Study on unsteady saturated-unsaturated flow subjected to variable soil water characteristic curve

Based on the Richards equation, a finite difference program is presented for modelling transient non-steady-state and nonlinear seepage flow through saturated unsaturated porous media of a stream aquifer system. Responding to the variability of the unsaturated aquifer's structure induced by the water table fluctuation, variable soil water characteristic curves are adopted for modelling groundwater seepage. The vertical seepage section is divided into saturated zone, transition zone and vertical seepage zone from bottom to upper. Horizontal flow and vertical flow occur synchronously in the saturated zone and the transition zone, and there is only vertical movement of seepage in the vertical seepage zone. Finite difference and linear shape function are used for the spatial discretization, and the Picard scheme is used for temporal discretization. Basic information responding to the general mesh structure is generated automatically, then, the automatic and homogenous meshes adapting to the seepage free surface can be obtained. Based on a sand-box experiment, the water table and runoff volume of free surface seepage responding to soil water characteristic curve is discussed. The results show that, compared to the fixed soil water characteristic curve, the variable curve provides more accurate simulation results.