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In Mexico City, more and more residents are watching their taps go dry for hours a day. Even when water does flow, it often comes out dark brown and smells noxious. A former political leader is asking the public to “prioritize essential actions for survival” as the city’s key reservoirs run dry. Meanwhile, 2,000 miles south in the Colombian capital of Bogotá, reservoir levels are falling just as fast, and the city government has implemented rotating water shutoffs. The mayor has begged families to shower together and leave the city on weekends to cut down on water usage.

The measures come as a so-called heat dome sitting atop Mexico is shattering temperature records in Central America, and both Central and South America are wasting beneath a drought driven by the climate phenomenon known as El Niño, which periodically brings exceptionally dry weather to the Southern Hemisphere. Droughts in the region have grown more intense thanks to warmer winter temperatures and long-term aridification fueled by climate change. The present dry spell has shriveled river systems in Mexico and Colombia and lowered water levels in the reservoirs that supply their growing cities. Officials in both cities have warned that, in June, their water systems might reach a “Day Zero” in which they fail altogether unless residents cut usage.

In warning about the potential for a Day Zero in the water system, both cities are referencing the famous example set by Cape Town, South Africa, which made global headlines in 2018 when it almost ran out of water. The city was months away from a total collapse of its reservoir system when it mounted an unprecedented public awareness campaign and rolled out strict fees on water consumption. These measures succeeded in pulling the city back from the brink.

Six years later, Cape Town stands as a success story in municipal crisis management, but experts say its playbook will be hard for Mexico City and Bogotá to replicate. Instead of focusing primarily on changing public behavior, these cities will need to make big investments to improve aging infrastructure and shore up their water supplies. How they fare in these endeavors will in turn inform future efforts to make the world’s fast-growing cities resilient to increasing climate volatility.

“The bigger question, and what’s relevant for other cities, is now that we’ve experienced this, what can we do going forward to make sure that this doesn’t happen again?” said Johanna Brühl, a water expert at the nonprofit Environment for Development in South Africa who has studied Cape Town’s water crisis.

Coining the very phrase “Day Zero” was part of Cape Town’s solution to a water crisis that many officials had seen coming for years. As reservoir levels fell between 2015 and 2017 amid a drought, city leaders released dozens of statements urging residents to reduce water usage, but no one........

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