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Evaluating the variability of ecosystem water vapour fluxes
and spatiotemporal effects of extreme events

Final Report Summary - ECOWAX (Evaluating the variability of ecosystem water vapour fluxes <br/>and spatiotemporal effects of extreme events)

This project was ’Evaluating the variability of ecosystem water vapour fluxes and spatiotemporal effects of extreme events’ (ECOWAX) using a synthesis dataset of the global measurement network FLUXNET in combination with a novel biophysical process model (BESS – Breathing Earth System Simulator) to upscale ecosystem fluxes globally. The research objectives of ECOWAX were (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.

During the course of the project, the following main work was performed: (1) Analyses of global water vapour fluxes across biomes and climates. The results of these analyses were presented to the scientific community during the GEWEX conference in The Hague (Netherlands) in July 2014 and Bern (Switzerland) in April 2015. A manuscript with these results is in preparation. (2) Improvements were implemented into the BESS model, including topographic effects for radiative transfer and a routine for chlorophyll fluorescence. Subsequently, new model runs of the improved version of BESS were initiated. (3) A regional case study was performed that analysed the impact of spring drought on ecosystem water and carbon fluxes in Switzerland. The results of these analyses were published in a peer-reviewed journal, along with dissemination activities. (4) In 2012, the US experienced one of the most severe droughts during the last century. The ECOWAX project compiled and analysed a large-scale synthesis dataset to quantify the impact of this exceptional, continental-scale drought event on ecosystem carbon and water fluxes. The main results from these analyses are currently in review and further peer-reviewed articles for high-profile international journals are in preparation.

The results of ECOWAX show, among others, that (1) tropical ecosystems have globally the highest rates of water vapour fluxes and the largest proportion of available energy converted to latent heat. (2) Grasslands and forests respond differently to spring drought due to different adaptive strategies: while forests increase their water use efficiency during spring drought, grasslands do not. (3) The severe summer drought across the US in 2012 caused widespread reductions in summer water vapour fluxes and carbon uptake. However, increased carbon uptake during the warmer spring partially compensated for the reductions in summer carbon uptake and reduced the impact on the annual carbon budget. The warmer spring also depleted water resources earlier and resulted in a positive heating feedback during summer.

Based on these results, the ECOWAX project indicates that (1) the seasonal timing of drought largely determines the impact on ecosystem water and carbon fluxes, i.e. drought impacts are different during spring and summer, and differ among grasslands and forests. (2) Climate change induced warmer temperatures during spring will increases carbon uptake but at the cost of depleting water resources earlier, which results in a heating feedback during summer that could enhance the intensity of heatwaves. (3) Tropical ecosystems play the most important role for the global hydrological cycle, by having the highest absolute rates of water vapour fluxes and the largest proportion of available energy converted to latent heat.

The outcome of the ECOWAX project will contribute to a better understanding of ecosystem water cycling, particularly during extreme events such as large-scale droughts. This knowledge will assist in the development of mitigation options to secure water supply for human society in a changing climate.

The project website is available at http://www.gl.ethz.ch/research/bage/ecovax.html and will be updated with results once these are published in peer-reviewed international journals.