Logically, it would come from the nearby Pacific Ocean. But in the west of South America, the Andes rise as a 6000 meter high barrier. Rain clouds from the Pacific therefore have little chance of reaching the forests in the Amazon basin.

Of course, some of the clouds are formed by the evaporation of water from the countless rivers. But this is not enough to allow an average of over 2000 millimeters of rain per year to fall over the huge area of forest.

That leaves only the Atlantic. But how do the rain clouds manage to make the journey, in extreme cases over 5,000 kilometers long, to the furthest corners of Amazonia? The answer: with the help of the forests in a unique water cycle.

Here in the Amazon rainforest there is an invisible river, a continuous flow of fresh water that we cannot see with the naked eye. And this despite the fact that it carries more water than all the rivers of the Amazon basin combined!

That's because the forests of the Amazon basin are the largest contiguous rainforests on earth, and they make their own weather.

It all begins over the Atlantic Ocean. Under the rays of the hot tropical sun, vast quantities of water vapor rise skyward and condense into clouds. East winds then drive them over the forest, where they rain down. 26% of the rainwater does not even reach the ground, but evaporates from the leaves to form water vapor.

Water arriving at the ground is quickly absorbed by the roots and then conducted via the trunk into the crowns. On average, 760 liters of water are "exhaled" per tree per day, which is about 190,000 liters per hectare of forest per day!

So the remaining 74% of the fallen rain soon rises again above the forest as water vapor and forms rain clouds. These are driven a little further west by the wind, it rains, and the whole process begins again. In total, this cycle repeats itself about five times until it reaches the Andes. This way, the rain can reach even the farthest corners of Amazonia all the way from the Atlantic.

In fact, the plants of the Amazon basin release 20 billion tons of water into the atmosphere every day, creating the largest river of fresh water on Earth!

This unique system influences the entire world climate and once again emphasizes the importance of protecting these forests. The only thing that maintains this water cycle is the vast extent of the contiguous rainforests.

This is why deforestation in eastern Amazonia is so dramatic. Because when the water masses from the Atlantic rain down over deforested areas, 80 percent of the water immediately flows back into the rivers. Only 20 percent is transported further inland. Thus, not enough rain reaches the western areas of Amazonia. Subsequently, the rainforest with its biodiversity disappears and gives way to a steppe landscape.

Text: Fabian Mühlberger and Henriette Wessel
Further sources: "Lebenswelt Regenwald", Spitzer et al. 2004.