Earlier this year, an obscure United Nations document, the World Water Development Report, unexpectedly made headlines around the world. The report made the startling claim that the world would face a 40 percent shortfall in freshwater in as soon as 15 years. Crops would fail. Businesses dependent on water would fail. Illness would spread. A financial crash was likely, as was deepening poverty for those just getting by.

The UN also concluded that the forces destroying the world’s freshwater supply were not strictly meteorological, but largely the result of human activity. That means that with some changes in how water is managed, there is still time—very little, but enough—for children born this year to graduate from high school with the same access to clean water their parents enjoyed.

Though the UN looked at the issue across the globe, the solutions it recommended—capturing rainwater, recycling wastewater, improving sewage and plumbing, and more—need to be implemented locally. Some of the greatest challenges will come in cities, where bursting populations strain systems designed to supply far fewer people and much of the clean water available is lost to waste and shoddy, centuries-old infrastructure.

We’ve looked at eight cities facing different though representative challenges. The amount of water in the Earth’s atmosphere is more or less fixed, meaning that as populations and economies grow, what we have needs to be clean, available, and conserved. Economies, infrastructure, river systems and climates vary from place to place, and the solutions will have to as well. Here is how eight of the world’s major cities are running out of water, and trying to save it.

TOKYO

Tokyo shouldn’t have a water problem: Japan’s capital enjoys average precipitation similar to that of Seattle or London. But all that rainfall is compressed into just four months of the year, in two short seasons of monsoon and typhoon. Capturing and storing so much water in such a short period in an area four times as dense as California would be a challenge anywhere. One weak rainy season means droughts—and those are now coming about once every decade.

Betting on the rain will be a precarious strategy for the world’s most populous city and its suburbs, home to more than 30 million people. When the four rivers feeding Tokyo run low, crisis conditions arrive fast. Though efficient, 70 percent of Tokyo’s 16,000-mile-long plumbing system depends on surface water (rivers, lakes, and distant snowpack). With only 30 percent of the city’s water coming from underground aquifers and wells, there are not enough alternative sources to tap during these new cyclical droughts.

The Japanese government has so far proved forward-thinking, developing one of the world’s most aggressive programs for capturing rainwater. In Sumida, a Tokyo district that often faces water shortages, the 90,000-square-foot roof of Ryogoku Kokugikan arena is designed to channel rainfall to a tank, where it’s pumped inside the stadium for nonpotable use.

Somewhat more desperate-seeming is a plan to seed clouds, prodding the environment to do what it isn’t doing naturally. Though tested in 2013 with success, the geo-engineering hack is a source of controversy; scientists debate whether the technique could produce enough rain to make much of a difference for such a large population.

MIAMI

Though most Americans’ concern with water shortage in the U.S. is firmly focused on California at the moment, a crisis is brewing in the last place you’d figure: South Florida, which annually gets four times as much rain, on average, as Los Angeles and about three times as much as San Francisco.

But according to the U.S. Geological Survey, the essential Biscayne Aquifer, which provides water to the Miami–Dade County area, is falling victim to saltwater intrusion from the Atlantic Ocean. Despite the heavy rains replenishing the aquifer year-round, if enough saltwater enters, all of it will become unusable.

The problem arose in the early 20th century, after swamps surrounding the city were drained. Osmosis essentially created a giant sucking effect, drawing the Atlantic into the coastal soils. Measures to hold the ocean back began as early as the 1930s, but seawater is now bypassing the control structures that were installed and leaking into the aquifer. The USGS has made progress mapping the sea water intrusion, but ameliorating it seems a ways off. “As sea level continues to rise and the demand for freshwater increases, the measures required to prevent this intrusion may become more difficult [to implement],” the USGS noted in a press release.

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LONDON

London faces a rapidly growing population wringing every last drop out of centuries-old plumbing. Water managers estimate they can meet the city’s needs for the next decade but must find new sources by 2025—even sooner than the rest of the world, by the U.N.’s measure. London’s utility, Thames Water, looked into recycled water—aka “toilet-to-tap”—but, being English, found it necessary first to politely ask people if they’d mind.

At least four urban districts in California use recycled water, which is treated, re-treated, and treated again to be cleaner than conventional supplies before being pumped into groundwater or other supply sources. The so-called “yuck factor” could be an impediment to this solution spreading to London and elsewhere.

CAIRO

Five thousand years ago, an ample water supply and a fertile delta at the mouth of the Nile supported the growth of one of the world’s great civilizations. Today, while 97 percent of Egypt’s water comes from the great river, Cairo finds itself downstream from at least 50 poorly regulated factories, agricultural waste, and municipal sewage systems that drain into it.

Though Cairo gets most of the attention, a UNICEF-World Health Organization study released earlier this year found that rural areas to the city’s south, where more than half of Egyptians live, depend on the river not just for irrigation and drinking water but also for waste disposal. Engineer Ayman Ramadan Mohamed Ayad has noted that while most wastewater discharged into the Nile upriver from Cairo is untreated, the river’s enormous size has historically been sufficient to dilute the waste to safe levels (and Cairo’s municipal system treats the water it draws from the river). Ayad argues, however, that as the load increases—with 20 million people now discharging their wastes to the Nile—this will no longer be possible. The African Development Bank recently funded programs to chlorinate wastewater before it’s dumped in the river, but more will need to be done.

