into a hydrophobic medium (van Ommen et al., 1988; Hendrickx et al., 1993; Ritsema et al., 1993; Ngu-of drainage in the vadose zone by inducing finger-like flow paths in soils with or without macropores. Recent studies have identified the yen et al., 1999;Carrillo et al., 2000;Wang et al., 2000b), major factors governing fingered flow to be the combined effects of soil air compression during infiltration (White et al., capillary hysteresis, the existence of a threshold water-entry value in a porous medium, and a positive matric potential gradient behind the rated infiltration under low application rates (Selker wetting front. This situation typically occurs during redistribution et al., 1992; Yao and Hendrickx, 1996; Wang et al., following high-rate infiltration, a common occurrence in hydrology. 1998b; Geiger and Durnford, 2000). In addition to these The conditions favoring instability can also develop during infiltration recognized situations, another condition predicted to into a fine-over-coarse layered soil, into hydrophobic or air-entrapped cause unstable flow in uniform porous media is redistrisoils, or even in a homogeneous coarse-textured soil if the infiltration bution following the cessation of infiltration (Raats, 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 demon-1973; Philip, 1975). Diment and Watson (1985) and Tastrate that if the matric potential gradient (dh/dz ) becomes positive mai et al. (1987) demonstrated that redistribution causes during redistribution, a perturbation at the wetting front will cause unstable flow in coarse-textured, oven-dry, uniform mafinger flow. However, if dh/dz remains negative, the perturbation will terials. However, Diment and Watson's (1985) experibe dissipated. The analysis is used to predict a critical depth of irrigaments in a small slab box showed that redistribution tion (I c ) beyond which the flow should become unstable. A series of "stabilized" when the initial water content was increased point-source and line-source infiltration experiments were conducted to only a few percent of saturation. In contrast, recent using a slab-box filled with uniform sands. The results confirmed that experiments of Wang et al. (2003a, 2003b) in the field as soon as I c is exceeded, a finger was formed at the bottom of the and with a large slab box showed that redistribution is wetting front, channeling the flow and stopping water movement in the surrounding areas. We discuss this phenomenon's implications unstable even in a very wet uniform sand. Nicholl et al. for practical irrigation and leaching designs.
Unstable flow causes major uncertainties in the characterization
