Cape Town, South Africa, a city roughly the size of Los Angeles, is about to run out of water — completely.

Authorities are warning that as soon as May 11 — which they’ve ominously dubbed “Day Zero” — the drought-stricken city will have to cut off taps to all homes and most businesses, leaving nearly all of the city’s 4 million residents without access to running water.

Residents will then have to go to roughly 200 collection points scattered across the city to collect strictly rationed water. People will be allowed just 25 liters — about 6.5 gallons — of water a day. That’s all the water they’ll have for drinking, bathing, flushing toilets, and washing their hands.

Some services like hospitals, clinics, and schools will be exempt from the cutoff and will continue to have access to running water. But the overwhelming majority of the megacity’s residents will have to work with their tiny daily allotment.

Experts say the possibility of civil unrest is high. South Africa is the most unequal country in the world in terms of income, and Cape Town has the highest murder and robbery rates in the country.

So what’s going on? How does a major, modern city in 2018 completely run out of water? And what will happen if and when it does?

How did things get this bad in Cape Town?

The story of how Cape Town went dry is remarkable because up until a few years ago, the city was held up as an example of a place with particularly sophisticated water conservation policies. Cape Town has been proactive in coming up with new ways to conserve water since at least the turn of the millennium, experts say.

For example, the city worked hard to fix leaks in the pipes that distribute water across the city. Leaky pipes are not a trivial matter — on average around the world, leaky pipes account for between 30 and 40 percent of a city’s lost water, Shafiqul Islam, an expert on water management at Tufts University, tells me. Cape Town has reduced the amount of water it loses through leaks to about half of that. And in 2015 — just three years ago — Cape Town even won a prestigious international award for its water conservation policies.

But 2015 also marked the beginning of a devastating three-year drought unlike anything the city had seen in more than a century. The drought exposed a key problem in the city’s water supply: its near-total reliance on rainwater. Unlike many other cities, which can draw their water supplies from various sources like underground aquifers or through desalination plants, Cape Town gets more than 99 percent of its water supply from dams that rely on rain.

Cape Town’s government thought its dams were big enough to deal with a drought — but they weren’t designed to deal with a once-in-a-century type of drought. While they were completely full just a few years ago, the dams now stand at about a quarter capacity.

There’s also a political dimension to the crisis. Cape Town and the province it’s in, the Western Cape, are governed by a party called the Democratic Alliance. But the national government (and every other provincial government in the country) is run by another party, the African National Congress (ANC).

Analysts say that partisan differences helped lay the foundation for Cape Town’s sluggish response to the drought in its early stages, and made it harder for the local and national government to form a united front once the water shortage became evident.

William Saunderson-Meyer, a South African journalist, points out that the national government had a clear incentive to drag its feet in helping the city get enough water: It may stand to benefit politically from a botched response to the shortage.

“Many ANC politicians would love to see the liberal ruling Democratic Alliance tarnished by failure in the Cape, perhaps opening the way to the ANC recapturing the province in 2019,” he writes at Reuters.

The flat-footed policy response to the drought has made the crisis even larger than it would’ve been otherwise.

Warding off Day Zero will require a lot of collaboration — and luck

Day Zero is not inevitable. It is possible that Cape Town’s population can collectively reduce its water usage quickly enough to prevent water levels in the city’s dams from dropping so low that water needs to be cut off.

Day Zero is supposed to kick in when they the dam levels drop to 13.5 percent. Theoretically, it’s possible that water levels won’t hit that point. The projected date for Day Zero was recently moved from mid-April to May 11 because of a decline in water usage from the agricultural sector in the province surrounding Cape Town.

But a key factor is urban usage — how much water people use at homes and business in the city. And though the government has warned of Day Zero and attached fines to exceeding its daily limits in recent months, urban usage hasn’t declined significantly.

In January, the government said that no individual should be using more than 87 liters (23 gallons) of water a day, but a majority of the city’s residents went ahead and did so anyway.

“Despite our urging for months, 60 percent of Capetonians are callously using more than 87 liters per day,” Cape Town Mayor Patricia de Lille said during a press briefing on January 18. “It is quite unbelievable that a majority of people do not seem to care and are sending all of us headlong towards Day Zero.”

This month, Cape Town’s government has called for residents limit their water use even more — to 50 liters a day. The fines have increased as well, particularly for people who are using far too much water.

Cape Town residents not only need to collectively decide to start complying with the city’s usage restrictions, they need to do it until the city gets its long-awaited winter rains. According to Kevin Winter, an urban water management expert at the University of Cape Town, the city has seen its winter rains come later and later in the year over the past decade. While in the 1970s, the rains reliably started around April, recent patterns suggest that this year, substantial rain is more likely to come around July.

