Tomographic Measurements of Pore Filling at Infiltration Fronts

For certain porous media and initial conditions, constant flux infiltrations will produce a nonmonotonic saturation profile. This nonmonotonicity is purported to be a cause of gravity driven fingering, cannot be described by standard models of unsaturated flow, and is likely due to the wetting front being sharp at the pore-scale. Here we report micro-computed tomography measurements of the porous medium and measure which pores are filled by water and air at the initial wetting front as a function of flux. We find that at high fluxes all the pores are filled with water; for intermediate fluxes the pores along the edge of the column become unsaturated; and for low fluxes the pores in the bulk of the experimental column become unsaturated. This suggests that the transition between a sharp and diffuse front as a function of flux is abrupt, and that the unsaturated conditions observed at intermediate and higher fluxes are primarily an edge effect of the column.