Unstable finger-like flow in water-repellent soils during wetting and redistribution - The case of a point water source

Wetting and redistribution of water in air-dried wettable and water-repellent soils was investigated by tracking the spatial and temporal moisture-content variation in a transparent flow chamber. Water was applied to the soil surface as a point source at three application rates. Contrary to spontaneous penetration into the wettable soil, water ponded on the water-repellent soils prior to penetration. The interplay of water-repellency degree and water-application rate had a substantial effect on the shape and dimensions of the plume. The horizontal expansion of the wetting front in repellent soils beyond the ultimate pond width was small and did not obey the t^1/2 horizontal capillary-dependent flow. Moisture content throughout the plume in the repellent soils was high and gradually increased with depth (saturation overshoot), indicating an unstable flow shaping finger-like wetting fronts. Moisture redistribution was mainly in the vertical direction, leaving a wet region at the location of the plume tip when redistribution started.