مهدی رحمتی

Current climate change requires a rethinking and updating of all relations between hydrology and the environment, mainly where changes have the strongest impacts: arid and semi-arid regions. Often, these environments are characterized, all over the world, by an uneven distribution of water resources, posing limitations in development and socio-political issues. Our knowledge of ecohydrology is forcibly incomplete due to the complex nature of ecosystems and interactions among their compartments, which are constantly changing under different stresses: from deforestation to urbanization, from water pollution to water scarcity, from air pollution to climate change, and from soil erosion to soil pollution. Furthermore, the hydrology of arid and semiarid regions is a combination of characters: water supply/losses irregularly distributed in time and space, heterogeneous topography and landscape, and high anthropogenic pressure. Hence, since water distribution follows the prevailing conditions, the development of relationships among ecosystems and hydrology is needed at different layers of knowledge to overcome all barriers for optimizing water use and water use efficiency at suitable time and space scales. In the face of these difficulties, it is evident that there is a necessity to increase, improve and advance our knowledge about the interactions between ecology and hydrological regimes of arid and semiarid areas, and, in particular, of: • Soil water retention and availability; • Water supply and evapotranspiration spatial and temporal variability; • Surface and groundwater transfer processes; • Water flows between soil and vegetate surfaces at catchment scale. Water is evidently central to the framework of ecosystems, both for the theoretical comprehension of processes and for practical applications in solving environmental issues in the twenty-first century. The works here presented develop basic principles and case studies around the world that show the importance of