A pan-European analysis of drought events and impacts

A drought is a slowly developing natural phenomenon that can occur in all climatic zones, and propagates through the entire hydrological cycle with long-term economic and environmental impacts. Climate change has made drought one of the greatest natural hazards in Europe, affecting large areas and populations. Different definitions of drought exist, i.e. meteorological, hydrological, and agricultural droughts, which vary according to the time horizon considered and differ in the variable used to define them. Just as there is no single definition of drought, there is no single index that accounts for all the types of droughts. As a consequence, capturing the evolution of drought dynamics and associated impacts across different temporal and spatial scales remains a critical challenge.

In this work, we analyze existing standardized drought indexes in terms of their ability in detecting drought events at the pan-European scale using data from HydroGFD2.0 reanalysis and E-HYPE hydrological model simulations over the time period 1993-2018. We firstly compare the frequency and mean duration of drought events detected by different indexes to identify the river basins mostly affected by droughts and to assess similarities and differences in the information provided by different indexes. We then compare them with the drought impacts recorded in the Geocoded Disasters (GDIS) dataset to examine agreements and discrepancies between index-detected droughts and impact data.

Preliminary results show that different indexes generally agree in pointing out that Southern England, Northern France, and Northern Italy are the regions that experienced the highest number of drought events, whereas other regions, such as Southern Spain, experienced intense droughts events, which are not consistently indicated by all indexes. In terms of drought duration, the areas affected by the longest droughts are instead the Baltic Sea region and Normandy. Clustering the 35408 European basins according to dominant hydrologic processes reveals that the variables mainly controlling the drought process vary across clusters and depends on the characteristics of each cluster. While substantial agreement exists between observed impact and detected drought, several areas without GDIS records show critical index values. Such asymmetry can be explained by incomplete reporting in GDIS but also due to some non-physical hydrometeorological factors influencing drought dynamics, such as controlled water infrastructure, that are not adequately captured by standardized indexes. These findings suggest the need of adjusting the formulation of drought indexes to the specific characteristics of different river basins in order to improve drought detection and management.