Abstract
AbstractGroundwater 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.
Highlights
At the global scale, groundwater is by far the largest freshwater storage of the active hydrological cycle
Consistent to that, the model had less power to simulate the anomalies in mountainous regions, where aquifers are mostly fractured rocks and the model cannot capture the small but important valley aquifers which are often smaller than model resolution
A country‐scale groundwater model was tested for its potential to simulate locally different responses of groundwater to meteorological drought across Germany
Summary
Groundwater is by far the largest freshwater storage of the active hydrological cycle. In the theory of drought propagation (e.g., Changnon, 1987; Van Loon, 2015) the meteorological drought signal (i.e., the precipitation deficit) translates into a delayed and attenuated soil moisture deficit (soil moisture drought), discharge deficit (hydrological drought), and decreased groundwater heads (groundwater drought) To compare these different hydrological fluxes and storages and their variability in space it has become common practice to express drought severity as a (standardized) relative deviance instead of an absolute value. Standardized drought indices (e.g., McKee et al, 1993; Palmer, 1965) and percentile thresholds (e.g., Andreadis et al, 2005; Tallaksen et al, 2009) are examples for relative drought metrics that are frequently used for many variables and applications While this large‐scale comparative approach necessarily neglects small‐scale local conditions, the relative metrics allow for a better comparison in space. An anomaly‐based comparison of drought characteristics for different variables and regions helps to identify large‐scale drivers of drought patterns and is a prerequisite for assessments of climate sensitivity, predictions of drought impacts, and proactive drought governance
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