Wetting Front Instability in Layered Soils

Abstract Percolation of a one‐dimensional wetting front in homogeneous soil is known to be stable. On the other hand, in a layered soil whose upper layer is finer and less conductive than the coarser layer beneath, the wetting front becomes unstable and breaks into narrow wetting columns, or “fingers,” in which flow is three‐dimensional. The instability originates at the interface but takes some time to develop, hence the wetting fingers appear some distance below the interface. The fingers move downward at a velocity equal to the saturated conductivity divided by the saturated volumetric water content of the bottom layer. The fingers consist of a saturated inner core percolating downward surrounded by an unsaturated outer layer. Experimental results indicate that the width of the fingers is independent of the flow rate and of irregularities at the interface. The number of fingers per unit area of interface is directly proportional to the flow rate. Instability of the wetting front in layered soil following rain or irrigation makes it possible for smaller volumes of water to penetrate deeper than would have been possible had the wetting front remained stable.