Altered energy partitioning across terrestrial ecosystems in the European drought year 2018

Drought and heat events, such as the European drought 2018, interact with the exchange of energy between the land surface and the atmosphere, potentially affecting albedo, sensible and latent heat fluxes, as well as CO2 exchange. Each of these fluxes may aggravate or mitigate the drought, heat, their side effects on water scarcity and productivity, and global warming. We utilized measurements of 56 eddy covariance sites across Europe to examine the dependence of fluxes on drought intensity and the ecosystem types forest, grassland, cropland and peatland. Each component of the surface radiation and energy balance observed in 2018 was compared to available data per site during a reference period 2004-2017. Based on anomalies in precipitation and reference evapotranspiration, we classified 46 sites as drought-affected. These received on average 9% more solar radiation and released 33% more sensible heat to the atmosphere compared to the mean of the reference period. In general, drought decreased evapotranspiration and net CO2 uptake by 2 and 16%, respectively. However, the response of these fluxes differed characteristically between ecosystems; in particular the general increase in evaporative index was strongest in peatlands and weakest in croplands. Changes in albedo and absolute ground heat storage were comparatively small.