Effects of air pressure on water flow in an unsaturated stratified vertical column of sand

By the use of a vertical stratified column of unsaturated soil the effects of air pressure on constant flux infiltration and gravity drainage were studied experimentally. Water content was measured by γ ray attenuation; water pressures were obtained simultaneously at 10 different levels in the column by using tensiometers connected to pressure transducers. In addition, the differential ports of some transducers were connected by a ‘scanivalve’ either to the external atmosphere or to the soil air phase by using hypodermic needles inserted through the column wall. The soil column consisted of three layers: fine sand over coarse sand over fine sand. The wall of the column was also provided with a series of adjustable air ports to allow experiments either with or without lateral air movement. It was shown that (1) the local soil air pressure can differ significantly from the external atmospheric pressure, e.g., +50 mb for rain with an intensity of 3 cm/hr or −15 mb for gravity drainage, and (2) the water flow depends not only on the boundary conditions in terms of water content and/or water pressure but also on the air pressure conditions. We conclude that the air pressure must be taken into account in determining the soil water suction, and we suggest that the flow equations must be written in terms of two‐phase immiscible fluid flow.