Dynamics of Water Movement near Boundaries of the Vadose Zone

Abstract

Processes at boundaries of the unsaturated soil water zone were investigated: At the upper boundary evaporation at the soil-atmosphere interface, and at the lower boundary the dynamic capillary fringe. For studying the upper boundary, an evaporation experiment at the representative elementary volume (REV) scale was considered and modelled numerically. A model with a diffusive boundary layer and a 1D Richards' description including vapour transport fitted well to experimental data. It showed a boundary layer dominated regime in the wet range and a regime where dynamics is controlled by soil hydraulic properties in the dry range. The model could successfully be used to determine soil hydraulic properties from the corresponding evaporation experiment by inverse modelling using a Monte-Carlo Levenberg-Marquardt approach. For the lower boundary, light transmission and NIR imaging spectroscopy methods were developed and employed to measure the micro- and macroscopic water distribution in response to transient boundary conditions in a semi-2D sand medium in a Hele-Shaw cell with high temporal and spacial resolution. The analysis showed that coupled multi-phase and dynamic non-equillibrium effects are essential to understand water movement in dynamic capillary fringes, and sub-REV processes play an important role in the dynamics.