Effect of hysteresis on water flow in a sand column with a fluctuating capillary fringe

The transport of water and solutes from the topsoil to groundwater is sensitive to the mixing regime near and within the capillary fringe. The capillary fringe is the transition zone between the saturated and unsaturated regions of the vadose zone. This study was conducted to describe variations of water content and matric potential in the presence of a capillary fringe. We used a sand column with a fluctuating water table to explore the dynamics of the phase distributions. A column 57 cm in length and 5.3 cm in diameter was packed with a sand mixture. The water pressure fluctuated at the bottom and forced the capillary fringe to move within the column. The column was installed on a balance for measuring the total water mass in it. Water content and potential were measured at different soil depths using TDR probes and tensiometers. The first series of experiments was conducted without irrigating. In a second series water was added to the surface at a constant rate by means of a sprinkling system. The variations of water content and matric potential were increasingly dampened and shifted in time with increasing distance from the capillary fringe. To describe the dynamics of water distribution it was necessary to account for hysteresis. Hysteresis dampened the water dynamics and caused a highly asymmetrical response of water content to the symmetrically oscillating lower boundary condition. The water dynamics were simulated with the computer code HYSTFLOW [Stauffer, F., 1996. Hysterestic unsaturated flow modelling. In: Proceedings of the Second International Conference on Hydroinformatics, Hydroinformatics ʹ96, Zürich, Switzerland. Balkema, Rotterdam, pp. 589–595] which is based on Richardsʹ equation, the Brooks and Corey hydraulic functions and a modified Mualem [Mualem, Y., 1984. A modified dependent-domain theory of hysteresis. Soil Sci. 137 (5), 283–291] hysteresis model. The water dynamics including the hysteretic behaviour were well described by the simulations.