Water Flow in an Unsaturated Soil with a Step‐Type Initial Water Content Distribution

Abstract

AbstractWater flow against the water content gradient in soil columns composed of two layers of a given soil, with the upper layer at a higher water content than the lower layer, has been reported in the literature. From experiments with columns involving air gaps, control and variation of the temperature gradient, removal of water by evaporation, porous membranes and air‐dry soil, water content and pressure head measurements, it was concluded that vapor movement was the dominant mechanism when water was removed by evaporation. A vapor pressure gradient, originating from the temperature gradient created by the evaporation of water from the soil, provided the driving force for vapor transfer against the water content gradient. Liquid transfer predominated when water was removed isothermally. Initially water moved from the wetter layer into the drier soil in response to the pressure head gradient. After a time, the direction of the pressure head gradient reversed without a corresponding reversal in the water content gradient and water flowed out of the drier soil against the water content gradient.Two phenomena were instrumental in reversing the pressure head gradient. First, disturbance of the soil during the handling procedures caused an increase in the pressure head without a change in the water content. Second, since the driver layer initially gained water and dried thereafter, hysteresis also reduced the pressure head difference between the soil layers. Then removal of only a small quantity of water from the wetter soil made the pressure head smaller in the wetter soil than in the drier layer and the direction of the pressure head gradient reversed.