Coupled heat and water transport in frozen soils

The effect of freezing on soil temperature and water redistribution was examined in four Mesocosms maintained at different initial water content profiles. An innovative experimental setup involving use of a frozen base layer acting as a proxy to permafrost beneath an active layer made up of packed and undisturbed peat cores was used. The experimental setup was successfully validated for its ability to maintain one dimensional change in temperature and soil water content in frozen soil. There was a substantial amount of water redistribution towards the freezing front, enough to create an impermeable frozen, saturated zone within 15 cm of the soil surface. The water movement was likely due to the potential head gradients between colder and warmer regions created by temperature effects on matric potential of frozen soil. In addition, there is enough evidence that water migration in form of vapour contributed to moisture movement towards the freezing front. Initial moisture profiles appeared to have a significant effect on the freezing induced soil water redistribution likely through differences in moisture dependant hydraulic conductivity. Initial soil moisture profiles also affected the rate of freezing front movement. Frost propagation was controlled by latent heat for long periods, while soil thermal conductivity and heat capacity appeared to control the rate of frost migration once the majority of water was frozen. Using the observations of this study, a conceptual model was proposed to explain freezing of an active layer on a permafrost plateau assuming a variable moisture landscape at onset of winter. Further, a one-dimensional model based on coupled cellular automata approach was developed. The model is based on first order conservation laws to simulate heat and water flow in variably-saturated soil. Inside the model, Buckingham-Darcy’s -and Fourier’s heat laws are used to define the local interactions for water and heat movement respectively. Phase change is brought about by the residual energy after sensible heat removal has dropped the temperature of the system below freezing point. Knowing the amount of water that can freeze, the change in soil temperature is then modeled by integrating along the soil freezing curve. This approach obviates the use of apparent heat capacity term. The 1D model is successfully tested by comparing with analytical and experimental solutions

Effects of Freezing on Soil Temperature, Freezing Front Propagation and Moisture Redistribution in Peat: Laboratory Investigations

There are not many studies that report water movement in freezing peat. Soil column studies under controlled laboratory settings can help isolate and understand the effects of different factors controlling freezing of the active layer in organic covered permafrost terrain. In this study, four peat Mesocosms were subjected to temperature gradients by bringing the Mesocosm tops in contact with subzero air temperature while maintaining a continuously frozen layer at the bottom (proxy permafrost). Soil water movement towards the freezing front (from warmer to colder regions) was inferred from soil freezing curves, liquid water content time series and from the total water content of frozen core samples collected at the end of freezing cycle. A substantial amount of water, enough to raise the upper surface of frozen saturated soil within 15 cm of the soil surface at the end of freezing period appeared to have moved upwards during freezing. Diffusion under moisture gradients and effects of temperature on soil matric potential, at least in the initial period, appear to drive such movement as seen from analysis of freezing curves. Freezing front (separation front between soil zones containing and free of ice) propagation is controlled by latent heat for a long time during freezing. A simple conceptual model describing freezing of an organic active layer initially resembling a variable moisture landscape is proposed based upon the results of this study. The results of this study will help in understanding, and ultimately forecasting, the hydrologic response of wetland-dominated terrain underlain by discontinuous permafrost