Damped groundwater response to recharge: From spectral analysis to regional modeling

• Unsaturated zone thickness controls regional variations in recharge damping. • Long-term climatic signals are more pronounced in the deepest monitoring wells. • Low-pass filtered recharge improved hydrologic-groundwater model coupling. Transient groundwater models, in which recharge is estimated from a water balance equation or conceptual hydrologic models, without explicitly solving for unsaturated flow, often struggle to reproduce the periodicity of climate-controlled hydraulic heads. The problem stems from the fact that the precipitation signal is smoothed due to low pass filtering and lagged in time as it propagates through the unsaturated zone toward the water table. We present a workflow that leverages the established theory and the empirical evidence of climatic signal damping in the subsurface to model heads in a region with variable unsaturated zone thickness. To this purpose, we analyzed frequency spectra and periodograms of groundwater level (GWL) fluctuations in 288 observation wells in Brandenburg (NE Germany) and compared them against an analytical solution based on Richards’ equation. We found that the unsaturated zone thickness exerts the primary control on GWL fluctuations spectra. The annual GWL periodicity dominates lowland aquifers, characterized by the water table depths of < 5 m. In plateau aquifers, for which the water table depth reaches 15–40 m, GWL exhibits 6–9-year periodicity, possibly related to teleconnections, e.g., the North Atlantic Oscillation. The derived functions of recharge damping and delay with depth were utilized to correct the recharge computed by the mesoscale Hydrologic Model (mHM) before applying it as a boundary condition in a regional groundwater model. This improved the frequency spectrum and amplitudes of the simulated heads, enabling a further use of such model for forecasting long-term groundwater dynamics under changing climate conditions.