Water Movement in Horizontally Layered Soils

The water movement in layered soils where fine‐textured soil overlays a coarse textured soil has become very important in recent years due to its effect on the movement of pollutants and the distance to which water can percolate in a short time. Previous experiences have shown that when the sublayer is more pervious than the top layer, the wetting front becomes unstable through the second layer and water moves in the form of separate columns rather than uniform unsaturated flow. The purpose of this research was to determine the ratio of hydraulic conductivities at which the wetting front loses its one‐dimensionality and breaks into “fingers.” The result showed that up to a hydraulic conductivity ratio of 20, the wetting front is one‐dimensional and movement through the second layer is unsaturated. When the hydraulic conductivity ratio exceeds the value of 20, the wetting front loses its one‐dimensionality and the water moves through the second layer in the form of wet columns called “fingers.” While previous experiences have shown that an increase in initial moisture content will reduce the instability of the wetting front, the increase in hydraulic conductivity ratio increases the instability. This may explain the formation of fingers in initially wet layered soils. The experiments described herein were conducted on air‐dry soils. In soils with higher initial moisture content, the wetting front could remain stable at hydraulic conductivity ratios higher than 20.