Oblique Porous Composite as Evaporating "Cap": Do Desert Dunes Preserve Moisture by Capillary Barriers and Tilt of Their Slopes?

Evaporation from desert sand dunes is studied practically using laboratory columns and mathematically using both analytical and numerical modeling. On outcropping cliffs of the dunes of Buwshar (Oman), microstratification, with layers' thickness of 1-2 mm, was observed. Loose sand and consolidated core samples were collected for laboratory experiments. Imbibition into initially dry sand columns and ensuing steady state evaporation from a capillary fringe above a horizontal water table were studied. The evaporating, negative-pressure topsoil of the columns was slanted mimicking the cliffs' tilt. In the columns plugged by a thin composite "cap" made by tooling the dune stratified clod, evaporation was impeded and reduced as compared with a homogeneous column. The same outlet evaporating boundaries were considered in HYDRUS-2D simulations, which showed that 2-D evaporation from a homogenous column was significantly higher than that from the column with a stratified "plug" at the top. Analytical solution for a 2-D tension-saturated homogeneous trapezium was obtained by conformal mappings of pentagon in the physical plane onto a rectangle in the Zhukovsky plain, via a reference plane. This solution combines the versatility of the Toth model, where topology of flow is controlled by an isobaric boundary of a flow tube, and of the Vedernikov model, which assumes a constant hydraulic conductivity in the negative pressure zone of a capillary fringe. Analytical formulae also manifest a decrease of the flow rate with the increase of the angle of tilt of an evaporating isobar.