Unstable Flow during Redistribution: Controlling Factors and Practical Implications

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.