Analysis of unsaturated/saturated water flow near a fluctuating water table

Periodically changing boundary conditions influence the water flow at the interface of the unsaturated/saturated zone. In this study we investigate periodically time-variant lower boundary conditions in a one-dimensional soil column with a water table, using numerical solutions of Richards equation. The signals used for the simulations are triangular and rectangular wave functions as well as a sine function with periods ranging from minutes to years and amplitudes of 25, 50, and 100 cm. The upper boundary was described by a zero or a constant downward flux of 0.01 and 1.0 cm d−1. The hydraulic properties were described by the van Genuchten–Mualem model with parameters representing soils from the textural classes sand, loamy sand, sandy loam, and loam. Water content and pressure head profiles exhibited sharper fronts for upward movement of the water table than for downward movement. This also resulted in asymmetric variations of matric head and saturation as a function of time. By analyzing the simulation results in terms of the maximum, minimum, and average height of the water table, we have shown that the water table was strongly damped for the upward movement but weakly damped for the downward movement. We present a criterion for determining the period and amplitude of the boundary condition in which the damping of the water table movement is negligible. The unsaturated zone above the water table was characterized by the height at which the water flow changed from a transient to a steady state. By applying scaling [Youngs, E.G., 1990. Application of scaling to soil-water movement considering hysteresis. In: Elrick, D.E., Hillel, D. (Eds.), Scaling in Soil Physics, Principles and Applications. SSSA Special Publication No. 25, pp. 23–37] and using a macroscopic length scale derived from steady unsaturated water flow, we found that the variation of the simulation results caused by textural differences was greatly reduced.