The Amazon is famous for the mighty river that snakes through its forests, carrying more water than any other river on Earth. But high above the canopy, another vast flow of water is moving across South America - almost entirely invisible.
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Known as “flying” or “aerial rivers”, these atmospheric highways carry moisture released by the rainforest for hundreds or even thousands of miles, helping to generate rain both within the Amazon and far beyond it.
At the heart of this extraordinary system are the forest’s trees. Through their roots, trees draw water from the soil and transport it upwards through their trunks to their leaves. Much of it eventually escapes as water vapour through tiny pores in the leaves, in a process known as transpiration. Water also evaporates directly from soil and other wet surfaces. Together these processes are known as evapotranspiration.
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The quantities involved can be astonishing. One 2022 study measuring water use by Amazonian trees recorded figures ranging from about 11 to 190 litres per tree per day.
Across the rainforest, the numbers become enormous. Scaled across approximately 5.5 million sq km of rainforest, that is equivalent to about 13 to 18 cubic kilometres of water being transferred from the
land into the atmosphere every day.
Once airborne, water vapour is carried by winds. As moist air rises and cools, it condenses into tiny droplets, forming clouds and eventually falling as rain. Some of that water is then absorbed by
trees and returned to the atmosphere, allowing the cycle to begin again.
It is why the Amazon is sometimes said to “make its own rain”, although Dr Wei Weng, Assistant Professor at National Taiwan University, whose research examines atmospheric moisture transport and
the links between forests and water, says the reality is more nuanced.
“The Amazon Basin is extremely large, so the degree to which different regions depend on moisture recycled within the basin varies considerably,” she says. “Some areas rely more strongly on the basin’s
own evapotranspiration, while others depend more on external moisture sources.”
The Atlantic Ocean remains an important source of water. Depending on location, Wei says the proportion of rainfall dependent on evapotranspiration from within the Amazon can range from almost zero to about 50 per cent.
Moisture can also be recycled repeatedly as it moves across the continent, with rain falling, being taken up by vegetation and later released back into the atmosphere.
“Moisture can be recycled multiple times as it moves across the continent,” Dr Wei explains, although the first recycling cycle is generally considered the most important in terms of its contribution to rainfall.
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But not all of this water falls back over the rainforest. Vast quantities are transported across South America through what have become known as flying or aerial rivers. Rather than being literal rivers in the sky, Dr Wei describes them as “preferential pathways of moisture transport” which persist over long periods, reflecting the combined effects of winds and other atmospheric processes.
“Moisture originating from the Amazon can be transported to many parts of South America however, its dominant preferential pathway is generally directed toward the La Plata Basin,” she added. Bolivia and Peru are among the regions particularly dependent on moisture supplied by the Amazon Basin, while Amazonian moisture also contributes to rainfall elsewhere across the continent.
Perhaps most remarkable is that these aerial and terrestrial flows can move in opposing directions.
While surface rivers generally carry water eastwards towards the Atlantic, atmospheric moisture brought in from the ocean travels inland and is repeatedly recycled as rainfall.
For Dr Wei, this is one of the most surprising things scientists have uncovered about the Amazon. "The opposing directions of surface rivers and flying rivers together facilitate continuous water exchange
between the land surface and the atmosphere,” she says. But remove enough trees and this remarkable circulation can begin to change. “A single tree is unlikely to have a measurable regional effect but
large-scale forest clearing can substantially alter moisture recycling, particularly when it occurs in critical upwind source regions,” Wei explains.
In previous modelling, she found that deforestation in upwind areas of Ucayali, Peru, was associated with an estimated 5 to 13 per cent reduction in annual rainfall in the downwind region. Annual runoff fell by an estimated 19 to 50 per cent.
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Across the Amazon more widely, her modelling suggests extensive deforestation could reduce annual rainfall by approximately 6 to 18 per cent. This raises fears of a feedback loop in which forest loss contributes to drier conditions, leaving surviving forest more vulnerable to drought and degradation.
Scientists continue to debate exactly where an Amazon “tipping point” might lie, but Dr Wei warns where trees disappear could be as important as how many are lost. “Deforestation in critical upwind areas of the flying-river system is likely to have a disproportionately large effect on downwind moisture supply,” she says, potentially accelerating ecological change in particularly vulnerable regions.
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The reverse may also be possible. Strategically restoring forests in important upwind areas could strengthen atmospheric moisture recycling and improve water availability downwind. This extraordinary movement of water helps explain why rainforests can flourish so far from the ocean. “The Amazon forest is one of the greatest jewel boxes of biodiversity on Earth,” says Dr Wei. “Part of its secret is water.
“Trees release moisture into the atmosphere, and trade winds carry this moisture deep into the continent, helping forests survive even thousands of kilometres from the ocean.
"What makes the Amazon especially fascinating is the way its surface rivers and flying rivers work together. This creates a remarkable circulation between the land and the atmosphere - a kind of natural water system that helps keep the Amazon green, wet and extraordinarily rich in life,” she added.
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