Moisture Recycling

node · earth · Amazon Basin

The process by which rain that has already fallen on the Amazon is returned to the atmosphere by the forest and falls again further inland. A mature Amazonian tree does not simply shed water; it lifts it. Roots reaching several metres down draw on soil moisture that shallow-rooted vegetation cannot reach, and the canopy transpires it at a rate of the order of three to four millimetres per day even in the dry season, when a pasture in the same place would have gone dormant. The first quantitative demonstration came from stable isotopes: Salati and colleagues showed in 1979 that the deuterium and oxygen-18 signature of rainfall changes systematically from the Atlantic coast inland in a way that ocean-derived vapour alone cannot produce, and inferred that roughly half of the basin's rainfall is water recycled within it. That is the mechanism the entire Journey turns on. It is also the mechanism that makes the forest's dry season survivable - the trees keep the atmosphere humid enough to bring the wet season back early, a process sometimes described as the forest starting its own rainy season.