Modeling the thermal dynamics of the active layer at two contrasting permafrost sites

The thermal and hydraulic dynamics of unsaturated active layers are described in a one-dimensional numerical forward model. Hydraulic and thermal transport processes are coupled in a set of partial differential equations based on Richards' equation, conductive and convective heat flow and a phenomenological description of soil freezing. The model is applied to the detailed data sets of two rather different field sites, one in the Arctic on Svalbard and one on the Tibetan Plateau. Soil temperatures and water contents as well as important quantities like the thaw depth and the duration of the isothermal plateau can be reproduced. To examine the influence of different heat transport processes, three scenarios of different complexity are studied. We show that heat conduction is the dominant process at both sites. While representing this process is sufficient for rough thaw depth estimates, a more detailed representation is necessary for an accurate representation of the active layer thermal dynamics. With our detailed model, characteristic deviations between measurements and simulations can still be observed. As possible explanations we discuss downward vapor migration in the upper soil layer and mechanical deformations.