Water and heat coupling processes and its simulation in frozen soils: Current status and future research directions

To date, most studies on coupled-water-and-heat processes in frozen soils haves focused on the mechanism of changes in frozen soil and the contribution of climate change, hydrological processes, and ecosystems in cold regions. Several studies have demonstrated considerable improvements in the accuracy of simulating water and heat transfer processes in cold regions. However, substantial differences remain among the different models and parameterizations because of the lack of observations and in-depth understanding of the water and heat transfer processes. Hence, it is necessary to summarize recent advances in the simulation of water-and-heat-coupling processes and challenges for further research. Therefore, we present a theory-focused summary of progress in this field considering the aspects of water flow and coupled-water-and-heat transfer. The simulation progress is discussed in terms of physical process models; one type of model only considers the heat conduction transfer processes without water flow, and the other considers coupled-water-and-heat transfer processes. Aspects of model deficiencies related to non-conductive heat transfer and soil water transfer processes in the frozen soil are also summarized. Moreover, the major parameterizations are reviewed, including phase changes, freeze–thaw fronts, thermal conductivity, hydraulic conductivity, snow processes, surface parameterization schemes, ground ice, and lower boundary conditions. While models and parameterizations can suitably capture local-scale water and heat transfer processes in frozen soil, their applications are spatiotemporally constrained, requiring further improvement. We provide a theoretical basis for further studying water and heat transfer processes in frozen soil and suggest that future research should enhance the accuracy of frozen soil parameterization and improve the understanding of the coupling of water and heat transfer processes based on improved observation techniques and high-resolution data.