Development of An Enthalpy-based Frozen Soil Model and Its Validation in A Cold Region in China

Abstract:

A physically-based frozen soil model was developed based on the Water and Energy Budget-based Distributed Hydrological model (WEB-DHM) for the simulation of water and energy transfer in cold regions. In order to simulate the soil freezing/thawing processes stably and efficiently, a two-step algorithm is applied to solve the non-linear energy governing equations: 1) the thermal diffusion equation is used to simulate the heat fluxes between soil layers without considering liquid-ice phase change; 2) a freezing/thawing scheme is used to derive soil temperature, liquid water content and ice content from enthalpy conservation, mass conservation, and freezing point depression equations. In the algorithm, a parameterization set is adopted to update hydraulic and thermal properties by considering the presence of ice and low soil temperatures. The performance of the frozen soil model was validated at point scale in a typical mountainous permafrost region of Binggou Watershed, Heihe Basin, Northwest China. Results show that the model can achieve a convergent solution at a typical time step (hourly) and layer sizes (centimeters) of current land process models. It is able to reproduce the observed soil freezing/thawing processes and hydrological processes. The simulated profiles of soil temperature, liquid water content, ice content and thawing front depth are in good agreement with the observations and the characteristics of permafrost. The freeze-thaw cycle in frozen soil evolution was continuously represented by the contour map of soil temperature and ice content of all soil layers. Therefore, this model can be coupled with hydrological, ecological and climate models to deepen our physical understanding in permafrost regions.