Similarities and differences in groundwater responses to droughts across Germany

Regional groundwater resources are crucial for water supply, maintaining environmental stream flows and the integrity of aquatic ecosystems. At the same time, however, these resources are increasingly threatened by the effects of climate change. More extreme weather conditions like exceptional droughts are expected to increase water stress by increasing water demand and decreasing water availability even in humid regions such as Germany. To identify consistent long-term trends and areas vulnerable to droughts, it is important to characterize and understand similarities and differences in groundwater dynamics across sites. Herein, we analyze groundwater head responses to climatic variability at more than 6500 groundwater wells over the last 30 years in Germany to identify response clusters. Principal component analysis (PCA) revealed that about two-thirds of the observed groundwater level variability across all wells can be explained by five typological time series. These time series represent different response patterns to climatic forcing capturing distinct dampening effects and time lags, with different weights (loadings) being assigned to each groundwater well. The subsequently identified clusters of the wells reveal clear regional structures suggesting underlying spatial controls. In the next step, groundwater drought characteristics (e.g., severity, duration) will be considered and linked to climate and landscape properties. This study may help to understand regional controls and to identify zones vulnerable to groundwater droughts where water supplies are at risk in the long term and mitigation measures are needed.