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Forests can be used to counteract drought by enhancing local rainfall – but this does not work everywhere. This is demonstrated by PhD research conducted by Jolanda Theeuwen from Utrecht ľ¹Ï¸£ÀûÓ°ÊÓ and Wetsus. The research is part of a broader program with the ultimate goal of restoring the water cycle in arid regions, such as Egypt’s Sinai Desert, through regreening.
This study examined how trees contribute to local moisture recycling – specifically in regions with a Mediterranean climate. Trees extract water from the soil and release it into the air as water vapor. This can lead to cooling and rainfall – either nearby or farther away – a process known as moisture recycling. However, if evaporation increases in relatively dry areas due to regreening and the lost moisture does not return locally as rain, it can instead contribute to drying.
Globally, on average, only 1 to 2% of evaporated water returns as rainfall within 50 km; in Mediterranean regions, this percentage is similar. In the Mediterranean Basin, however, the average is somewhat higher, likely due to its complex topography. In Mediterranean regions, forest cover appears to be positively associated with local recycling.
To further investigate the potential for enhancing local rainfall through regreening in the Mediterranean Basin, simulations were conducted. These suggest that if sufficient soil moisture is available, forests could contribute to cooling and wetting; in drier conditions, however, they may instead enhance drought.
Forests in coastal and mountainous areas in particular seem to offer potential for increasing local rainfall. These insights form the foundation for future research into targeted regreening efforts aimed at restoring the water cycle.