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Originally published Saturday, May 18, 2013 at 6:08 AM

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Cutting rain forests may undercut hydro output

The elimination of swaths of rain forest decreases rainfall, and is likely to reduce the energy output of hydroelectric projects in countries such as Brazil, according to a study published by the National Academy of Sciences.

The New York Times

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The loss of tropical rain forests is likely to reduce the energy output of hydroelectric projects in countries like Brazil that are investing billions of dollars to create power to support economic growth.

That is the conclusion of a group of experts whose findings, released Monday, run counter to the conventional understanding of deforestation’s impact on watersheds.

For years, scientists and engineers have noted an increase in river flows when the trees along streams are removed. The water in the soil, which would otherwise have been taken up by the tree roots and sent into the atmosphere, instead moves directly into streams and rivers.

At the same time, large areas of tropical forest create rain clouds as moisture from their leaves evaporates. So the elimination of swaths of these forests decreases rainfall. Cut down enough trees, the scientists argue, and the indirect impact of lost rainfall outweighs the direct impact of removing trees.

The study, published by The Proceedings of the National Academy of Sciences, predicts that extensive deforestation will leave less water in the rivers to generate hydropower from projects like Belo Monte, which is currently under construction on the Xingu River in Brazil and will be the world’s third-largest hydropower complex.

The Belo Monte project, whose massive scope and impact on the landscape has led to opposition, is expected to generate at least 4,400 megawatts of electricity, the study said. The project’s overall capacity would be more than 11,000 megawatts, but because of wide variations in seasonal flows of the Xingu River, the lower output is what developers guarantee.

But the study warns that by 2050 as much as 40 percent of this power could be lost because of the reduced rainfall caused by regional deforestation.

Loss of tropical rain forests in the Amazon basin, Central Africa, Indonesia and other parts of the world has been a pressing environmental issue for two decades, but the debate has been framed largely in two ways. First, that the loss of the forests accelerates worldwide climate change by removing a large carbon sink that absorbs carbon dioxide from the atmosphere. Second, that the deforestation destroys the livelihoods of indigenous communities.

The idea that deforestation could lessen rainfall and thus economically harm a country like Brazil, which gets more than 80 percent of its energy from hydropower, is less familiar.

Noting the established connection between the loss of trees and an increase in river flow, Claudia M. Stickler, the paper’s lead author, said researchers in the Amazon basin “saw effects where the conventional wisdom did not hold true.”

“They removed so much forest that it reduced rainfall and reduced the stream flow,” she added.

Her co-author, Daniel Nepstad, who like Stickler works at the Amazon Institute for Environmental Research in Brasilia, said that rain forests create rain because they “are in the equatorial sun, evaporating a huge amount of water that goes up through the stems and into the leaves and out into the atmosphere.” That moisture, he added, “feeds rain clouds.”

In some eastern and southern tributaries of the Amazon, he added, “the cycle has changed.” The Xingu River, he said, is already near a tipping point where the increased flows caused by the loss of tree roots will be nullified by the overall loss of rainfall.

The authors conclude that “as tropical rain forest nations turn increasingly to hydropower to meet growing demands for ‘green’ electricity, it is important” that planners take into account the links between forest cover and stream flows.

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