The controlling factors of drought propagation from the hydrological to the terrestrial energy and carbon cycles in Europe

Like those recently experienced in 2018 and 2022, European droughts significantly alter ecosystem processes, such as photosynthesis and evapotranspiration. Quantifying these large-scale alterations and understanding their drivers is essential to studying the drought impacts on ecosystem performance, water resource management, and carbon emission budgeting. However, to this date, because of differing definitions of drought events and complex interactions among other hydrological compartments, such as groundwater, across multiple time scales, research has left open questions on the controlling factors of drought propagation – across time, space, hydrological compartments, and the terrestrial energy and carbon cycles. In this work, based on pan-European simulations of the land surface model CLM5, we identified drought events with a generalized clustering algorithm. Notably, we considered water deficits in multiple compartments of the hydrological cycle (groundwater, soil moisture, evapotranspiration, and vapor pressure deficit). Further, we distinguished these droughts' direct and lagged effects by aggregating water deficits across various time scales. We highlight statistics and trends of the identified drought events, their drivers, and their impact on photosynthesis and evapotranspiration, with an increasingly severe groundwater deficit and more considerable propagation to inhibited ecosystem processes. However, in the shorter time scales, atmospheric droughts are the primary driver of photosynthesis and evapotranspiration anomalies. This study presents a novel multi-scale and multivariate approach to droughts, paving the way for holistic and more precise considerations of their impacts on ecosystems.