Congo Rainmaker

Evapotranspiration is the largest source of water vapor in the Congo rainforest during the spring rainy season.

Photo of rainforest showing densely packed palms and other trees.

The Congo rainforest’s dense trees provide enough moisture through transpiration to trigger the spring rainy season.

Axel Fassio/CIFOR CC BY-NC-ND 2.0

The Congo Basin, which covers large swathes of seven countries in equatorial Africa, contains the world’s second-largest rainforest. It faces rapid deforestation but has been much less studied than the more-accessible Amazonia. Now, research has identified an unusual hydrological phenomenon that may make the Congo rainforest especially vulnerable to the compounding impacts of deforestation.

Researchers at the University of California Los Angeles, the Jet Propulsion Laboratory, and the California Institute of Technology analyzed a suite of atmospheric data collected by satellite over the Congo. One variable—the levels of the heavy hydrogen isotope deuterium in water vapor—indicates how much moisture blew in from the ocean and how much was released by the basin’s vegetation in a process called evapotranspiration (ET). Another variable, solar-induced fluorescence, signals photosynthetic activity, which causes ET.

These data, together with precipitation and wind measurements, show that the Congo Forest itself produces an unusually large share of the rain that falls on it— an estimated 75 to 85 percent, versus the 50 to 62 percent of Amazonian moisture that is recycled. ET was already known to be the main source of precipitation during the Congo’s winter and summer dry seasons, when oceanic winds are weak. But deuterium levels indicate that ET also produces about 83 percent of atmospheric moisture during February, the transition to the spring rainy season.

The researchers note that their research raises additional questions: Would the spring rainy season disappear or substantially weaken if ET were substantially reduced by rainforest loss? Are the mechanisms for the onset of spring rainy season significantly different from those of the fall rainy season? Why does ET contribute more to the atmospheric moisture in the Congo than in the Amazon basin? In a subsequent interview, researcher Rong Fu noted, “the Congo is dryer than the Amazon to begin with. It could be more volatile. It is the only area we know that is so dependent on evapotranspiration.” Whether that transpiration is necessary to trigger the vital spring rains, she said, “is the subject of our next paper.” (JGR Biogeosciences)

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