Soil water content dependent wetting front characteristics in sands

The initial soil water content affects wetting front instability. A series of experiments were conducted where water was infiltrated into a 20/30 sand with initial volumetric water contents of 0, 0.005, 0.01, 0.015, 0.02, 0.03, 0.04 and 0.047 cm3/cm3. Both water content and matric potential were measured. Water content was measured with Synchrotron X-rays at the Cornell High Energy Synchrotron Source (CHESS) and matric potential with fast responding tensiometers. The flow field changed gradually from unstable fingered flow at 0% initial water content to a wetting pattern that can be described with Richards’ equation without hysteresis. For unstable flow, the matric potential and water content decreased a short distance behind the wetting front. Except in the initially dry soil, the finger pattern began to widen immediately after infiltration. The pressure and water content of the wetting front were hyperbolically related to the initial water content. In general, this research showed that in coarse soils when an unstable wetting front occurs both the matric potential and water content drop behind the wetting front while, when a stable Richards’ type wetting front forms, the matric potential and water content increase (slightly) behind the wetting front.