The hydrology of the humid tropics

655 Fresh water is under pressure in the humid tropics from population growth, land use and climate change, all of which are influenced by humans. These pressures are likely to have profound consequences. Here we present a research vision for the humid tropics as an outcome of a community workshop held in Hawaii in March 2011. This report includes various perspectives from the international scientific community and examines the key role that hydrology plays in the functioning of the humid tropics in this age of human impact on all facets of the natural environment. Processes that operate within the hydrological cycle are expected to accelerate as temperatures rise and the capacity of the air to carry moisture increases. Understanding of key interactions is limited geographically and relies heavily on model-based scenarios rather than observations. Better understanding of interactions within the hydrological cycle is needed for the hydrological community to actively address adaptation and mitigation strategies for anthropogenic and climate changes. Of particular concern are interactions among atmospheric moisture fluxes and vegetation, soil water and energy balances, near-surface and subsurface processes (including groundwater resources), and stream flow and the transport of solutes and sediments (Fig. 1). Here we outline the state of knowledge, highlight critical research needs and suggest research strategies that would aid understanding of hydrology in the humid tropics in the context of rapidly changing environmental conditions. We identify three main research needs related to: (1) moisture cycling — we call for more explicit attention to integrated measuring, modelling and understanding of water fluxes across the land–atmosphere continuum; (2) catchment processes — we highlight the need for a clear understanding of how human landscape alterations affect evapotranspiration and runoff ratio in diverse environments, as well as the need for more information on biogeochemical cycling; and (3) long-term data acquisition and organization — we call for integrative field campaigns that The hydrology of the humid tropics