But if the current water restrictions don’t work and Day Zero does happen, things could get dicey in Cape Town, fast.

Day Zero could cause chaos in Cape Town

Day Zero is going to pose some big logistical challenges. Each of the water collection points will serve roughly 20,000 residents, many of whom will have to trek out to the sites daily.

South African police and military forces will be deployed to guard collection points, but with such small amounts of water provided at such a limited number of sites across the city, things could get rough. “The government’s first and foremost priority is going to be to try to quell anarchy,” says Patrick Reed, an expert on sustainable water management at Cornell University.

While wealthy residents are already installing private water tanks in their homes, lower-income residents don’t have any obvious solution for getting any more than their tiny daily allotment. And for the very poor who may live far from a distribution point and lack access to reliable transportation, just fetching that water is a challenge.

People would have to risk missing work or making less money just to be able to ensure they could get water to their homes.

Reed sums up the dilemma: “Would you be willing to get fired to get your water?”

Shafiqul Islam, the expert on water management at Tufts University, says that while it’s hard to predict outbreaks of violence in crises like this, the conditions in Cape Town are a natural tinderbox.

“Will it happen? We do not know. Is it likely to happen? Yes,” Islam says.

Cape Town is in the unenviable situation of being the first major city in the modern era to face the threat of running out of drinking water.

However, the plight of the drought-hit South African city is just one extreme example of a problem that experts have long been warning about – water scarcity.

Despite covering about 70% of the Earth’s surface, water, especially drinking water, is not as plentiful as one might think. Only 3% of it is fresh.

Over one billion people lack access to water and another 2.7 billion find it scarce for at least one month of the year. A 2014 survey of the world’s 500 largest cities estimates that one in four are in a situation of “water stress”

According to UN-endorsed projections, global demand for fresh water will exceed supply by 40% in 2030, thanks to a combination of climate change, human action and population growth.

It shouldn’t be a surprise, then, that Cape Town is just the tip of the iceberg. Here are the other 11 cities most likely to run out of water.

1. São Paulo

Brazil’s financial capital and one of the 10 most populated cities in the world went through a similar ordeal to Cape Town in 2015, when the main reservoir fell below 4% capacity.

At the height of the crisis, the city of over 21.7 million inhabitants had less than 20 days of water supply and police had to escort water trucks to stop looting.

It is thought a drought that affected south-eastern Brazil between 2014 and 2017 was to blame, but a UN mission to São Paulo was critical of the state authorities “lack of proper planning and investments”.

The water crisis was deemed “finished” in 2016, but in January 2017 the main reserves were 15% below expected for the period – putting the city’s future water supply once again in doubt.

2. Bangalore

Local officials in the southern Indian city have been bamboozled by the growth of new property developments following Bangalore’s rise as a technological hub and are struggling to manage the city’s water and sewage systems.

To make matters worse, the city’s antiquated plumbing needs an urgent upheaval; a report by the national government found that the city loses over half of its drinking water to waste.

Like China, India struggles with water pollution and Bangalore is no different: an in-depth inventory of the city’s lakes found that 85% had water that could only be used for irrigation and industrial cooling.

Not a single lake had suitable water for drinking or bathing.

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3. Beijing

The World Bank classifies water scarcity as when people in a determined location receive less than 1,000 cubic metres of fresh water per person a year.

In 2014, each of the more than 20 million inhabitants of Beijing had only 145 cubic metres.

China is home to almost 20% of the world’s population but has only 7% of the world’s fresh water.

A Columbia University study estimates that the country’s reserves declined 13% between 2000 and 2009.

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And there’s also a pollution problem. Official figures from 2015 showed that 40% of Beijing’s surface water was polluted to the point of not being useful even for agriculture or industrial use.

The Chinese authorities have tried to address the problem by creating massive water diversion projects. They have also introduced educational programmes, as well as price hikes for heavy business users.

4. Cairo

Once crucial to the establishment of one of the world’s greatest civilisations, the River Nile is struggling in modern times.

It is the source of 97% of Egypt’s water but also the destination of increasing amounts of untreated agricultural, and residential waste.

World Health Organization figures show that Egypt ranks high among lower middle-income countries in terms of the number of deaths related to water pollution.

The UN estimates critical shortages in the country by 2025.

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5. Jakarta

Like many coastal cities, the Indonesian capital faces the threat of rising sea levels.

But in Jakarta the problem has been made worse by direct human action. Because less than half of the city’s 10 million residents have access to piped water, illegal digging of wells is rife. This practice is draining the underground aquifers, almost literally deflating them.

As a consequence, about 40% of Jakarta now lies below sea level, according to World Bank estimates.

