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 inve stigate periodically time-variant lower boundary conditions in a one-d imensional soil column with a water table, using numerical solutions o f Richards equation. The signals used for the simulations are triangul ar and rectangular wave functions as well as a sine function with peri ods 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 fl ux of 0.01 and 1.0 cm d(-1). The hydraulic properties were described b y the van Genuchten-Mualem model with parameters representing soils fr om 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 res ulted in asymmetric variations of matric head and saturation as a func tion of time. By analyzing the simulation results in terms of the maxi mum, minimum, and average height of the water table, we have shown tha t the water table was strongly damped for the upward movement but weak ly damped for the downward movement. We present a criterion for determ ining the period and amplitude of the boundary condition in which the damping of the water table movement is negligible. The unsaturated zon e above the water table was characterized by the height at which the w ater flow changed from a transient to a steady state. By applying scal ing [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 simu lation results caused by textural differences was greatly reduced. (C) 1998 Elsevier Science B.V. All rights reserved.