多年冻土活动层浅层包气带水-汽-热耦合运移规律

Coupled water-vapor-heat transport in shallow unsaturated zone of active layer in permafrost regions

活动层水热状态直接影响多年冻土和寒区工程的稳定性。已有研究大多基于附面层理论研究冻土温度场变化,较少研究液态水和水汽运移过程及其对冻土温度场的影响。结合多年冻土活动层包气带水-热耦合迁移的物理过程和内在机制,以温度和含水率为基本变量,建立了考虑液态水和水汽相变、水分对流传热和水汽运移的土壤-地表-大气能量平衡及土壤内部水热变化的耦合模型,分析了真实野外气象条件下活动层液态水和水汽运移规律。计算结果表明:白天温度梯度水分向土壤内部运移,夜间温度梯度水分向土壤表层运移;暖季温度梯度水分以向土壤内部运移为主,冷季温度梯度水分以向地表运移为主;就全年而言,活动层各个深度处水汽运移作用大于15%,水势梯度水汽运移极小可以忽略不计,特别是浅层土壤在无降雨状态下,水分运移以温度梯度水汽运移为主;水势梯度液态水通量受降雨影响明显,在降雨事件期间和之后,液态水和水汽下渗占主导地位,降雨降低表层土壤温度、减小土壤热传导通量,有利于土壤热稳定性。

The thermal-moisture dynamics of the active layer directly affects the stability of permafrost and engineering projects in cold regions. Previous studies mainly focus on the thermal stability of permafrost based on the adherent layer theory. The migration process of liquid water and water vapor and its effects on the active layer are still lack of consideration. Considering the physical process and mechanism of liquid water-vapor transfer in unsaturated soils, a new heat and mass transfer model in saturated-unsaturated partially frozen soil was developed, in which the moisture migration in both vapor and liquid phases and heat transfer by means of conduction, convection and phase change were accommodated. The established water-vapor-heat transport model was used to analyze the water-vapor-heat transport in the shallow unsaturated zone of active layer under the in-situ meteorological conditions. The results show that the liquid water and water vapor are driven by the temperature gradient flow downward during daytime but upward at night, the liquid water and water vapor are driven by the temperature gradient flow upward in warm season but downward in cold season. Vapor water accounts for more than 15% of the water flux in the active layer and the water vapor driven by the pressure head can be neglected throughout the year. The moisture transport is mainly controlled by temperature gradient in sunny days. During and after rainfall events, rainfall infiltration is significantly enhanced and liquid water and water vapor mainly infiltrate downward. Rainfall can significantly decrease surface soil heat flux, heat conduction and soil temperature, which mitigate the permafrost degradation process.