In April 2024, more than 9 million residents of Bogotá, Colombia’s capital city, were told to collect rainwater – if the city was lucky enough to experience a storm.

Fed by the Guatiquía River, the Chingaza reservoir system, which supplies the area with 70% of its water, had reached critically low levels.

To make what was left stretch through a dry spell with no clear end in sight, authorities divided the city into nine zones. Every day, one of the zones would go dry for 24 hours. No toilet would flush. No glass of water would be filled from the tap. Dishes would have to go unwashed.

Bogotá Mayor Carlos Galan told residents they should be prepared to live with the water restrictions for a year.

“The call is to take care of every drop of water,” the mayor’s office said, according to CBS News.

A month later, 2,000 miles away in Mexico, the Cutzamala system of reservoirs reached historic lows. The water reserves supply a substantial portion of water to Mexico City’s 22 million residents, who faced mandatory rationing.

Bogotá and Mexico City’s stories mirror those of cities across the globe. The amount of water stored in lakes worldwide has drastically and steadily decreased since 1992, according to a 2023 study published in the prestigious research journal Science. During those 30 years, freshwater lakes collectively lost an average of 600 cubic kilometers of water storage annually – 17 times the volume of Lake Mead, the largest reservoir in the United States.

A global crisis

The cause is a combination of human-caused overuse and unprecedented shifts in the climate, the researchers found.

Increasing temperatures, accelerated evaporation, and unpredictable shifts in rain and snow patterns and the runoff these events create have made urban water sources increasingly unstable.

These factors, coupled with unsustainable water consumption, are responsible for about half the water losses over the last 30 years. They’ve pushed cities around the globe closer to Day Zero, when water supply would be depleted and taps would run dry.

But understanding which of these stressors is having the biggest impact on each water system is the cornerstone for creating solutions.

The cautionary tale of the Aral Sea

In living memory, the Aral Sea, which straddles the Uzbekistan-Kazakhstan border, was the fourth-largest lake in the world.

That decision made the Aral Sea a poster child for what happens when humans overuse water in arid regions.

Today, the lake covers one-tenth of the surface area that it did in 1920. Five and a half million hectares of the lake – an area the size of Lake Michigan – is dry. If rusting fishing boats didn’t stud the taupe-colored desert, there would be little reason to believe the surrounding area is a lake bed.

A similar story is unfolding for dozens of critical waterways, from the Maipo River in Chile, the basin of which provides 80% of Santiago’s water, to the Colorado River in the United States, which supplies water to 40 million people. Both rivers have been overpromised and overused by sprawling cities and agricultural operations.

How does climate change affect water supplies?

Even the best-laid plans can be thwarted by the uncertainties climate change brings. Fueled largely by hotter temperatures, evaporation is a significant threat to drinking water supplies worldwide.

Evaporation in reservoirs and natural lakes increased by nearly 60% between 1985 to 2018 – more than scientists previously thought, according to a study published in Nature Communications in 2022.

In central Argentina, a salt lake called Laguna Mar Chiquita, or “Lake Little Sea,” shows how increased pressures from climate change interact with overconsumption. Lack of rainfall and high temperatures have meant the three rivers that feed Lake Mar Chiquita deliver less water. Without rainfall to irrigate their crops, farmers divert more water than usual from the lake so their plants won’t die. Evaporation, exacerbated by heat, adds pressure to the lake. In the summer of 2022 and 2023, Argentina set a record with 10 heat waves.

The Colorado River, which supplies water to seven Western states, nearly 30 Native American tribes, and two states in Mexico, faces similar threats. The river irrigates 15% of American agriculture, including around 90% of vegetables grown in winter. The American West is a quarter century into a megadrought that hasn’t been remedied by two consecutively wet winters.

As a result, more water is being used than is coming in.

The U.S. Department of the Interior monitors Lake Mead, which is fed by the Colorado River. If experts expect reservoir levels to dip below a certain point, the federal agency releases less water, leaving downstream shareholders across two countries with less water. In 2022, this happened for the first time.

Arizona, Nevada, and Mexico have all faced cuts to their shares of the Colorado River in recent years, a trend expected to continue in 2024.

“A warmer atmosphere is a thirstier atmosphere, and without a compensating increase in precipitation, which has not occurred, humans and ecosystems will be left with less water,” hydroclimatologist Park Williams of the University of California, Los Angeles, told the Los Angeles Times.

What can cities do about dwindling water supplies?

There have already been a series of small, continued efforts that have been able to stave off Day Zeros.

Cutting water use is the most obvious line of defense. It’s a scenario people around the world watched play out during a narrowly averted crisis in Cape Town, South Africa, in 2018. Starting in 2015, dwindling reservoir storage for Cape Town’s water supply brought the city perilously close to Day Zero. In response, city officials implemented harsh restrictions that cut water consumption by 50% of 2015 levels. Those efforts, coupled with the return of seasonal rains, kept the city’s water supply from running out.

Some nations have already implemented changes to their water infrastructure in an effort to reduce the amount lost to evaporation and overuse, such as Las Vegas’ nonfunctional turf ban.

In southern Brazil’s Passaúna reservoir, floating solar panels have the potential to be a win-win. The shimmery sheets reduced evaporation in the reservoir by 60%, while at the same time powering homes, businesses, and industry with renewable energy. Floating covers and shade balls can have the same evaporation-thwarting effect, especially for reservoirs in arid climates. Though these technologies show promise for decreasing evaporation, ongoing research is evaluating side effects, such as impaired water quality.

“Understanding the primary stressors on lake water losses is often a prerequisite for management solutions,” said Fangfang Yao, a postdoctoral fellow at the Cooperative Institute for Research in Environmental Sciences who led the Science study, told Yale Climate Connections.

And addressing climate change is a necessary part of the solution, too. “Reducing greenhouse gas emissions nationally and internationally will help alleviate further water losses due to evaporation,” Yao said.