Water vapour fluxes are a key variable in the biosphere-atmosphere exchange of water in terrestrial ecosystems and have strong interactions with the climate system. However, research on the effects of climate change has been largely focused on carbon cycling and therefore, many open questions remain, in particular with respect to the long-term variability of water vapour fluxes and to spatiotemporal patterns in ecosystem responses to extreme events. The project ECOWAX intends to close this knowledge gap and contribute to a better understanding of ecosystem water cycling, thus assisting in the development of mitigation options to secure water supply for human society in a changing climate.The overarching goals of ECOWAX are (1) to assess the intra- and interannual variability of ecosystem water vapour fluxes across biomes and climate, and (2) to evaluate spatiotemporal effects of extreme events on these fluxes. To achieve these goals, ECOWAX will combine water vapour fluxes measured in situ from a new synthesis dataset of the global measurement network FLUXNET with a novel biophysical process model (BESS – Breathing Earth System Simulator) to upscale ecosystem fluxes globally for spatiotemporal analysis.The research objectives of ECOWAX are (1) to investigate the range and magnitude of intra- and interannual variability of water vapour fluxes across biomes and climates using the FLUXNET synthesis dataset, (2) to improve the novel biophysical model BESS to overcome current limitations, (3) to upscale water vapour fluxes globally with the improved version of BESS and validate the model results with site data from FLUXNET, and (4) to assess the spatiotemporal variability of water vapour fluxes and the effects of extreme events on these fluxes in Europe during the last decade.The outgoing host for ECOWAX will be the lab of Prof. Dennis Baldocchi at the University of California, Berkeley (USA) and the return host will be Prof. Nina Buchmann at ETH Zurich (Switzerland).