Rainfall Amounts Affected By Deforestation

April Flowers for redOrbit.com – Your Universe Online

Tropical rainfall can be significantly affected by deforestation, a new study finds, which can have potentially devastating impacts for people living in and near the Amazon and Congo forests.

The research team, a collaboration between the University of Leeds and NERC Centre for Ecology & Hydrology, found that for a majority of the Earth’s tropical land surface, air passing over extensive forests produces at least twice as much rain as air passing over little vegetation. In some cases, rainfall was increased thousands of kilometers away by these forests.

The research team estimated that the destruction of tropical forests would reduce rain across the Amazon basin by up to 21 percent in the dry season by 2050 by combining observational data with predictions of future deforestation.

In the study published in Nature, lead author Dr Dominick Spracklen from the School of Earth and Environment at the University of Leeds said, “We were surprised to find that this effect occurs strongly across more than half of the tropics. We found that the Amazon and Congo forests maintain rainfall over the periphery of the forest basins – regions where large numbers of people live and rely on rainfall for their livelihoods. Our study implies that deforestation of the Amazon and Congo forests could have catastrophic consequences for the people living thousands of kilometers away in surrounding countries.”

This debate over the effects of vegetation on rainfall has been raging for hundreds of years. It is well established that plants put moisture back in the air through their leaves by a process known as evapotranspiration, but the quantity and geographical reach of rainfall generated by large forests has been unclear, until recently. While there is plenty of anecdotal evidence that forests significantly increase rainfall, until now there has been a lack of observational evidence.

Spracklen and colleagues used newly available NASA satellite observations of rainfall and vegetation, along with a model that predicts atmospheric wind flow patterns to explore the impact of Earth’s tropical forests.

“We looked at what had been happening to the air over previous days — where it came from and how much forest it had travelled over,” Dr Spracklen said.

The team investigated the journey of air masses arriving over different parts of the forest, to see the cumulative amount of leaf cover the air had moved over during the previous ten days, not just the amount of vegetation it was over when it rained. This revealed that the more vegetation the air had travelled over, the more moisture it carried and more rain was produced.

“The observations show that to understand how forests impact rainfall, we need to account for how air has interacted with vegetation during its journey through the atmosphere often over thousands of kilometers. This has significant implications for how policy makers should consider the environmental impacts of deforestation, since its effects on rainfall patterns may be felt not only locally, but on a continental scale,” said Dr. Stephen Arnold from the University of Leeds.

Dr Spracklen said the findings showed the importance of initiatives to protect tropical forests. “Brazil has recently made progress in slowing the historically high rates of deforestation across the Amazon and our study emphasizes that this progress must be maintained if impacts on rainfall are to be avoided. The Amazon forest maintains rainfall over important agricultural regions of Southern Brazil, while preserving the forests of the Congo Basin increases rainfall in regions of Southern Africa where rain fed agriculture is important. Increased drought in these regions would have severe implications for their mostly subsistence farmers.”

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