Where does the rain in the Amazon rainforest come from?
Logically, one would expect it to come from the nearby Pacific Ocean. However, in the west of South America, the Andes rise up as a 6,000-metre-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. However, this is not enough to produce an average of over 2,000 millimetres of rainfall per year over the vast forests.
That leaves only the Atlantic. But how do the rain clouds manage to cover the distance – in extreme cases over 5,000 kilometres – to the remotest corners of the Amazon? 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 is despite the fact that it carries more water than all the rivers of the Amazon basin put together!
This is because the forests of the Amazon basin are the largest contiguous rainforests on Earth, and they create their own unique weather patterns.
It all begins over the Atlantic. Under the rays of the hot tropical sun, vast quantities of water vapour rise into the sky and condense into clouds. East winds then drive these clouds over the forest, where they fall as rain. 26 per cent of the rainwater does not even reach the ground, but evaporates straight away from the leaves back into water vapour.
Water that reaches the ground is rapidly absorbed by the roots and then channelled up through the trunk into the crowns. On average, 760 litres of water are ‘exhaled’ per tree per day – that’s around 190,000 litres per hectare of forest every day!
The remaining 74 per cent of the rainfall therefore soon rises again above the forest as water vapour and forms rain clouds. These are driven a little further west by the wind, it rains, and the whole process begins anew. In total, this cycle repeats itself around five times before reaching the Andes. This is how rain can travel from the Atlantic to the remotest corners of the Amazon.
In fact, the plants of the Amazon Basin release 20 billion tonnes of water into the atmosphere every day, thereby creating the largest flow of fresh water on Earth!
This unique system influences the global climate as a whole and once again highlights just how important it is to protect these forests. The only thing that sustains this water cycle is the vast expanse of contiguous rainforests.
This is why deforestation in the east of the Amazon is so devastating. For when the water masses from the Atlantic fall as rain over deforested areas, 80 per cent of the water immediately flows back into the rivers. Only 20 per cent is transported further inland. As a result, not enough rain reaches the western regions of the Amazon; the rainforest, with its biodiversity, disappears and gives way to a steppe landscape.
Text: Fabian Mühlberger and Henriett Wessel
Further sources: ‘Lebenswelt rainforest’, Spitzer et al. 2004.