Regreening for natural water production: Local atmospheric moisture recycling in Mediterranean regions

Climate change and land-use change, such as deforestation, are intensifying water scarcity worldwide. Mediterranean-type climate regions are particularly prone to drying. While regreening is widely promoted for carbon sequestration, its hydrological effects remain unknown in the Mediterranean-type climate regions. This thesis explores whether regreening can enhance local rainfall through terrestrial moisture recycling—the process by which evaporated water returns as precipitation over land. The study focuses on five major Mediterranean-type climate regions around the world. Using the output of a moisture-tracking model, Chapters 2 and 3 quantify local evaporation and precipitation recycling, revealing that 1%–2% of water typically recycles within ~50 km. Recycling is highest in coastal and mountainous areas. While local evaporation recycling increases with precipitation, local precipitation recycling is higher in drier regions. Furthermore, in Mediterranean-type climate regions, areas with vegetation tend to have higher local moisture recycling, suggesting that vegetation may enhance the local water cycle. Chapter 4 examines how forests influence atmospheric boundary layer dynamics and assesses the potential for convective precipitation in the Mediterranean Basin using a simple process-based model. The results suggest that forests have a cooling and moistening effect in areas with sufficient soil moisture, but may exacerbate drying where soil moisture is low. Forests may contribute to local rainfall in coastal and mountainous areas in the Mediterranean Basin. In the final chapters, the thesis highlights that forestation can also enhance rainfall downwind, emphasizing the relevance of understanding moisture flows and using available tools to guide forestation projects for targeted rainfall enhancement. Furthermore, the importance of interdisciplinary collaborations and stakeholder engagement to translate scientific knowledge into practical regreening initiatives is addressed. Finally, the outcomes of all chapters are combined to propose an engineering-oriented approach for planning forestation projects aimed at increasing rainfall – natural water production. This approach includes atmospheric and hydrological modeling, along with socio-economic assessments, offering a conceptual framework for future development. Overall, this thesis provides a scientific basis for using regreening to enhance rainfall in Mediterranean-type climate regions, while emphasizing the need for integrated strategies to ensure sustainable restoration.