Explain how deforestation leads to reduced rainfall?

The Amazon rainforest is being cut down so rapidly that the local climate changes in less than a decade, drastically reducing rainfall for parts of Brazil.

Data from satellites show deforestation is spreading southward in Mato Grosso state at a rate of about six football fields per minute during the early part of this century. In central Mato Grosso, the deforestation rate is low and rainfall has been nearly constant over the last 40 years, but in the southern part of the state — where most future deforestation is expected to occur — there’s been a drastic decline.

“Cutting down forests changes the climate, even if you put that forest back again,” said University of Leeds hydrology expert Chris Clark.

After deforestation, “the landscape becomes much less efficient at using water,” he said. “You just get a lot more rainfall runoff and that’s what we’re seeing in southern Mato Grosso.”

The study was published in the journal Geophysical Research Letters.

Mato Grosso is home to both the Amazon rainforest and Brazil’s soybean belt. The researchers said they chose to study it because deforestation is proceeding differently in that state than elsewhere in Brazil, and could represent a preview of things to come.

The team analyzed nearly 40 years of changes in the area’s rainfall and vegetation using data from weather stations and NASA satellites.

When trees are cut down, sunshine heats the ground and the air above it, causing water to evaporate from both surfaces. Trees also convert rainfall into moisture they store in their leaves, trunks and roots. The forest’s vast network of roots helps maintain soil stability and reduce erosion by causing fallen leaves to mix with the soil.

When forests are removed, all that vegetation is no longer there to absorb the heat and sunlight, and the ground dries out more quickly.

“Biomass is a major factor controlling rainfall,” said University of Maryland atmospheric scientist Stanley B. Goldenberg. “When you take away vegetation from an area, it has consequences for water runoff and how much rain falls.”

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