Direct Measurement of Unsaturated Conductivity including Hydraulic Nonequilibrium and Hysteresis

The unsaturated hydraulic conductivity function is the dominant material property for modeling soil water dynamics. Because it is difficult to measure directly, it is often derived from the water retention characteristic combined with a geometric model of the pore space. In this study, we developed an automated, simple multistep flux (MSF) experiment to directly measure unsaturated conductivities, K (ψ m ), at a number of water potentials, ψ m , using the experimental setup of classical multistep outflow (MSO) experiments. In contrast to the MSO experiment, the MSF experiment measures the conductivity directly at a spatially constant water potential assuming macroscopically homogeneous materials. Additionally, the proposed method reveals the hysteresis of K (ψ m ) with respect to increasing and decreasing water potentials as well as the temporal dynamics of K (ψ m ) during transient-flow conditions. This temporal behavior is explained by the dynamics of fluid configurations at the pore scale during drainage and imbibition leading to hydraulic nonequilibrium. It may provoke a systematic underestimation of hydraulic conductivity using inverse optimization of K (ψ m ) based on classical MSO experiments. The new approach will improve the determination of K (ψ m ) and it provides an experimental tool to quantify the effects of hydraulic nonequilibrium under transient conditions.