Visualization of microscale phase displacement proceses in retention and outflow experiments: nonuniquensess of unsaturated flow properties

Methods to determine unsaturated hydraulic properties can exhibit random and nonunique behavior. We assess the causes for these behaviors by visualizing microscale phase displacement processes that occur during equilibrium retention and transient outflow experiments. For both types of experiments we observe the drainage process to be composed of a mixture of fast air fingering 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 yields macroscale effective behavior, measured unsaturated flow properties are also a function of these controls. Such results suggest limitations on the current definitions and uniqueness of unsaturated hydraulic properties.