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.