Unstable Flow during Redistribution: Controlling Factors and Practical Implications
Unstable Flow during Redistribution: Controlling Factors and Practical Implications
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DOI：
10.2136/vzj2004.0549
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发表时间：
2004-05
影响因子：
2.8
通讯作者：
Zhi Wang;W. Jury;A. Tuli;Dong‐ju Kim
中科院分区：
文献类型：
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作者：
Zhi Wang;W. Jury;A. Tuli;Dong‐ju Kim
不稳定的流动通过在有或没有大孔的土壤中诱导指状流动路径，导致包气区排水特征的重大不确定性。最近的研究已经确定，控制指状流的主要因素是毛细管滞后、多孔介质中存在的入水阈值以及润湿锋后面的正基质势梯度的综合影响。这种情况通常发生在高速渗透后的重新分配过程中，这在水文学中很常见。在渗透到细过粗的层状土壤、疏水性或滞留空气的土壤中，甚至在渗透率较低的均匀粗质地土壤中时，也会出现有利于不稳定的条件。本文对均匀土壤中出现不稳定流所需的条件进行了分析。我们证明，如果基质势梯度 (dh/dz) 在重新分布期间变为正值，润湿前沿的扰动将导致指状流动。然而，如果 dh/dz 保持负值，扰动就会消失。该分析用于预测灌溉的临界深度 (Ic)，超过该深度水流就会变得不稳定。使用填充均匀沙子的板箱进行了一系列点源和线源渗透实验。结果证实，一旦超过 Ic，润湿锋底部就会形成指状结构，引导水流并阻止周围区域的水运动。我们讨论了这种现象对实际灌溉和淋滤设计的影响。
Unstable flow causes major uncertainties in the characterization of drainage in the vadose zone by inducing finger‐like flow paths in soils with or without macropores. Recent studies have identified the major factors governing fingered flow to be the combined effects of capillary hysteresis, the existence of a threshold water‐entry value in a porous medium, and a positive matric potential gradient behind the wetting front. This situation typically occurs during redistribution following high‐rate infiltration, a common occurrence in hydrology. The conditions favoring instability can also develop during infiltration into a fine‐over‐coarse layered soil, into hydrophobic or air‐entrapped soils, or even in a homogeneous coarse‐textured soil if the infiltration rate is low. An analysis of the conditions necessary for the onset of unstable flow in a uniform soil is provided in this paper. We demonstrate that if the matric potential gradient (dh/dz) becomes positive during redistribution, a perturbation at the wetting front will cause finger flow. However, if dh/dz remains negative, the perturbation will be dissipated. The analysis is used to predict a critical depth of irrigation (Ic) beyond which the flow should become unstable. A series of point‐source and line‐source infiltration experiments were conducted using a slab‐box filled with uniform sands. The results confirmed that as soon as Ic is exceeded, a finger was formed at the bottom of the wetting front, channeling the flow and stopping water movement in the surrounding areas. We discuss this phenomenon's implications for practical irrigation and leaching designs.