"Ecohydrology is the study of the two-way interaction between the hydrological cycle and ecosystems. More broadly, it is the science of the linkages between life and water on Earth. On the one hand, the space and time variability of the hydrological cycle controls the water availability for ecosystems; on the other hand, ecosystems, especially through transpiration by vegetation, control the main pathway by which water returns to the atmosphere from land. The terrestrial water cycle also drives some of the dynamics of soil organic matter, 7 8 CHAPTER INTRO DUCTION Abiotic processes (water and energy cycles) Biotic processes (carbon and nutrient cycles, vegetation and soil microbes) 2-way interaction Society Agroecosystems (60% of rainfall goes in transpiration in agricultural land Rule of thumb: 1 kg of bread requires 1 m3 of water) 2-way interaction Figure 1.1: Ecohydrology is characterized by the feedbacks between hydrological and ecological processes, involving numerous biotic and abiotic components, as well as by the growing interactions with the social components, especially ftwowayg through agroecosystems. microbial biomass, and the related nutrient cycling. These in turn not only a ect the vegetation dynamics but also impact the hydraulic and thermodynamic properties of soil, thereby directly acting on the partitioning of water and energy uxes at the land-atmosphere interface. The interaction between water balance and plants is responsible for some of the fundamental di erences among biomes (e.g., forests, grasslands, savannas) and for the developments of their space-time patterns. Thus one of the rst objectives of ecohydrology is to understand the intertwined dynamics of climate, soil, and vegetation. This interaction is especially interesting in water-limited ecosystems, where water is a limiting factor not only because of its scarcity but also because of its intermittent and unpredictable appearance. Many important practical issues depend on the quantitative understanding of the ecohydrological processes, including environmental preservation and proper management of soil and water resources. A solid scientic foundation of sustainable development and eco-agriculture must take into account the two-way interaction between biosphere and hydrosphere, as well as include a quantitative description of the human interference with these processes (see Fig. 1.1)"--