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Reforesting areas of the world comes with major benefits, but adding billions of trees can come with unintended consequences.

A new study details how China’s massive regreening effort has altered the country’s hydrology, shifting more precipitation to some areas while drying out others.

The main driver behind this change is an increase in both evaporation and transpiration—a process where plants release water vapor through pores called stomata.

It’s no secret that China is particularly adept at building things. The world’s largest dam? In China. Largest high-speed rail network? That’d be China. Largest wind farm? China. Solar Array? Argentina. Really? Nope, it’s also China. Largest ancient landmark? You get the picture.

Alongside all of that building, China has also been growing. Taking inspiration from that last infrastructure accolade, China began growing the Three-North Shelterbelt, or “The Great Green Wall,” in 1978 as an effort to combat soil erosion and decrease desert storms. The project, the country’s state-sponsored media announced, was finally complete as of last year.

According to Reuters, China grew 116,000 square miles of trees, increasing the country’s total forest coverage from 10 percent in 1949 to roughly 25 percent in 2024. But a new study published in the journal Earth’s Future, shows that all those additional trees (roughly 78 billion since the early 80s, by some estimates) come with some unforeseen consequences for China’s water distribution.

Scientists from Tianjin University, China Agricultural University in Beijing, and Utrecht University in the Netherlands found that between 2001 and 2020, increased vegetation reduced water resources in both the eastern monsoon region and the northwestern arid region. That’s a big deal, considering these areas make up roughly 74 percent of China’s total land area, according to Live Science.

According to the study, regreening efforts like the Great Green Wall—along with other tree-planting initiatives, such as Grain for Green Program and the Natural Forest Protection Program, with both started in 1999—have increased evapotranspiration, which is a portmanteau of evaporation and transpiration (the process through which plants release water vapor through tiny pores known as stomata).

“These shifts caused changes in precipitation, directing more moisture to the Tibetan Plateau, which saw an increase in water availability,” the authors wrote. “In contrast, eastern and northwestern China experienced a decrease in water availability, with the northwest losing the most due to substantial moisture moving to the Tibetan Plateau.”

By studying these rapid land use/cover changes (LUCC), the authors also note that certain zone transitions—such as grasslands-to-forests or croplands-to-grasslands—impacted evapotranspiration, precipitation rates, and water availability at varying rates. For example, grasslands transformed into forests increased evapotranspiration and precipitation, but negatively impacted water availability.

Unfortunately, China’s water availability isn’t conveniently distributed for its population. According to the study, the country’s northern regions contain roughly 46 percent of its population and more than half of the arable land, but only 20 percent of water availability. The authors argue that these altered hydrological cycles need to be taken into account when planning future reforestation efforts.

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“Our findings highlight that land cover changes can redistribute water resources between regions,” the authors wrote. “Understanding these effects is crucial for planning sustainable land and water management in China.”

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