Water Recycling in Rainforests Influences Rainfall

Published: Wednesday September 12 2012, 15:03:40

A new study shows that the amount of rainfall can be influenced by the degree of forest cover. While there is plenty of anecdotal evidence that forests significantly increase rainfall, until now there has been a lack of observational evidence. Vegetation has been shown to influence precipitation patters because of evapotranspiration. This is the process by which water that is taken in by a plant via its roots and is lost to the atmosphere by its leaves. This pump-like process adds moisture to the atmosphere which in turn influences the energy balance and trace-gas fluxes. If forests are replaced by cropland, this diminished the atmospheric humidity which has the potential to suppress rainfall.

Generally air passing over land is dryer than when passing over the oceans, what this work shows is the role of vegetation since air masses passing over densely vegetated areas were significantly more moist. In fact air passing over extensive forests produces at least twice as much rain as air passing over little vegetation. In some cases these forests increased rainfall thousands of kilometres away.

This new study uses satellite images to link observations and climate model predictions to assess on a pan-tropical scale the effects on tropical rainfall. This exercise explores the cause-effect relationship of vegetation on tropical rainfall; and what it revealed was a strong correlation between the amount of vegetation exposed to the air and the amount of rainfall from that air. To understand the relationship in detail, the team investigated air masses passing over different parts of the forest, to assess 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). What this showed was air that travelled over greater areas of vegetation carried more moisture and more rain was produced. This relationship provides evidence of feedback to the water cycle as well as clarifying the quantity and geographical reach of rainfall generated by large forests.

The role played by vegetations is important to regional rainfall; however land use changes can alter the precipitation locally. The tropical forests of the Amazon are estimated to lose 40% coverage by 2050, under current conditions (1997-2002). Combining this deforestation scenario with present forest coverage was a way to estimate potential changes to future rainfall. It was estimated that future rainfall would be 12% less during the wet-season and 21% less during the dry season. This highlights the sensitivity of tropical forests to loss of area; indeed for the Rio de la Plata basin a 4% decrease in annual rainfall was estimated.

This study 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. 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 rainfed agriculture is important. Increased drought in these regions would have severe implications for their mostly subsistence farmers.

Additional information:

The paper, “Observations of increased tropical rainfall preceded by air passage over forests” by D.V. Spracklen, S.R. Arnold and C.M. Taylor is published in Nature (2012), doi: 10.1038/nature11390
The study was funded by the Natural Environment Research Council, read more here
The University of Leeds issued a press release for this story.

A further paper discusses this article: Environmental science: The rainforest’s water pump by Luiz E. O. C. Aragão, is published in Nature (2012) doi:10.1038/nature11485; available online 05 September 2012 click here