Cape Town, South Africa’s largest city, will run out of potable water on April 22, 2018, Time reports.

A drought that climatologists call a “once in a millennium” event has wiped out the city’s reservoirs, which rely heavily on rainwater.

In order to keep water running for the city’s four million people, Cape Town’s mayor, Patricia de Lille, rolled out a rationing scheme meant to ensure that water lasts until the expected rainy season in May.

The plan called for all households to use only 23 gallons of water per person, per day. In practice, this means showering for no longer than two minutes, flushing the toilet as little as possible, leaving plants and lawns unwatered and washing clothes and dishes only when absolutely necessary.

Unfortunately for Cape Town’s citizens, only around 54 percent of residents have followed the mayor’s plan, which has led to the day the water will run out to be moved up to April 22: a date the city has taken to calling “Day Zero.”

The city’s government already has a plan in place for Day Zero — all non-emergency taps will be shut off, and the government will set up more than 200 water collection sites protected by police. At these sites, citizens will be able to receive 6.6 gallons of water per day, the daily amount recommended by the World Health Organization.

In addition, de Lille told Al Jazeera that the government has already begun figuring out ways to get water that aren’t dependent on rainfall. These include desalination plants, water reclamation and drilling into aquifers.

All of these projects, however, will take years.

Although none of them will be in place before Day Zero, they are all necessary to Cape Town’s long term survival. The city is growing rapidly; its population has doubled in the last 20 years, which has — and will continue to — put a strain on its water supply.

Alternative water sources are also needed because scientists like the University of Cape Town’s Kevin Winter say that it could take up to three years’ worth of rain to refill the struggling reservoirs.

And though climate scientists are calling the current drought one for the history books, they also have said that from here on out, climate change will likely make South Africa an increasingly dry place.

On Twitter, de Lille asked residents both not to worry, and also for everyone to do what they could in order to save water, writing that the government was “working around the clock to bring new water supplies online.”

Cape TownSouth-Africa