Large‐Scale Assessment of Delayed Groundwater Responses to Drought

Groundwater is a vital resource for freshwater supply during extended droughts and also a key storage governing drought propagation through the hydrological cycle. Current drought monitoring lacks large‐scale estimates of groundwater droughts, but progress of country‐to‐global‐scale models in the last years suggests that they could now be valuable tools to study and monitor water availability during extended droughts. As a prerequisite the models would need to be able to depict the diverse groundwater response to precipitation well enough to distinguish spatial differences. Here we developed a high‐resolution transient groundwater model for Germany and tested its ability for representing the groundwater system dynamics with a focus on droughts. Validation of model results against streamflow‐separated baseflow and groundwater head observation confirmed the model's ability to generally represent the groundwater head dynamics over 40 years with lower model performance in mountainous regions where model resolution was too low to capture local valley aquifers. The precipitation accumulation time that has the highest correlation with groundwater anomalies increases with hydraulic conductivity and specific yield from few months in the Central German Uplands to several years in the porous aquifers of northern Germany. Corresponding to these differences, distinct meteorological drought types led to different simulated groundwater reactions across Germany. Given the importance of groundwater as a resource, large‐scale groundwater models are important tools for future studies on drought propagation as well as groundwater drought under climate change. Key Points In Germany, groundwater reacts on precipitation in few months in the uplands but with up to several years of delay in the lowland aquifers A country‐scale transient groundwater model including lateral flows confirms the diversity in observed responses of groundwater to drought Patterns in head anomalies during droughts are related to aquifer hydraulic conductivity and surface elevation