Pressurised water flow in fractured permafrost rocks revealed by joint electrical resistivity monitoring and borehole temperature analysis

Abstract. Rock slope instabilities and failures from permafrost are among the most significant alpine hazards in a changing climate and represent considerable threats to high-alpine infrastructure. While permafrost degradation is commonly attributed to rising air temperature and slow thermal heat propagation in rocks, the profound impact of water flow in bedrock permafrost on warming processes is increasingly recognized. However, quantifying the role of water flow remains challenging, primarily due to the complexities associated with direct observation and the transient nature of water dynamics in rock slope systems. To overcome the lack of quantitative assessment that inhibits thermal and mechanical modelling, we perform a joint analysis of electrical resistivity measurements and borehole temperature, combining datasets of monthly repeated electrical resistivity tomography acquired in 2013 and 2023, rock temperature measured in two deep boreholes (2016–2023), and site-specific temperature-resistivity relation determined in laboratory with samples from the study area. Field measurements were carried out at the permafrost-affected north flank of the Kitzsteinhorn (Hohe Tauern range, Austria), characterized by significant water outflow from open fractures during the melt season. Borehole temperature data demonstrate a seasonal maximum of the permafrost active layer of 4–5 m. They further show abrupt temperature changes (∼ 0.2–0.7 °C) during periods with enhanced water flow, which cannot be explained by conductive heat transfer. Monthly repeated electrical resistivity measurements reveal a massive decrease in resistivity from June to July and the initiation of a low-resistivity (< 4 kΩm) zone in the lower part of the rock slope in June, gradually expanding to higher rock slope sections until September. We hypothesize that the reduction in electrical resistivity of more than one order of magnitude, which coincides with abrupt changes in borehole temperature, provides certain evidence of pressurised water flow in fractures. This study shows for the first time that, in addition to slow thermal heat conduction, permafrost rocks are subject to sudden push-like warming events, favoring accelerated bottom-up permafrost degradation, and contributing to the build-up of hydrostatic pressures potentially critical for rock slope stability.