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

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 © 2025 The Author(s)