Modelling variably saturated flow and solute transport into sandy soil

The one-dimensional simultaneous transport of water and solute into sandy soil has been studied numerically. Under very dry initial condition, a steep pressure gradient and great variation of hydraulic conductivity occur across the wetting front during the infiltration of water. To compute the steep pressure gradient (and hence the Darcy velocity) accurately, various methods of estimation of the internode hydraulic conductivity (IHC) have been attempted. The use of a composite integration formula to calculate the integrated average expression of the IHC was found to be optimal, whereas the use of two-point Gauss quadrature led to significant error, which in turn caused apparent error in the solute profile. The transport of solute in sandy soil is dominated by advection. Oscillation or smearing of the solute profile easily results when conventional schemes are used. The numerical difficulty is overcome by using the six-point fourth-order (6P4O) conservative characteristics based scheme with a flux limiter.