How does forest restoration affect the water cycle?
Restoring tree ecosystems is an effective way to store atmospheric carbon and mitigate climate change. Many initiatives exist to increase global forest cover. However, the effects of these initiatives on water resources are still poorly understood. In this context, a new study conducted by Wageningen University and published in Nature Geosciences, in which Jean-François Bastin from Gembloux Agro Bio-Tech participated, attempts to provide some insight. In particular, this study calculated the impact of large-scale tree cover restoration on water fluxes and water availability on a global scale.
The results of the study show that a global increase in evaporation is expected with increasing tree cover. Anne Hoek van Dijke, PhD student in hydrology and remote sensing at Wageningen University and first author of the study points out, "By combining models characterizing water evaporation associated with vegetation and recent maps of forest cover restoration potential, we calculated where and to what extent the increased evaporation, associated with restoration, would return to the land surface as increased precipitation."
The results show that large-scale tree canopy restoration can locally increase annual evaporation by an average of nearly 10 liters for each square meter of restored forest. Locally, in the tropics in particular, this effect can be much greater, at nearly 250 liters per square meter. The problem is that not all of this water returns to the earth's surface. Only about 70% of the extra water in the atmosphere returns to the earth, while the remaining 30% is washed into the oceans by rain. On a global scale, this means that tree restoration results in a net decrease in water availability.
For individual watersheds, the impact of tree restoration is more complex. Following tree cover restoration, flows in major river basins would generally decrease (up to about 10%). But for other river basins (e.g., the Yangtze River and the Amazon), the reduction in flow will be close to zero because the negative impact of evaporation will be offset by increased rainfall. Interestingly, some of these basins may even gain water.
Future forest restoration
Hoek van Dijke: "In the study, we show that tree restoration will significantly alter water flows and availability. Future tree restoration strategies should take these hydrological effects into account. Smart tree restoration strategies are needed to secure water supply locally and remotely or to improve water supply in water-scarce regions."
