Visualization of microscale phase displacement processes in retention and outflow experiments: Nonuniqueness of unsaturated flow properties

Methods to determine unsaturated hydraulic properties can exhibit random an d nonunique behavior. We assess the causes for these behaviors by visualizi ng microscale phase displacement processes that occur during equilibrium re tention and transient outflow experiments. For both types of experiments we observe the drainage process to be composed of a mixture of fast air finge ring and slower air back-filling. The influence of each of these microscale processes is controlled by a combination of the size and the speed of the applied boundary step, the initial saturation and its structure, and small- scale heterogeneities. Because the mixture of these microscale processes yi elds macroscale effective behavior, measured unsaturated flow properties ar e also a function of these controls, Such results suggest limitations on th e current definitions and uniqueness of unsaturated hydraulic properties.