To make things worse, aquifers are not being replenished despite heavy rain because the prevalence of concrete and asphalt means that open fields cannot absorb rainfall.

6. Moscow

One-quarter of the world’s fresh water reserves are in Russia, but the country is plagued by pollution problems caused by the industrial legacy of the Soviet era.

That is specifically worrying for Moscow, where the water supply is 70% dependent on surface water.

Official regulatory bodies admit that 35% to 60% of total drinking water reserves in Russia do not meet sanitary standards

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7. Istanbul

According to official Turkish government figures, the country is technically in a situation of a water stress, since the per capita supply fell below 1,700 cubic metres in 2016.

Local experts have warned that the situation could worsen to water scarcity by 2030.

In recent years, heavily populated areas like Istanbul (14 million inhabitants) have begun to experience shortages in the drier months.

The city’s reservoir levels declined to less than 30 percent of capacity at the beginning of 2014.

8. Mexico City

Water shortages are nothing new for many of the 21 million inhabitants of the Mexican capital.

One in five get just a few hours from their taps a week and another 20% have running water for just part of the day.

The city imports as much as 40% of its water from distant sources but has no large-scale operation for recycling wastewater. Water losses because of problems in the pipe network are also estimated at 40%.

9. London

Of all the cities in the world, London is not the first that springs to mind when one thinks of water shortages.

The reality is very different. With an average annual rainfall of about 600mm (less than the Paris average and only about half that of New York), London draws 80% of its water from rivers (the Thames and Lea).

According to the Greater London Authority, the city is pushing close to capacity and is likely to have supply problems by 2025 and “serious shortages” by 2040.

It looks likely that hosepipe bans could become more common in the future.

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10. Tokyo

The Japanese capital enjoys precipitation levels similar to that of Seattle on the US west coast, which has a reputation for rain. Rainfall, however, is concentrated during just four months of the year.

That water needs to be collected, as a drier-than-expected rainy season could lead to a drought. At least 750 private and public buildings in Tokyo have rainwater collection and utilisation systems.

Home to more than 30 million people, Tokyo has a water system that depends 70% on surface water (rivers, lakes, and melted snow).

Recent investment in the pipeline infrastructure aims also to reduce waste by leakage to only 3% in the near future.

11. Miami

The US state of Florida is among the five US states most hit by rain every year. However, there is a crisis brewing in its most famous city, Miami.

An early 20th Century project to drain nearby swamps had an unforeseen result; water from the Atlantic Ocean contaminated the Biscayne Aquifer, the city’s main source of fresh water.

Although the problem was detected in the 1930s, seawater still leaks in, especially because the American city has experienced faster rates of sea level rise, with water breaching underground defence barriers installed in recent decades.

Neighbouring cities are already struggling. Hallandale Beach, which is just a few miles north of Miami, had to close six of its eight wells due to saltwater intrusion.

How Controlling Water Flow Prevents Flooding

Flooding occurs when water flows uncontrollably. This can happen naturally during periods of heavy rain, particularly around homes with inadequate drainage systems. Flood problems can also originate inside the home when pipes or appliances fail. This kind of flooding can occur in any property, not just those in flood zones.

Controlling the flow of water in all these cases ensures small leaks don’t turn into major floods that can devastate a property’s foundations and flooring and provide a fertile environment for mildew and mold.

Water-Controlling Devices for Homes and Businesses

Water-controlling devices help homeowners and businesses protect their assets from water damage. Innovative products like the Water Security System from Enolgas, an advanced single-point water monitoring unit, limit water flow so flooding cannot occur. This high-tech system uses motion and moisture sensors and a revenue-grade flow meter to determine whether water is flowing normally within a property. When excess flow is detected, the system automatically shuts the water off and notifies the owner.

More basic measures can also control water’s flow, directing it away from areas where it can cause damage. For example, backwater valves on sewer drains reverse the flow of waste water if sewer lines clog, ensuring it doesn’t travel through the home. Secondary sump pumps can take over and prevent flooding when primary units fail.

Water may have once seemed like an unstoppable force, but several products can help us take our power back and minimize the risk of flooding in our homes and businesses.

Unfortunately, that means the reliance on energy for water supply will increase. That’s a problem because currently 100% of the energy that’s used to generate water is acquired from outside of Singapore. As world-renowned water expert says: “Singapore has no energy … As Singapore becomes more and more self-sufficient (in water), it is becoming, energy-wise, more and more not self-sufficient. More and more energy (has to be) input in order to sustain the water sector,”

The desalination of water is an energy rich process that has a relatively high carbon footprint. Both of these must be reduced to decrease the dependency on energy to meet the water supply. Fortunately, Singapore is investing in solar energy, research, and power plant facility to meet the water demands of the future in a way that’s sustainable for the nation.