On the potential of modeling thermal diffusivity to identify water fluxes in permafrost rock slopes
Abstract
Purpose Permafrost is warming and thawing at a global scale because of climate change and this has consequences for slope stability. Despite numerous studies focusing on the evolution of permafrost, knowledge of the physical properties of frozen ground is based on a few in-situ measurements and laboratory experiments. There is a paucity of 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 thermal regime, an indicator of water-induced permafrost degradation, which are currently inducing increasingly deep-seated slope instabilities. Methods In this study, we identify non-conductive heat flow in mountain permafrost as a potential proxy for water fluxes, using borehole temperature data. We quantify thermal diffusivity based on a linear regression between d2T/dz2 and dT/dt (Nicholson & Benn, 2012) and examine the temporal evolution of thermal diffusivity in mountain permafrost boreholes, using the largest mountain permafrost database worldwide, the Swiss Permafrost Monitoring Network PERMOS. Deviations from the regression line can be used as a qualitative indication of non-conductive heat fluxes. Following Petersen et al. (2022), the empirical estimation of thermal diffusivity based on multiple linear regression, with additional consideration of dk/dz. In addition to the described approach, we calculate conductive heat fluxes using analytical and numerical modeling. Given the one-dimensional heat conservation equation, the non-conductive heat flux is quantified using the difference between the observed and modeled temporal temperature change. Conclusions The 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 mountain permafrost with different substrates such as talus slopes, ice-rich rock glaciers, and bedrock. From these preliminary results, we establish the possibility of further investigating non-conductive processes governed by thawing and/or water advection. Once concluded, this analysis will represent the basis for many other studies investigating the thermal and mechanical behaviour of mountain permafrost slopes.