The liquid phase held by capillary forces in crevices and grain contacts in unsaturated porous media may form a network of liquid-filled corners resembling the structure of foam. A recent study (Or and Assouline, 2013) has shown that gravity induced drainage of such domain resembles drainage dynamics of wet foam, thereby paving the way for using the rich mathematical tools developed for foam drainage. The soil foam drainage equation may offer an alternative to the Richards equation with the distinct advantage of avoiding the need for unsaturated hydraulic conductivity function while retaining information on conducting liquid cross-section areas (for colloid and pathogen transport). We have revised some of the solutions of Or and Assouline [2013], and developed new analytical expressions for drainage volumes and rates for different boundary conditions. Analytical predictions were in good agreement with numerical solutions of the foam drainage equation for unit gradient drainage or drainage following a pressure jump from hydrostatic equilibrium. The analytical expressions successfully describe drainage processes in natural soils. We address aspects of parameter estimation for real soils and provide new insights into unsaturated hydraulic conductivity functions at textural contrasts.

American Geophysical Union, Fall Meeting 2016, abstract #H41F-1385