Multiscale modeling of thermo-hydrologic processes in frozen soils

In cold regions, frozen soils usually act as a buffer that alters the hydraulic connections between land surface and groundwater system. Mechanistic understanding of the thermo-hydrologic (TH) processes in frozen soils is critical yet challenging in several aspects, such as multi-scale (e.g. column-, slope-, to watershed-scale), multi-phase (i.e. ice, water and soil), and multi-processes (i.e. flow, heat transfer and phase change). In this study, an in-house developed physically-based cryo-hydrogeological model (darcyTHFOAM) for simulating the coupled TH processes in frozen soils was implemented into an open-source, massively-parallel computational fluid dynamics (CFD) software with a user-friendly interface. As validations, two benchmark cases (under thawing) using synthetic porous media samples are simulated to demonstrate the accuracy of the current model. The proposed model is applied to simulate flow and heat transfer in 1D column sampled from the upper reaches of the Heihe River Basin (uHRB), 2D shady/sunny slopes extended from two automatic meteorological stations in the uHRB, and further a 3D small watershed on the northeastern Qinghai-Tibet Plateau (QTP). On-site hydrogeological properties are adopted to configure the numerical model. Soil moisture and temperature profiles are collected from field observations and utilized for cross-comparisons. Simulated results are in general agreement with the observation data, which shows that the model is capable of characterizing the complex TH processes in frozen soils and can be used in future predictions in larger scales.