A tree takes water from the ground, brings it to the leaves and then returns part of it to the air. Do it with millions of trees, over huge territories, for twenty years and that little natural pump begins to weigh even on the travel of the clouds. This is what would have happened in China, where decades of reforestation and prairie recovery have changed the water cycle between different regions of the country.
A study published in the journal reconstructs it Earth’s Futurewhich analyzed the changes that occurred between 2001 and 2020. In those twenty years, China’s forest area increased by approximately 226,000 square kilometers, an area almost the size of the United Kingdom. In the meantime, prairies, crops and bare land have also changed. And the water moved with them.
Trees return water to the air, and that water can travel
To understand what happened just follow a drop. Plants absorb water with their roots and release some of it through tiny pores in their leaves. Added to this is that which evaporates directly from the ground. The two processes together are called evapotranspiration. That water does not disappear: it becomes vapor, enters the atmosphere, is transported by the winds and can return to the ground in the form of rain even very far away.
According to the researchers, changes in Chinese vegetation have increased evapotranspiration by about 1.71 millimeters per year on average. Precipitation also increased, by approximately 1.24 millimeters. The recovery of rain, however, was not enough everywhere to compensate for all the water released into the atmosphere. The estimated average result is a decrease of approximately 0.46 millimeters per year in water availability.
Of course, the study is not saying that China “lacks” that amount of water in its taps. Scientists calculate water availability as the difference between rainfall and evapotranspiration. It is therefore a balance of the natural water cycle, reconstructed with data and models, not the level of an aqueduct.
Some of the water ended up on Tibet
The most interesting part comes when you look at the map. The effects vary quite a bit from one area to another. On the Tibetan Plateau, water availability would have increased by approximately 0.38 millimeters per year. In the east of the country, affected by the monsoon, it decreased by around 0.59 millimetres. The largest estimated loss is in China’s arid northwest: about 1.14 millimeters per year. How can one region lose water while another gains water? The winds take care of it.
The simulations show that some of the moisture released by vegetation in the northwest is pushed towards Tibet by atmospheric currents. The researchers estimate that land changes in that region have contributed to adding about 0.52 millimeters of precipitation per year to the Tibetan Plateau. The trees remain still. Water definitely less.
It wasn’t just the trees
Talking exclusively about reforestation, however, would be an understatement. The study considers all the major changes that have occurred on the Earth’s surface: forests, prairies, cultivated fields, urban areas, bare land and areas covered by water.
One of the most important transitions was precisely that from prairie to forest. According to the study’s calculations, this transformation would have produced approximately 8.44 billion cubic meters of additional evapotranspiration each year. About 5.08 billion would then return as rain, leaving a lower balance of about 3.36 billion cubic meters. In other areas, especially in Tibet and in the north-west, the recovery of the prairies is very important.
Behind these changes there are enormous public programs. China has been carrying out projects like the for decades Grain for Green and the Natural Forest Protection Program. Then there is the gigantic Three-North Shelterbelt Forest Program, also known as the “Great Green Wall”, started in 1978 to combat desertification, soil erosion and sandstorms. And no, the Great Green Wall has not been “completed”: the program is expected to continue until 2050.
Planting trees remains useful, but you have to choose where and which ones
The study therefore does not deliver a moral such as “trees consume too much water”. That would be a much simpler conclusion than the phenomenon the researchers observed.
A new forest absorbs carbon, protects the soil, can promote biodiversity and change the microclimate. At the same time it needs water. How much it uses and where that water ends up depends on the climate, the species planted, the soil and even the prevailing direction of the winds.
To arrive at these estimates, researchers from China Agricultural University, Utrecht University, Tsinghua University and Tianjin University combined satellite images, climate data and a model called UTrack, capable of tracking the movement of moisture in the atmosphere. It is therefore a model reconstruction. The same authors explain that not all exchanges between vegetation and atmosphere can be perfectly represented and that a forest, in addition to water, modifies soil temperature, energy and air movements.
The practical consequence is much less spectacular than “let’s plant billions of trees and we’ve solved it”: in arid areas we need to understand how much water is really available before choosing where to create new forests and which species to use. China will continue to revegetate territories for at least another two decades. After twenty years of data, however, one thing has become visible: when the landscape changes on a gigantic scale, it is not just the boundaries of greenery that move. Water also moves.
