Heat and water transfer processes on alpine meadow frozen grounds of Heihe mountainous in Northwest China

The hydrological process in frozen soil regions is very intensive to global warming.The coupled heat and mass transfer model for soil-plant-atmosphere system(Coup Model) is applied in the headwater region of the Heihe River,with an alpine meadow frozen ground landscape.The observed meteorological forcing and soil and vegetation properties are used in the Coup Model application for the period from September 10,2005 to September 10,2007.A 24-h time step is used in the model simulation.The results show that the simulated soil temperature and water content as well as frozen depth compare well with the observations.The coefficient of determination(R²) of 7 soil layers is 0.95 for the mean soil temperature,and 0.83 for the mean soil water content.An analysis is performed on the simulated heat and water transfer process from the deep soil layer to the upper one during the freezing seasons.At the beginning of the season,the water in the deep soil layer moves upward to the freezing front and releases heat during the freezing process.When the soil layer is frozen completely,there will be no vertical water exchanges between the soil layers,and the heat exchange process is controlled by the vertical soil temperature gradient.During the thawing period,the downward heat process becomes more active with the help of the increase of incoming shortwave radiation at the ground surface.There will be no confinement of the melt water in the thawing front.Instead,the melt water will be quickly dissolved in the soil.Eventually,the frozen soil heat and water transfer process vanishes and the unfrozen process returns.