Abstract: The unsaturated zone, which is the link between the s urface water and groundwater, plays a key role in controlling water exchange and solute transport in the soils. Infiltration in the unsaturated zone is in essence a problem of twophase
(air and water) immiscible displacement. A deep understanding of the airwater displacement process in the unsaturated zone is of great significance for the management and sustainable use of groundwater. A s eries of drainageimbibition
experiments are carried out to study the watercontent distribution in the unsaturated zone. The light transmission method is used to monitor water content in the 2-dimensional sandbox during the drainageimbibition cycles. It is found that the lateral fluctuation of water content in the drainage process (0.145-0.175) is greater than that in the imbibition process (0.150-0.165) due to the existence of unstable drainage processes at the pore-scale. The difference of water content between drainage and imbibition means that the mechanism happened at pore-scales should also be affected by water movement at darcy-scales.

Key words: unsaturated zone, water content distribution, drainage and imbi bition process, light transmission method