A Conceptual Model of Unstable Flow in Unsaturated Soil during Redistribution

Abstract

whose two most important attributes are finger diameter and the fraction of the total cross-sectional area occu-A conceptual model is presented that represents the development pied by fingers (hereafter called the finger flow fraction). of unstable flow in uniform soils during redistribution. The flow insta-Various attempts have been made to predict the finger bility results in the propagation of fingers that drain water from the diameter from soil properties and the water flow characwetted soil matrix until equilibrium is reached. The model uses soil retention and hydraulic functions, plus relationships describing finger teristics. They either involve an approximate equation size and spatial frequency. The model assumes that all soils are unsta-that is a function of the sorptivity (Parlange and Hill, ble during redistribution, but shows that only coarse-textured soils will 1976) or include a macroscopic surface tension associform fingers capable of moving appreciable distances. Once it forms, a ated with the large-scale curvature induced by the radius finger moves downward at a rate governed by the rate of loss of water of the finger (Chuoke et al. , 1959; Glass et al., 1989a). from the soil matrix, which can be predicted from the hydraulic con-Hydrostatic balance equations written at the wettingductivity function. Fingers are assumed to stay narrow as a result of front interface have periodic solutions that have a maxihysteresis, which prevents lateral diffusion. The draining front in the mum unstable wavelength, and this is used to calculate soil matrix between the fingers is assumed to cease downward movethe finger diameter. Both approaches involve associating ment because water pressure drops below the threshold water-entry one-half of the wavelength with the finger diameter. matric potential. The threshold potential is not present in the conven-

Unstable Flow during Redistribution in Homogeneous Soil

“…the fingers are unsaturated. Finger velocity declined We first sought to demonstrate that the finger pathways with time, as predicted by the model of Jury et al (2003), established in the first experiment and faithfully foland indicates that the rate of supply from the matrix is lowed in later experiments were not caused by soil hetercontrolling the advance of the fingers. Velocity decreased ogeneity, but rather were a random consequence of peras initial water content increased (Exp.…”

“…tube with draulic radius, and i is drainage rate. Equation [1] was 3-mm mesh screens at 10-and 30-cm heights above the botderived byWang et al (1998a) and used inWang et al tom). The sand was consolidated using a shaker, after which(2003) andJury et al (2003) to describe observed finger the surface was leveled and covered with cheesecloth.…”

Beech Forest Density Control on the Dominant Water Flow Types in Andic Soils

“…As opposed to that, the MCF included both heterogeneous matrix flow and fingering (Fig. 3c) that likely evolved due to soil microlayering and wetting front instability during redistribution (Hill and Parlange, 1972; Jury et al, 2003). Frequencies of the two flow types ( D ), as reproduced in the respective marginal tables, were used to examine the nature of their associations with factors A , B , and C (design variables).…”

The Pressure Head Regime in the Induction Zone During Unstable Nonponding Infiltration: Theory and Experiments

“…Jury et al (2003) suggested that a θ‐ h relationship with a water‐entry pressure head (below which the hydraulic conductivity during infiltration is zero) should allow Richards' equation to produce unstable wetting fronts. Egorov et al (2003) conclude the same but warn that such a modification fundamentally has no fastest growing perturbation of the wetting front, while experiments typically show fairly uniform finger widths/radii.…”