Conductivity and Hydraulic Nonequilibrium across Drainage and Infiltration Fronts

Recently, a new approach was introduced to directly measure unsaturated hydraulic conductivity in flux‐controlled experiments—the multistep flux experiment. Thereby an overshoot in matric potential h m across drainage and infiltration fronts was observed. We extended this experimental approach to simultaneously measure the volumetric water content θ within the sample and applied the method to a sand and a clay loam soil. The detailed trajectories within the h m –θ space were obtained during a number of decreasing and increasing steps in infiltration rate. This clearly demonstrates the type and magnitude of hydraulic nonequilibrium under transient conditions where water content and matric potential deviate from a well‐defined static relation. We also compared the directly measured hydraulic conductivities with those obtained from classical multistep outflow experiments and found that nonequilibrium dynamics might lead to an underestimation of hydraulic conductivity when obtained from an inverse solution of Richards' equation. We provide a qualitative explanation of nonequilibrium that depends on the structure of the material and the type and magnitude of external forcing. The new experimental setup is considered to be a valuable tool to actually quantify nonequilibrium effects. This will make it possible to represent this relevant phenomenon in future modeling concepts.