Book excerpt: Brazil possesses the largest freshwater resources worldwide. However, it is already facing severe water problems due to an adverse distribution of water availability and demand, which becomes especially apparent in the densely populated Southeast. This area is home to 75 % of Brazil's population and the location of two of the world's largest megacities, São Paulo and Rio de Janeiro. Their regional water demands serve various purposes, including drinking water, agricultural and industrial water, and hydropower supply for the metropole regions. At the same time, the region belongs to the Atlantic rainforest, the Mata Atlântica. The remaining patches of the pristine Mata Atlântica biome host worldwide outstanding biodiversity, which depends on the unique hydro-climatic conditions of the region. Nevertheless, little is known about the ecohydrological processes and dynamics in the region and their alteration under climate change. This, however, is an essential requirement for making informed decisions in river basin management that aims at safeguarding water-related ecosystem functions and services to simulate processes at different spatial and temporal scales. This dissertation contributes to the understanding of ecohydrological process dynamics in the Atlantic rainforest of Brazil. It creates a crucial prerequisite for science-based environmental planning and knowledge-based river basin management. The results indicate major challenges for water-resources management in the Southeastern Mata Atlântica for the upcoming decades. The methods and tools developed and used in this work can be easily applied and transferred to sub-tropical and tropical regions worldwide. They allow planning and mitigation for future climate, land-use, and management changes to sustain water-related ecosystem functions and services and preserve the beauty of unique biomes, such as the Mata Atlântica.