土壤蒸发的内部过程机制与水汽运动模拟研究

Soil evaporation is an important component of land water circle. In general, it is controlled by the soil properties and climate conditions. Usually most researches of soil evaporation focus on the evaporation calculation methods, the impacts of irrigation and management on soil evaporation, and the ratio of evaporation in evapotranspiration. The interior vapor transport beneath the soil surface in the soil evaporation process is ignored. In this work, we propose that the vapor transfer in the unsaturated soil under isothermal and nonisothermal conditions should be studied based on the enhanced diffusion theory of vapor. An enhancement factor model under water content gradient at isothermal condition will be developed. Combining it to the enhancement factor model under temperature gradient, the total soil water vapor transport model was established. It is capable of describing the vapor transport process and evaporations at different soil layers under transient conditions. During the experiment, the heat pulse technique and blue dye tracing will be used. From the dynamic monitoring of primary drying front, secondary drying front and dry surface layer, the interior dynamic process of soil evaporation will be illustrated. Both laboratory and field experiments will be conducted to study soil evaporation and vapor transport. Our measurements and models are useful in understanding the water vapor and liquid water dynamics in the soil evaporation process, and can improve the current models of coupled heat and water transport in the soil.

土壤蒸发是陆地水循环的重要组成部分，主要受土壤特性和大气条件共同控制。传统的土壤蒸发研究大部分关注于蒸发量计算、灌溉等管理措施对蒸发的影响、蒸发在蒸散中的比例等，很少考虑蒸发土体内部的水汽运移过程。本项目拟从土壤水汽运移研究入手，通过深入探讨等温及非等温条件下水汽在非饱和土壤中的促进扩散机理，建立等温水汽运移促进因子模型。在此基础上，将温度梯度下的水汽运移过程引入模型，发展得到土壤总水汽运移模型，实现蒸发条件下的土壤内部分层蒸发量及水汽运移过程模拟。研究将综合运用热脉冲示踪和染色示踪技术，通过对蒸发过程中初级干燥锋、次级干燥锋、干土层的动态监测，揭示蒸发条件下土体内部真实的水汽过程。研究结果有助于深入认识蒸发土壤内部的水汽和液态水动态，并为改进土壤水热耦合运动理论提供了一个新方法。

土壤蒸发过程与热量传输过程紧密相关，土壤内部的液态水在潜热的作用下才可发生蒸发相变，并最终以水汽扩散的形式运移到大气中去。考虑到土壤内部的孔隙结构复杂，大小量级差异巨大。本项目首先研究了控制水汽相对湿度的水吸力，进而依据气体分子在孔隙内的碰撞，获取了土壤内部孔隙平均线程。在此基础上，通过设计多质地土壤实验，分析了土壤内部蒸发动态和热量传输动态。通过对多质地土壤热量、液水、水汽三者的耦合观测实验及模拟，本项目取得了以下一些结果：（1）将多质地土壤的内部蒸发过程进行了定量化，阐明了土壤内部分层蒸发水汽动态及其演变；（2）通过理论和实验研究，提出了热通量板测定误差的主要来源即是对水汽传输的阻断；（3）将气体分子动理论与热物理理论结合，获取了土壤孔隙结构度量的平均线程，并阐述了土壤微孔隙内部气体分子碰撞在表征孔隙结构上的作用；（4）定量模拟了控制水汽相对湿度的水力学参数水吸力，探讨了基于部分范围测定数据从而外推预测干区水吸力的误差；（5）建立了土壤热特性与水吸力之间的定量关系，并定量评价了利用热特性预测水吸力的范围和效果。本项目的研究成果可为进一步地明确蒸发过程中的土壤内部水、汽、热动态，提高土壤水热耦合运移模拟精度提供理论基础和参考。