With an estimated share of about 65 percent, groundwater is one of the major drinking water sources in Europe, and is often essential for irrigated agriculture. Groundwater availability can also be especially important in times of surface water scarcity when groundwater abstraction may increase to alleviate negative effects to the livelihood of people. Drought events such as in 2011-12, 2015 and 2017-18 showed spatial coherence across several European regions in surface water deficits, and are likely to affect groundwater levels in a similar pattern. However, groundwater droughts may also show distinct spatial coherence based on their hydrogeological settings and recharge patterns as much as on the driving meteorology and available recharge. To assess whether the patterns in groundwater drought propagation are similar to the observed extent of major surface water droughts, a pan-European analysis of historic groundwater level data is required. However, no such study has been undertaken to date.
To address this gap, and to evaluate spatial patterns of groundwater drought response on a continental scale from the late 1900s to present, a pan-European Groundwater Drought Initiative (GDI) was formed. Within GDI, groundwater data from numerous supporting institutions was gathered and jointly analysed, covering a spatial extent from the Iberian Peninsula to Eastern Europe (Ukraine). Thus we produced the first comprehensive overview of historic groundwater droughts across Europe. The spatio-temporal analysis is based on the Standardised Groundwater Index (SGI), which allows for comparison of sites from disparate regions in a consistent manner. Spatial patterns in the drought response across the continental scale were analysed. First insights from the study highlight differences in groundwater system responses to the driving meteorology, such as fast and slowly responding sites and their spatial coherence. Delayed responses to major surface water droughts, like the 1975-1976 or 1996-1997 events, are observed at many of the analysed sites, though impacts differ based on the specific site setting. The further processing of this data will allow for more detailed comparison of the historic groundwater droughts with major surface water drought events, and the analysis of driving factors will increase our understanding of how catchment and local characteristics impact on groundwater responses. Additionally, areas particularly vulnerable to groundwater droughts will be identified, thus allowing for improved groundwater management.