks.