Heat and water movement in variably saturated freezing soils is a strongly
coupled phenomenon. The coupling is a result of the effects of sub-zero
temperature on soil water potential, heat carried by water moving under
pressure gradients, and dependency of soil thermal and hydraulic properties
on soil water content. This study presents a
one-dimensional cellular automata (direct solving) model to simulate coupled
heat and water transport with phase change in variably saturated soils. The
model is based on first-order mass and energy conservation principles. The
water and energy fluxes are calculated using first-order empirical forms of
Buckingham–Darcy's law and Fourier's heat law respectively. The
liquid–ice phase change is handled by integrating along an experimentally determined soil
freezing curve (unfrozen water content and temperature relationship)
obviating the use of the apparent heat capacity term. This approach highlights a
further subtle form of coupling in which heat carried by water perturbs
the water content–temperature equilibrium and exchange energy flux is used
to maintain the equilibrium rather than affect the temperature change. The model
is successfully tested against analytical and experimental solutions. Setting
up a highly non-linear coupled soil physics problem with a physically based
approach provides intuitive insights into an otherwise complex phenomenon