Forest restoration for green water

Tree restoration is an effective way to store atmospheric carbon and mitigate climate change. Many initiatives exist to increase global tree cover. However, the effects of these initiatives on water resources are still poorly understood. Locally, tree restoration will enhance evaporation. But impacts on precipitation reach far beyond country or even continent level: tree restoration in the Amazon can, for example, affect rainfall in Europe and Eastern Asia. A new study by Wageningen University & Research and Wetsus scientists – amongst others – has calculated the impact of large-scale tree restoration on water fluxes and water availability globally. They published their findings in Nature Geoscience today.

The researchers calculated the hydrological effects of “The global tree restoration potential”: a global map highlighting 900 million hectares where more trees could grow or be planted given local climate conditions and without encroaching on agricultural and urban land. The increase in evaporation resulting from the increased tree cover was calculated globally at a high – 50 by 50 km – resolution. The study used data-driven models that describe how much rainfall evaporates and how much goes to streamflow. Wetsus researcher and coauthor Jolanda Theeuwen: “This is one of the first paper describing the effect of reforestation on worldwide precipitation. A field that is rapidly gaining the interest as droughts are increasing, and the potential for locally recaptured water due to tree planting is promising.”

Local and global shifts in water availability

The results show that large-scale tree restoration can locally increase evaporation annually by nearly 10 liters on average for every square meter of restored forest. Locally, in the tropics particularly, this effect can be much more significant, with almost 250 liters for every square meter. Crucially, not all of this water returns to the land surface. Only around 70% of the extra water in the atmosphere returns to the land, while the remaining 30% is shed over the oceans through rain. On a global scale, this means that tree restoration results in a net decrease in water availability.

For individual river basins, the impact of tree restoration is more complex. Following tree restoration, streamflow for major river basins would generally decrease – by up to 10%. But for other river basins – e.g., the Yangtze and Amazon river, streamflow reduction will be close to zero because the negative impact of enhanced evaporation is compensated by increased rainfall due to forests in these areas. Interestingly, some of these basins possibly will even gain water.

Future forest restoration

Theeuwen: “With the knowledge gained here, we can start further detailing the effects of forest plantation on a local and global level. It shows that reforestation can help reclaim local water from the atmosphere depending on the climate. With it, Wetsus and the Natural Water Production theme are interested in using the precipitation as green water and other effects, like local cooling and the prevention of extreme weather. But that will have to wait until we better understand the effects.”