The pressure head regime in the induction zone during unstable nonponding infiltration: theory and experiments

Fingered flow rapidly moves water and pollutants from the root zone to the groundwater through a limited fraction of the unsaturated zone, limiting the possibilities for decay and adsorption. The onset of wetting front instability and the characteristics of the flow pattern under nonponding infiltration have received limited attention. We aim to theoretically and experimentally advance our understanding of pre-fingered flow, and contrast fingered flow under ponding and nonponding conditions. We developed a Green-Ampt based expression for the pressure head in a developing induction zone (from which fingers protrude) for the time before fingers developed. A uniform, nonponding water flux was applied to the surface of two-dimensional glass bead porous media with a dry region above a capillary fringe. Microtensiometers recorded pressure heads in the induction zone. The pressure head data confirmed both the theoretical early-time pre-finger model, and a model developed earlier for late-time lateral flow toward fully developed fingers. The physically more realistic constant flux boundary condition of our experiments gave larger finger spacings and travel times, compared to the frequently used set-up with ponding infiltration into a fine-over-coarse porous medium