Unexpected water content profiles under flux‐limited one‐dimensional downward infiltration in initially dry granular media

Abstract

Distinct water‐content profiles, having saturated θ‐bumps at and immediately behind wetting fronts, were observed in narrow columns (9 mm ID) of homogeneous air‐dried glass beads under downward one‐dimensional infiltration with applied water fluxes at lower than the saturated hydraulic conductivity. The infiltrating water content profiles and the position of the wetting front as a function of time were also calculated based on Richards' equation using an independently measured hydraulic conductivity function K(θ), and water retention characteristic θ(h), of the drainage process, and by applying a pressure boundary condition, hwe, which is almost atmospheric pressure, at the moving wetting front. The good agreement between the calculated profiles and the observed confirms that the dynamic water entry pressure hwe (where θ(hwe) is the saturated water content), does exist for air‐dried glass beads, although it is in contradiction to the modern theory of unsaturated water flow in porous media. This physical property of dry media creates an upward negative pressure profile behind the wetting front that would inevitably produce finger flow if the column diameter were larger. At the wetting front, the water content and pressure should be completely discontinuous at the level of pore size, and hence Darcy's equation cannot be applied across the interface between the air‐dried and saturated media.