Characterising the response of groundwater systems to major, continental-scale droughts: a multidecadal European case-study

Groundwater systems are susceptible to droughts. However, the relationship between driving meteorological droughts and resulting groundwater droughts is particularly complex due to spatially and temporally varying climate drivers and spatially heterogeneous catchment and aquifer characteristics, as well as the potential effects of longer-term groundwater overexploitation and of groundwater abstraction and management interventions during episodes of meteorological drought. Consequently, many previous studies of the propagation of meteorological drought to groundwater systems have typically been geographically limited in scope, focussing on characterising aquifer units within a catchment or basin, and/or temporally limited to specific episodes of drought. Based on a new European-wide dataset consisting of groundwater level data from over 6,000 sites, here we describe the results of a multidecadal analysis of the expression of major episodes of meteorological drought at the continental scale in groundwater systems, independent of local hydrogeological setting.

In this study, raw groundwater level time series are modelled using an impulse response function of precipitation to obtain monthly groundwater levels that are then standardised. Sites with long-term trends in groundwater level are identified and usually inferred to be associated with overexploitation or other anthropogenic influences. Cluster analysis of the modelled standardised hydrographs is used to identify spatially coherent ‘type’ groundwater hydrographs. These type hydrographs can be characterised by differences in the autocorrelation of the underlying groundwater hydrographs, but may also reflective continental-scale variations in the driving meteorology. Finally, episodes of groundwater drought are extracted from the type groundwater hydrographs and compared with the driving meteorological droughts. The data provides evidence for the coherent response of groundwater systems to droughts across large areas of Europe depending on driving meteorology and the ‘memory’ of the groundwater system, and drought events such as in 2011-12, 2015 and 2017-18 showed spatial coherence across different European regions.