Thermodynamic transport mechanism of water freezing-thawing in the vadose zone in the alpine meadow of the Tibet Plateau

High altitude, cold and dry climate, strong solar radiation, and high evapotranspiration intensity have created an extremely fragile ecological and geological environment on the Tibet Plateau. Since the heat in the vadose zone is primarily generated by the external solar radiation energy, and evapotranspiration is contingent on the consumption of vadose heat, the intensity of evapotranspiration is associated with the intensity of solar radiation and the heat budget in the vadose zone. However, the spatial and temporal variation of heat budget and thermodynamic transfer process of the vadose zone in the frigid region are not clear, which hinders the revelation of the dynamic mechanism of evapotranspiration in the vadose zone in the frigid region. With the moisture content of the vadose zone in the alpine regions being the research object, the paper conducts in-situ geothermal observation tests, takes meteorological characteristics into consideration, and adopts the method of geothermal gradient and numerical computation to analyse the temporal and spatial variation rule of heat budget and thermodynamic transmission process of the vadose zone in the high and cold regions. The results show there is a positive correlation between air temperature, ground temperature, and water content of the vadose zone in both thawing and freezing periods. According to the change law of geothermal gradient, the thermodynamic transfer process of the vadose zone has four stages: slow exothermic heating, fast endothermic melting, slow endothermic cooling, and fast exothermic freezing. From the surface down, the moisture freezing rate of the vadose zone is slightly higher than the melting rate. This is of great significance for understanding the evapotranspiration dynamic process of the vadose zone and protecting and rebuilding the ecological and geological environment in the high and cold regions.