| Wetting front instability creates a shallow
induction zone from which fingers emerge that rapidly transport water and
solutes downwards. How the induction zone affects finger location and spacing is
unknown. In the moist subsoil, fingers may well dissipate because the finger
tips no longer have to overcome the water entry value. Both flow regions were
investigated in a two-dimensional chamber with a fine-over-coarse glass bead
porous medium. A capillary fringe was created by upward wetting through
capillary rise. Upon ponding with dye-coloured water, fingers emerged,
propagated downward and diverged when reaching the capillary fringe.
Microtensiometers were installed in the induction zone, the fingers, and in the
capillary fringe. In the induction zone, a lateral sinusoidal pressure head
developed within minutes. Only in one of two experiments could the observed
pressure head pattern be satisfactorily reproduced by a steady-state model
assuming uniform induction zone properties and uniform infiltration. Later,
fingers emerged below the pressure head minima. The induction zone did not
affect finger properties. The pressure head in the induction zone was determined
by the depth of the finger tips. The water requirement of the fingers dictated
the lateral pressure head gradients. The pressure heads in the capillary fringe
supported the hypothesis that the flow stabilised and dissipated there.
Keywords: fingered flow, wetting front instability, unsaturated flow,
microtensiometers, induction zone, capillary fringe |