In the first paper of this series a detailed theory was presented in which the principles of hydrodynamic stability analysis were used to develop a linear perturbation equation for vertical water movement with nonsharp fronts. The second paper of the series discussed the application of this analysis to the stability of several soil water systems, some of which were potentially unstable. These included redistribution following infiltration, infiltration into a scale heterogeneous medium, and infiltration into a fine‐over‐coarse stratified profile. Although instability was expected for these systems and a noticeable trend toward instability was present, the numerical results did not predict the occurrence of such a condition. There was evidence to suggest that small amounts of initial water could have a significant effect on the damping out of instabilities that would occur under dry initial conditions. The present paper studies the effect of initial water content and grain size on wetting front instability by using a series of simple experiments. Results are presented for redistribution following infiltration for three porous materials, initially in a dry condition, and for three wetting depths prior to the onset of the redistribution phase. In addition, for the coarsest of these porous materials, a similar series of infiltration‐redistribution experiments is conducted with varying amounts of initial water present. Results are also presented for the effect of initial water content on infiltration into a stratified profile. The experiments show quite clearly that even small amounts of initial water have a strongly inhibiting effect on the development of frontal instability patterns.

The occurrence of wetting front instability during water movement in unsaturated porous materials under certain conditions is discussed and a brief review presented of relevant stability studies. The principles of hydrodynamic stability analysis are summarized and then used in developing a linear perturbation equation for vertical water movement with nonsharp fronts. A numerical approach for determining the stability parameters involving the eigenvalues of a matrix solution is outlined, and the numerical results are checked against an analytical solution for a profile exhibiting a uniform distribution of capillary potential.

In an earlier paper a detailed theory was presented in which the principles of hydrodynamic stability analysis were used to develop a linear perturbation equation for vertical water movement with nonsharp fronts. In the present study the analysis is applied to the stability of several soil water systems some of which are potentially unstable. The first stage in the analysis requires the generation of water content and pressure head profiles using a computer‐based numerical solution for the flow equation for unsaturated porous materials. These profiles permit the calculation of certain differential terms which appear in the specification of the coefficients required in the stability analysis. Results for the growth rate of imposed disturbances of wavelengths of 1, 5, and 25 cm together with the appropriate eigenvectors are presented for infiltration into a homogeneous profile, redistribution following infiltration, infiltration into a scale heterogeneous medium, and infiltration into a fine‐over‐coarse stratified profile. Although instability was expected for the redistribution, heterogeneous, and layered systems, the results, whilst showing a noticeable trend towards instability, did not predict the occurrence of such a condition. The reason for this is discussed, in particular the evidence which suggests that small amounts of initial water can have a significant effect on the damping‐out of instabilities that otherwise would occur under dry initial conditions. Reference is made to a projected further paper documenting the effect of initial water content on wetting front instability and to the need for additional numerical studies at very low initial water contents.