Specifically, if the “flying rivers” of transpiration during dry seasons are from deep water tables or shallow groundwater:

The Amazon is the world’s largest tropical forest, home to unmatched biodiversity and one of the planet’s longest rivers. Besides the Amazon River, the Amazon rainforest also features “flying rivers:” invisible streams of vapour that travel through the atmosphere, fuelling rainfall both within the forest and far beyond its boundaries.

The forests play a central role in this system. Much of the moisture that rises into the atmosphere comes from transpiration. Trees pull water from the soil through their roots, transport it to the leaves and release it as vapour. That vapour becomes rainfall — sometimes locally, sometimes hundreds of kilometres away.

In the dry season when rain is scarce, up to 70 per cent of rainfall in the Amazon comes from this moisture-recycling generated by the forest itself. This raises a key question: where do the trees find the water to keep the cycle going during the driest months?

The results were surprising. Most water used for transpiration in the dry season did not come from deep reserves, but from shallow soil. In a year without extreme drought or floods, 69 per cent of transpiration on the hill and 46 per cent in the valley came from the top 50 centimetres of soil.

Our research also found that water stored in the shallow soil had fallen on land recently, specifically during the dry season. In other words, the forest rapidly recycles the rain: it falls, infiltrates shallow soil, is absorbed by roots and is released back into the atmosphere, fuelling new rainfall — right when the forest needs water most.

This is important because:

This delicate balance is threatened by deforestation. When forests are cut down, fewer trees release moisture into the air through transpiration, reducing the formation of local and nearby rainfall during the dry season.

Forest loss weakens the very system that sustains rainfall — the recycling of water through transpiration. Our study shows that embolism-resistant trees play a central role by quickly returning dry-season rainfall to the atmosphere, where it fuels new rainfall.

The message is clear: without the forest, there is no rain, and without rain, no forest. The quick recycling of dry-season rain keeps the Amazon alive through its driest months. It also plays a crucial role in triggering the return of the wet season. If the forest loses its ability to recycle this water, the entire hydrological cycle risks collapse.