A simple heat-conduction method for simulating the frost-table depth in hydrological models

Hillslope runoff in permafrost regions covered by organic soil is strongly influenced by subsurface flow in the active layer, as well as surface flow where the active layer is very shallow. Flow rates in the organic-rich active layer are strongly dependent on the depth to the thawing front (i.e. frost table) and the corresponding soil hydraulic conductivity at that depth. Therefore, hydrological models for permafrost terrains need to simulate the thawing of the active layer accurately. In order to simulate the downward movement of the frost table, a simple heat-conduction model was proposed and compared to field data from a wet, organic-covered watershed in a discontinuous permafrost region of Canada. Ground heat flux was measured simultaneously using the calorimetric, gradient, and flux-plate methods to increase the confidence in data sets. The majority (>86%) of ground heat flux was used to melt the ice in frozen soil, and the soil temperature had a linear profile from the ground surface to the frost table when averaged over several days. Assuming a linear temperature profile, the proposed method calculates the daily rate of thawing from ground surface temperature and bulk thermal conductivity, where the latter is essentially determined by soil water content. Simulated depths to the frost table during three thaw seasons (2003-2005) matched closely with the observed data for two contrasting ground-cover types with distinctly different thaw rates. The method can be easily implemented in hydrological models, and will provide a useful tool for simulating hillslope drainage in organic-covered permafrost terrains, and for evaluating the effects of topography and land cover on the temporal and seasonal variability of the frost table. Copyright (C) 2007 John Wiley & Sons, Ltd.