Exploring how forest restoration affects water cycles

Historic forest on Telegrafenberg in Potsdam as part of a heritage site and science campus. The study shows that large-scale reforestation has implications beyond the borders of countries and even continents. Credit: Josef Zens/GFZ

How would afforestation and restoration of large areas around the world affect water flows around the world? A new study by Anne Hoek van Dijke, a researcher at Wageningen University, with input from Martin Herold, GFZ, provides some interesting answers. Impacts on rainfall go far beyond the national or even continental level: tree restoration in the Amazon can, for example, affect rainfall in Europe and East Asia. The study, published in nature geoscience on May 11, 2022, calculated the overall impact of large-scale tree restoration on water flows and water availability.

“Restoring and planting more trees is seen as a viable solution to improve carbon storage and the biodiverse functioning of ecosystems. With innovative data and analysis, our interdisciplinary analysis underscores that hydrological effects are important in knowing how and where these nature-based solutions are more appropriate for achieving more climate-smart and sustainable future landscapes,” says Martin Herold of the German research center GFZ. for Geosciences, who contributed to the study led by Anne Hoek van Dijke of Wageningen University & Research.

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 climatic conditions, and without encroaching on agricultural and urban land. The increase in evaporation resulting from the increase in tree cover was calculated globally at high resolution. The study used data-driven models that describe how much precipitation evaporates and how much goes to flow. Anne Hoek van Dijke, Ph.D. Hydrology and Remote Sensing Candidate from Wageningen University and Research states that “These models include a vegetation parameter for forest and non-forest conditions that has been calibrated over a range of different evaporation and flow measurements. Next, we calculated where and how much of the increased evaporation would return to the land surface as increased precipitation.

Local and global changes in water availability

The results show that large-scale tree restoration can locally increase annual evaporation by nearly 10 liters on average for each square meter of restored forest. Locally, in the tropics in particular, this effect can be much greater, with nearly 250 liters per square meter. Above all, all this water does not return to the surface of the earth. Only about 70% of the additional water in the atmosphere returns to land, while the remaining 30% is dumped on the oceans by rain. On a global scale, this means that tree restoration leads to a net decrease in water availability.

For individual watersheds, the impact of tree restoration is more complex. Following the restoration of the trees, the flow of the main river basins would generally decrease (up to about 10%). But for other river basins (e.g. Yangtze and Amazon) the reduction in flow will be close to zero as the negative impact of increased evaporation is offset by increased rainfall due to forests in these areas. . Interestingly, some of these pools may even gain water.

The study presents the results under current climatic conditions. In a warmer climate, the potential for tree restoration would decrease. Additionally, future climate change may increase annual evaporation and precipitation, which will affect global atmospheric circulation patterns.


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More information:

Anne Hoek van Dijke et al, Changes in regional water availability due to global tree restoration, nature geoscience (2022). DOI: 10.1038/s41561-022-00935-0

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Helmholtz Association of German Research Centers

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Exploring how forest restoration affects water cycles (2022, May 11)
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