Presenter: Dr. Ce Zheng, Chang'an University
Co-author(s): Prof. Xiuhua Liu, Chang'an University
Sub-theme
4. Supporting Aquatic Ecosystem Health and Functions
Topic
4-7. Groundwater and ecosystem
Body
In cold regions, freeze-thaw cycles play a critical role in many engineering and agricultural applications and cause soil water flow and heat transport studies to be much more complicated due to phase changes involved. A fully coupled numerical module for simulating the simultaneous movement of water, vapor, and heat during freezing-thawing periods was developed and incorporated in the Hydrus-1D software in this study. To avoid numerical instabilities caused by a sudden increase in the apparent heat capacity during a phase change, a new approach based on the available energy concept was adopted to adjust soil temperature when the freezing temperature is reached. The proposed freezing module's performance was then validated using experimental data collected at the Mu Us Sandy Land in northwestern China with typical seasonal freezing/thawing processes of 2017-2018 winter and 2018-2019 winter. Results showed that the model could efficiently obtain a convergent solution and that simulated soil moisture and temperature variations captured the observed data well. To further validate the model’s applicability, observation data at the Inner Mongolia Grassland site and the Tibetan Plateau Grassland site were collected as well, and the fitted soil moisture and temperature corresponded well with the measured data. Driven by soil matric potential and temperature gradients, both liquid water and water vapor flowed towards the freezing front. The isothermal liquid flux was the most significant component of overall flow in most soil depths except in the frozen layer, where it decreased by 1-5 orders of magnitude from values before freezing. Instead, the thermal vapor flux was the dominant moisture transfer mechanism in the frozen layer and contributed about 10% to the ice formation, indicating the significance of considering vapor flow. Since the condensation and accumulation of water vapor can greatly contribute to the vegetation under soil drought and freezing stress, it is of great significance to maintain the desert vegetation ecosystem, where the soil water is critical to the vegetation growth in the fragile ecological areas. The vegetation module can be combined with the coupled model water, vapor, and heat transferring. Further studies can be implemented to explore the specific impact of liquid water and vapor on the surface vegetation in seasonal frozen region, which could deepen the theoretical fundamentals and the practical tasks in the seasonal frozen soil areas.