Modeling thermal diffusivity in permafrost rock slopes to identify non-conductive heat fluxes

Permafrost is warming and thawing globally because of climate change, which has consequences for slope stability. Despite numerous studies focusing on permafrost evolution, knowledge of the physical properties of frozen ground is based on a few in-situ measurements and laboratory experiments. There are few observations on water fluxes in permafrost, which are rapidly changing due to active layer thickening, ground ice melt, talik formation, and modified permeability. Particular attention should be given to changes in the thermal regime, an indicator of permafrost degradation and deep water infiltration, which are currently inducing deep-seated slope instabilities. In this study, we use data from the 29 temperature boreholes of the Swiss Permafrost Monitoring Network PERMOS to quantify the thermal diffusivity in different permafrost rock slopes characterized by the three landforms: rock glacier, talus slope, or bedrock. We apply statistical and numerical modeling approaches and calculate the thermal diffusivity for each instrumented depth in a two-month window that iterates by one day. The thermal diffusivity at each instrumented depth is additionally inverted for each calendar year using analytical modeling to validate the results. This systematic analysis of the PERMOS borehole temperature data, with three independent methods, allows us to derive a well-constrained range for the thermal properties of different substrates in mountain permafrost. Isolating spatial and temporal anomalies in thermal diffusivity, we can further investigate non-conductive processes governed by thawing and/or water advection. Given the one-dimensional heat conservation equation, the non-conductive heat flux can be quantified using the difference between the observed and modeled temperature change. Once concluded, this analysis will represent the basis for many other studies investigating the thermal and mechanical behavior of mountain permafrost rock slopes.