A detailed model to simulate heat and moisture transport in a frozen soil

The problem of moisture movement and storage in a frozen soil is complicated by water undergoing a phase change during freezing and thawing. The moisture migration and freezing processes are coupled and jointly influence the temperature field and the rate of transfer. Fluid properties change substantially as the temperature of the soil water system is lowered to and passes the freezing point of bulk water. With an unsaturated frozen system not only ice, soil and air are present, but also unfrozen water. Thin films of unfrozen water exist on the surface of soil particles. This interfacial water remains unfrozen because its properties vary from those of bulk water. As temperature is lowered the thickness of the unfrozen water film decreases. The quantity of unfrozen water presents in a frozen soil is also related to the soil type, i.e. fine-grained soils with large surface areas have greater quantities of unfrozen water. Water and heat fluxes are simulated with a one-dimensional numerical code developed by Milly (1991) and subsequently modified in order to consider the phase change and the thermal properties of ice. A peculiar feature of this code is the specification of the upper boundary condition in terms of atmospheric interaction: precipitation rate, air temperature, wind speed, absolute humidity of air, incoming solar radiation and long-wave radiation. Transient one-dimensional fluxes of soil water and heat in response to 3 months of atmospheric forcing are simulated for a site in Rosemount (Minnesota). Soil hydraulic and thermal properties have been estimated a priori from a combination of laboratory measurements (from the University of Minnesota, Agricultural experiment station), models, and other published information. The solution yields temperature, liquid water content, and ice content profiles along the column as a function of time. The approach is to use numerical simulations to interpret observed data. The long-term monitoring record of subsurface soil moisture and temperature in the present study provides initial conditions for the model and data to test model results.