Field-validated imaging of decadal and seasonal changes in permafrost bedrock using quantitative electrical resistivity tomography (Zugspitze, Germany/Austria)

Abstract. Ongoing permafrost degradation in alpine regions requires monitoring methods that accurately decipher spatial and temporal dynamics. Electrical resistivity tomography (ERT) is widely applied in bedrock permafrost, yet its outputs are often interpreted only qualitatively. Quantitative evaluation of ERT results, however, is crucial for improving process understanding and enhancing predictions of permafrost-related slope instability. In this study, we present a 17-year monitoring of permafrost rock slopes on Mount Zugspitze (Germany/Austria) with monthly ERT campaigns. ERT data are combined with rock temperatures at four depths to establish field-based temperature–resistivity calibrations and to validate existing laboratory-derived relations. Both approaches agree well in the freezing range; however, field calibrations tend to yield higher resistivities at subzero temperatures and reveal substantial spatial heterogeneity. Incorporating reciprocal measurements refines the existing error model, increases image resolution, and improves the identification of subsurface features. Over ten years, the measured rock temperature increased by 1 °C, accompanied by a 25 % decrease in resistivity. The permanently frozen surface decreased by 40 %, with degradation rated up to −4.2 kΩmy⁻¹. Extrapolating these trends would result in the loss of 65 % of permafrost within a decade. Thermal forcing controls the degradation; however, the observed conditions and projected increases in heatwaves suggest that newly unfrozen and connected fracture networks will enhance advective heat transfer. This is expected to accelerate permafrost thawing, thereby increasing the risk of slope instability. With these results, we demonstrate that ERT monitoring can yield high-quality quantitative insights into long-term permafrost evolution and effectively track bedrock permafrost degradation across both decadal and seasonal timescales.