Effect of soil water repellency on moisture distribution from a subsurface point source

Wetting and redistribution of water in air-dried wettable, slightly repellent, and strongly repellent soils was investigated by tracking the spatial and temporal moisture-content variation in a transparent flow chamber. Water was applied as a subsurface point source. The degree of water repellency had a substantial effect on the plume's shape, dimensions, and internal moisture-content distribution. The high uniform moisture content in the repellent soil's plume surrounded by a narrow transition layer within which moisture content was sharply decreasing indicates that unstable flow shapes a finger-like wetting front. Moisture redistribution in the repellent soils was limited and took place mainly in the vertical direction. The repellent soils were wetted immediately, very likely by local positive pressure buildup induced by the hydraulic resistance imposed by the initial >90° contact angle. The plume shape dimensions and internal moisture-content distribution should be considered in the design and operation of subsurface drip irrigation in water-repellent soils.