Multiphase models of unsaturated flow: Approaches to the governing equations and numerical methods

Fluid flow in the unsaturated zone may be viewed as a two-phase flow problem. In this light, several numerical solution methods may be employed to simulate the simultaneous movement of air and water. These include a two-pressure approach and several methods based on a global pressure-saturation formulation of the governing equations. The global pressure-saturation description leads naturally to a characteristic type of solution algorithm. For one-dimensional problems, this method has demonstrable numerical advantages. Implementation of this method in realistic, multi-dimensional problems remains to be demonstrated.