Two major flow mechanisms for preferential flow in unsaturated zone are the existence of macro pores or structures and wetting front stability (fingering flow). Do you have any field evidence or theoretical arguments to tell which of the two mechanisms is more important?
I suppose for cohesive soils fissures and cracks play an important role at the start of wetting until these are closed due to swelling.
Eris Uygar This is one part of the story. I think, the macro pores are important only near ground surface. For the the deep unsaturated zone, the fingering flow is more important.
May be I can speculate that, if there is heterogeneity in the soil saturation (in a soil mass), it would perhaps cause fingered flow due to variation in the soil suction potential in that soil upon wetting, which results in creation of paths of increased hydraulic conductivity (I mean affinity to conduct water). In that case, the connectivity of these paths (tortuosity of the unsaturated or partially saturated soil structure) will be a factor affecting the rate of conductivity.
I think these could help you;
They're available on the ResearchGate Server for download.
The occurrence and development of figuring flow (wetting front instability) in unsaturated zone are very much like those of turbulence flow in pipes. The environment disturbance may determine transition from laminar flow to turbulent flow. However, once fully developed, the turbulent flow is not significantly impacted by the disturbance any longer. Along the same line, the heterogeneity may impact the initial stage of fingering flow, but not the fully developed fingering flow that is mainly determined by the nonlinear nature of the unsaturated flow.
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