The global press recently warned that as early as June 2024, Mexico City, home to 22 million people, could face “Day Zero”—the complete loss of fresh water at the taps. The situation on the ground, although dire, is more nuanced. “Day Zero is a bit of an exaggeration,” says Juan Bezaury, a former associate director at the Nature Conservancy in Mexico. Instead, Bezaury says, Mexico City, North America’s largest city, is facing the exhaustion of the Cutzamala aqueduct system, which brings the metropolis up to 25 percent of its water, from reservoirs across the surrounding state of Mexico. Scientists say the persistent drought much of the country has suffered is depleting its reservoirs. “Climate change is changing the [likelihood] of these extreme events,” says Sarah Kapnick, chief scientist at the U.S. National Oceanic and Atmospheric Administration.
Even if total shutdown is an exaggeration, the threat of water shortages is high, and it is rising worldwide as well. Cape Town, South Africa, came perilously close to a Day Zero in 2018. Cities around the globe, from those in Colombia to India, are at great risk of losing fresh water. In April, Bogotá began rationing in response to drought.
Most of the rest of Mexico City’s water is drawn from the aquifer on which it is built, which brings its own risk. “The more the aquifer dries up, the more Mexico City sinks,” Bezaury says. Some parts of the city are sinking by up to 20 inches per year. He adds that Mexico City is already pumping more than twice the water from the aquifer that can be replaced by surface water infiltration. This deterioration cannot solely be attributed to climate, however. The population of the area has risen relentlessly. Centuries of development have compromised the aquifer’s replenishment. “We paved the hell out of [the Valley of Mexico],” Bezaury says, “and there is almost certainly no infiltration.” Most of the rainwater lingering on the surface evaporates.
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Overreliance on aquifers is part of a global trend. “Water [sources] that have been deep underground have been an insurance policy” against drought, Kapnick says, “but they are finite resources.”
Piecemeal construction as the city grew also created an inefficient and unwieldy water system that is difficult to repair, says Gabriel Eckstein, director of the Energy, Environmental and Natural Resource Systems Law Program at Texas A&M University. Bezaury says that improper maintenance of the region’s infrastructure has led to a system that loses up to 40 percent of its water to leaks.
Mexico City might be able to learn some lessons from elsewhere. In 2018 Cape Town’s 4.5 million people were hit by a drought that was much worse than any in the past. The city was already approaching the limits of water extraction, and it was almost entirely dependent on surface reservoirs. “The major problem in the Cape Town case was having one [type of] source of water that as a result of climate change was at much greater risk than it had been prior,” says Barton Thompson, a senior fellow at the Woods Institute for the Environment at Stanford University. He says all of Cape Town’s reservoirs were dangerously low because of a lack of rain. Even though Cape Town had “spent years conserving water,” the city’s continued growth left the system unable to adjust.
To prevent collapse, Cape Town “doubled down” on conservation efforts, Thompson says. Measures included restricting domestic and commercial use of water and reducing water consumption by farms surrounding the city. The restrictions were particularly onerous for businesses such as restaurants and hotels, forcing them to find alternative water sources. The Westin Hotel built its own desalination apparatus to take advantage of the seawater infiltrating its basement.
Conservation efforts pushed out the impending Day Zero date, staving off reservoir exhaustion until the winter rains came. Despite the reprieve, though, Cape Town is still in a precarious position. Cape Town was only saved because of “a combination of extreme conservation and luck,” Thompson says.
Other water-stressed areas around the world are trying to implement sustainable solutions. “The key thing cities can do to avoid these Day Zero situations is to diversify their water sources,” Thompson says, adding that “as water becomes scarcer, cities become far more creative in the way they try to diversify their supply.” San Diego has invested heavily in desalination, as well as in water recycling—filtering suspended solids and bacteria from a community’s sewage and storm runoff and using ultraviolet light to sterilize the filtered water. San Diego is also looking to exploit new external sources beyond the Colorado River, such as the nearby San Luis Rey River and local groundwater basins.
Las Vegas, which Thompson calls “one of the cities that has done the most with the least,” is trying creative arrangements with other cities. Las Vegas has a disproportionately small allotment of water from the Colorado River relative to its population, so it is funding desalination and wastewater-recycling projects in Los Angeles in exchange for a share of the distant city’s Colorado River allotment.
Israel and Singapore have been the most successful in addressing their water scarcity, Eckstein says. Both nations have invested heavily in desalination and water recycling; desalination now provides 70 percent of Israel’s potable water, and the country recycles 90 percent of its wastewater, which far exceeds the levels recycled by any other country. Further, Israel has exploited efficient irrigation methods such as drip and micro drip irrigation, which are significantly more efficient than traditional flood irrigation. Agriculture typically uses a lot of water, so even relatively small reductions can free up large amounts of water for other purposes.
Kapnick notes that some places have reduced the loss of rain runoff by building infrastructure with materials such as permeable pavement, which allows water to seep into the ground, and by painting roofs white to reflect sunlight, reducing urban temperatures and thus water lost to evaporation. Nature-based solutions are helping, too. Cape Town lessened reservoir losses by removing invasive, water-hungry plants growing along the edges and replacing them with indigenous, drought resistant species, Thompson says.
Some of these solutions may prove elusive in Mexico City. For example, the cost of pumping desalinated water from the coast to Mexico City “would be exorbitant,” Eckstein says. “Water is eight pounds per gallon,” he points out, noting that California’s State Water Project, which transports water from the Sacramento region to Southern California across similar elevation gradients, consumes up to five percent of the state’s electricity. Bezaury estimates that completely reconstructing Mexico City’s water grid could cost up to $6 billion.
The experts agree that regardless of the outcome of any single drought or water shortage, long-term resilience will require significant advanced planning. “Climate change has made new rules of the game,” Bezaury says. Adaptation is essential.
Source link : https://www.scientificamerican.com/article/0424–kinard-mexico-city-run-out-of-water/
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Publish date : 2024-05-10 03:00:00
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