An etiological hypothesis is offered to explain the occasional occurrence of fingering during infiltration and redistribution. The basic hypothesis is that a spatially distributed flow field, such as a planar wetting front, tends to constrict where the flow accelerates. This constriction may cause the flow field, if it is wide, to break into discrete, concentrated (partial volume) currents. Thus, initially parallel streamlines tend to converge and thereby form spatially separated currents when the velocity increases along the direction of flow. This can occur particularly during infiltration, at the transition from a less permeable toplayer to a more permeable sublayer, if the conductivity of the sublayer at the suction of water entry exceeds the flux through the toplayer. This simple hypothesis fits the criterion of Occam's razor and should lend itself to experimental testing.
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Hillel et al. (1988) studied this question.