On the demand side, more than 80 percent of the water taken from the Nile each year is used for irrigation, mostly the inefficient method of just flooding fields, which loses significant amounts to evaporation. Two years ago, initial steps were taken to modernize irrigation techniques upriver. Those programs have yet to show much progress, however.

SÃO PAULO

When it rains in Brazil, it pours. In São Paulo, where in an average year it rains more than it does in the U.S. Pacific Northwest, drains can’t handle the onslaught, and what could be the source of desperately needed drinking water becomes instead the menace of urban floodwater.

With the worst drought in a century now in its second year, São Paulo’s reservoirs are at barely a quarter of capacity, down from 40 percent a year ago. Yet the city still sees heavy rainstorms. But reservoirs outside the city are often polluted and are too small even at capacity to supply the metropolitan area of 20 million. Asphalt covering the city and poor drainage lead to heavy floods on city streets after as little as a quarter-inch of rain. It’s hard to believe a drought is under way if your house is ankle-deep in water, so consumers haven’t been strident about conservation. The apparent paradox of flooded streets and empty reservoirs will likely fuel an ongoing debate over proposed rationing.

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BEIJING

Poor air quality isn’t the only thing impinging Beijing citizens’ ability to enjoy a safe environment. The city’s second-largest reservoir, shut down in 1997 because of pollution from factories and agriculture, has not been returned to use.

Ensuring the cleanliness of its water is even more crucial in China than elsewhere, as there is little it can afford to lose: With 21 percent of the world’s population, China has only 6 percent of its freshwater—a situation that’s only going to get worse, as it’s raining less in northern China than it was a century ago, and glaciers in Tibet, once the largest system outside the Antarctic and Greenland and a key source of drinking water in the country’s south and west, are receding even faster than predicted. The UN Environment Programme estimates that nationally, Chinese citizens can rely on getting just one-quarter to one-third of the amount of clean water the rest of the world uses daily.

Hope emerged, however, from a 2013 study from Montreal’s McGill University, which found that an experimental program targeting farmers outside the capital showed promising results over nearly two decades. The vast Miyun reservoir, 100 miles outside Beijing, had seen its reserves reduced by nearly two-thirds because of increasing irrigation demands—while becoming polluted by agricultural runoff. Revenue from a tax on major water users in Beijing was spent paying farmers upstream from Miyun to grow corn instead of rice, which requires more water and creates more runoff.

Over the following 15 years, the study authors wrote, “fertilizer runoff declined sharply while the quantity of water available to downstream users in Beijing and surrounding areas increased.” Farmer income was not significantly affected, and cleaner water downstream led to higher earnings for consumers in the city despite the tax.

BANGALORE

Earlier this year, a report by India’s comptroller and auditor general found that the southern city was losing more than half its drinking water to waste through antiquated plumbing systems. Big losses from leaks aren’t uncommon—Los Angeles loses between 15 and 20 percent—but the situation in Bangalore is more complicated. A technology boom has attracted new residents, leading to new housing construction. Entire apartment blocks are going up faster than local officials can update the plumbing to handle additional strain on the water and sewage systems.

Bangalore’s clean-water challenges illustrate a dynamic that’s repeating itself across the world’s second-largest nation. India’s urban population will grow from 340 million to 590 million by 2030, according to a 2010 McKinsey study. To meet the clean-water needs of all the new city dwellers, the global consulting firm found, the government will have to spend $196 billion—more than 10 percent of the nation’s annual GDP. (McKinsey has a potential financial interest in India’s infrastructure, so its numbers may be inflated).

In Bangalore, they’re already behind schedule. The newspaper The Hindu reported in March that a 2002 plan to repair the existing system and recover the missing half of Bangalore’s freshwater had yet to be implemented.

MEXICO CITY

Gravity always wins. At more than 7,000 feet above sea level, Mexico City gets nearly all its drinking water by pumping it laboriously uphill from aquifers as far as 150 miles away. The engineering challenge of hauling that much water into the sky adds to the difficulty of supplying more than 20 million residents through an aging system. Mexico City’s public works loses enough water every second—an estimated 260 gallons—to supply a family of four for a day, according to CONAGUA, Mexico’s national water commission. CONAGUA estimates that between 30 and 40 percent of the capital’s potable water is lost to leaks and spills. The good news is that leaks can be fixed.

Water quality remains a worry, however. Unsurprisingly, companies selling bottled water have done very well in Mexico. The economy growing around the lack of potable water has attracted companies such as Coca-Cola and France’s Danone, whose Bonafont (“good spring”) brand is advertised in Mexico as a weight-loss aid. (Toting a bottle will help you “feel thinner anywhere,” according to a popular television ad).

Meanwhile, disputes over who will get access to underground supplies have turned violent: In February 2014, residents of the town of San Bartolo Atepehuacan, on Mexico City’s outskirts, clashed with police over a waterworks project they feared would divert local springs to the city’s business district. At least 100 people were injured and five arrested as the disturbances continued for more than three months.

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