土壤斥水性调控非饱和土壤入渗时指流形成的机制研究

Fingered flow often occurs in unsaturated infiltration in soils either repellent or subcritically repellent. Since its first observation, fingered flow has been a key issue in soil physics. Experiments show that the finger exhibits a saturation overshoot in its tip that remains a great challenge to the standard infiltration theories. We postulate a pore scale process, slug cascade in soil pores, plays a critical role on the induction of fingered flow. We also conjecture that soil repellency greatly affects the slug cascade in soil pores and is one main mechanism for water accumulation behind the wetting front. This work intends to prove these hypotheses from the following aspects, (1)Tthe slug cascade in soil pores will be modeled as slug-train flow in a vertically oriented sinusoidal capillary tube. To model the propagation and evolution of the slugs, a mathematical model will be set on the basis of Navier-Stokes equation and the mass conservation law. Experiments on slug-train flow will be conducted in sinusoidal capillary tubes for a range of repellencies. With newly developed visualization techniques in conjunction with digital microscopy, the propagation of water menisci in the capillary tube will be recorded, which will be simulated by using the slug-train flow model. In return, the theoretical model will be improved to give reliable predictions of slug propagations; (2) The previous model will be modified to simulate unsaturated infiltration in repellent soils. In the model, the soil structure will be represented as a bundle of sinusoidal capillary tubes with a pore size distribution. With this new model, the slug propagations in each capillary tube will be simulated, from which the soil saturation profile can be obtained by averaging the faction of pores filled with water over a representative volume; (3) Slab chamber infiltration experiments will be performed to examine the fingered flow formation. In experiments, the light transmission method will be used to record the wetting front propagation and the transient soil saturation profiles. The theoretical model will be examined with these experiments, and possibly will be improved further to reveal the mechanism of fingered flow, which can inspire new ideas for the study of water transport in repellent soils.

斥水性土壤（包括亚临界斥水性土壤）入渗时很容易发生指流现象。探究指流的形成机制一直是土壤物理学的一个重点课题，然而,现有的理论模型在模拟指流的饱和度凸起等特征时遇到诸多困难。在本项目中，我们提出“微观孔隙内水滴的瀑布式下落运动”是引发指流的一个重要机制，其中土壤斥水性是调控水滴运动模式的关键因子。为此，本项目将开展以下三个方面研究：①建立描述正弦毛细管内水滴瀑布式下落运动的数学模型，重点研究斥水性对先行水滴的阻滞作用以及对后续水滴追赶时间的影响; ②在非均匀正弦毛细管束模型中，考虑毛细管内水滴瀑布式下落运动机制，建立描述斥水性土壤入渗的数学模型，模拟水滴瀑布式下落后在湿润锋形成的堆积效应，研究土壤斥水性对饱和度凸起的调控作用; ③开展斥水性土壤入渗实验，验证和优化描述指流的理论模型。本项目有望在一定程度上揭示指流形成的物理机制，促进土壤水分运移理论的发展。

斥水性土壤（包括亚临界斥水性土壤）入渗时很容易发生指流现象。探究指流的形成机制一直是土壤物理学的一个重点课题，现有理论模型在模拟指流的饱和度凸起特征时遇到诸多困难。在本项目中，我们提出“微观孔隙内水滴的瀑布式下落运动”是引发指流的一个重要机制，其中土壤斥水性是调控水滴运动的关键因子。围绕验证这一物理机制，本项目开展了以下三个方面研究：①开展了毛细管内水滴下落运动的观测实验，建立了描述正弦毛细管内水滴瀑布式下落运动的数学模型，该模型包含动态接触角、孔径非均匀性和毛细管壁粗糙度等影响因子，模型能够很好地模拟水滴下落实验观测到的水滴液面位置的实时变化和水滴下落速度；②建立连接微观孔隙水分运动物理过程与宏观土壤入渗过程的数学模型，采用非均匀正弦毛细管束（ＢＣＴＭ模型）表征土壤结构，用毛细管束内的连续水分运动和非连续水滴运动分别表征土壤的充分供水入渗和非充分供水入渗，结合毛细管束模型对液面位置函数进行体积平均，从而获得不同时刻的土壤剖面含水量分布。然后应用所建立的ＢＣＴＭ模型模拟土壤稳定入渗实验，并将模拟结果与连续介质模型的ＨＹＤＲＵＳ软件模拟结果对比，结果表明在模拟土壤接触角较小的亲水性土壤入渗时，BCTM模型与HYDRUS模拟的剖面含水量分布可以很好复合，而对于接触角较大的斥水性土壤，模拟结果分歧较大，BCTM模型较连续介质模型可以更直接地分析土壤斥水性对入渗过程的影响。 ③结合毛细管内非连续水滴下落运动的数学模型和BCTM模型，模拟土壤非稳定入渗实验。模拟获得的土壤剖面含水量分布具有明显的非单调性凸起这一特征结构，从而验证了本项目提出的物理机制。本项目从微观孔隙尺度上提出并验证了指流发生发育的物理机制，项目研究结果对土壤水分运移理论的发展具有一定的促进作用。