The Atacama Desert in northern Chile sits in the rain shadow of the Andes Mountains. Though it borders the Pacific Ocean, a persistent cold flow known as the Humboldt Current keeps moisture levels in the air relatively low. Clouds form, but quickly dissipate. As a result, rain comes rarely and in small amounts– a few millimeters per year, on average in some parts. In other parts, decades-old weather stations have never recorded any precipitation. Outside of a handful of valleys in Antarctica, the Atacama is the driest place on Earth. The inhospitable landscape of sand, bare rock, and salt flats is so extreme and otherworldly that it’s used as a proxy for Mars by researchers.
Yet still, people live there–mostly in a smattering of coastal cities and towns. Iquique, the oceanside regional capital, is home to more than 230,000 people. Just inland and upslope from Iquique is the fast-growing municipality of Alto Hospicio, which has ballooned to more than 140,000 people (up from fewer than 100,000 in 2012) amid a lithium mining boom.
Fresh water comes from an underground aquifer, which hasn’t been meaningfully refreshed by rainfall for nearly 10,000 years. As more people rely on the aquifer, it’s drying up. Eventually, there will be nothing left. Desalination plants, which remove salt from ocean water, can fill some of the need, but they are expensive and energy intensive to run, especially for low-income cities like Alto Hospicio. Most desalination plants in the region service mining operations, not people.
An alternative, as-of-yet untapped, and inexpensive water source could help resolve the burgeoning water crisis. And it’s water that’s been hiding in plain view. Fog harvesting is a sustainable, simple method for collecting moisture from low-lying clouds. It’s long been used in rural areas around the world to support isolated villages of a few hundred people. But a new study suggests it could work on a much grander scale.
An analysis published February 20 in the journal Frontiers in Environmental Science suggests fog harvesting could meet the needs of Alto Hospicio’s informal settlements, providing as much as 300,000 liters per week to 10,300 people, who mostly live disconnected from the formal water distribution system. Currently, they rely on the disappearing aquifer, but that water is delivered to them via trucks instead of pipes, upping the cost and reducing reliability and accessibility. For inhabitants of these settlements, the water supply is even shakier than for the rest of the city’s residents, and thus fog offers an even bigger opportunity. Beyond drinking water, fog harvesting could also be used to irrigate green spaces in the region, or to fuel hydroponic agriculture–offering people a cheap source of locally grown, fresh food.
On its own, “this water is not going to save the city,” says Virginia Carter, lead study author, a geographer, and an assistant professor at the Universidad Mayor in Chile. But fog is a resource that might make a real difference, she says. “It could contribute, and in many places it might be important,” Carter explains, especially as Alto Hospicio continues to grow and climate change makes the existing water supply even more tenuous.
Fog harvesting relies on a low-tech set-up. Usually, a fine plastic mesh, like the type that might be used to shade heat-sensitive garden beds, is strung across two support poles a few feet in the air. A gutter beneath the mesh channels the moisture that condenses on the panel into a storage container, so it can be readily collected. The more panels and greater surface area of mesh used, the more water is harvested.
Unlike the aquifer beneath the Atacama, fog water is a potentially renewable resource. Low clouds routinely get churned up from the Pacific and blow overland. Without the mesh, the moisture evaporates in the dry air as the temperature rises each day, but with it, the fog would be only temporarily waylaid– providing valuable fresh water to people, before it’s treated and cycled back into the ocean.
To demonstrate that fog harvesting would be worthwhile for Alto Hospicio and northern Chile more broadly, Carter and her co-authors combined a year of observational measurements with satellite imagery and mathematical modeling of the region’s fog cycles. The researchers set up two one-square-meter “standard fog collectors” at different altitudes, along with a weather station to keep tabs on air moisture, temperature, wind speed, and other variables. They also used remote sensing to map fine-grained altitude and fog density across the province. Finally, they synthesized this and other data from existing fog collector projects into a mathematical model intended to estimate how much fog could be harvested at different times.
They found that the fog is highly seasonal, appearing from May through October (the Southern Hemisphere’s winter into spring). It peaks in June during the night and early morning, and all but disappears in the warmer months and by midday. In the zones immediately around Alto Hospicio, fog collectors would yield an estimated average of 2.5 liters of water per square meter of mesh during the fog season, according to the study.
At this rate, it would take 17,000 square meters of collectors (just over three football fields’ worth) to yield 300,000 liters a week–the same volume of water currently delivered via truck to Alto Hospicio’s informal settlements each week. However, Carter notes this is a conservative estimate, as certain areas to the north of the city have much more fog potential than the average, producing more than 5 liters per meter of mesh per day. If fog collectors were placed strategically, then just 200-300 fog collectors (each encompassing about 20 square meters) could reliably provide hundreds of thousands of liters for at least half the year, she explains. Complementary storage tanks or ponds could stretch the fog water into a year-round resource. “This is kind of a dream. To develop something like that for Alto Hospicio,” Carter says.
If providing drinking water to 10,000+ people proves too big of an initial goal, smaller pilot projects might offer a proof of concept. Carter and her colleagues suggest fog harvesting could also be used to irrigate public parks or provide water to hydroponic farms, with less initial investment. Just 110 square meters of mesh would be needed to fulfill the city’s green space needs. One square meter of fog collection could yield more than 15 kg of leafy greens each year.
Before the dream becomes a reality though, further work is needed. The scientists would like to verify their model estimates with more on-the-ground measurements, to home in on the best locations for fog collectors. Carter says she also hopes to test the quality of the harvested water, and determine what type of treatment would be needed to make it safe for human consumption. Fog can carry exhaust particles, bacteria, and microplastics–like any other natural water source. Yet, even untreated, the water could still have applications in agriculture or mining.
And already the research is advancing. Carter and her colleagues plan to release a publicly accessible, detailed map of fog for all of northern Chile later this year, based on their model. She hopes that local and national government officials take notice. “This study is a very clear example of how scientific knowledge [can] contribute to public decision-making and policies,” she says. “We have a problem: We have no water, and it’s getting worse. But on the other hand, there’s a solution. There’s this water sitting and waiting. We just need a logical way to harvest it.”
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