An alternative to primary variable switching in saturated–unsaturated flow computations

It is well-known that the mixed moisture content-pressure head formulation of Richards’ equation performs relatively poorly if the pressure head is used as primary variable, especially for problems involving infiltration into initially very dry material. For this reason, primary variable switching techniques have been proposed where, depending on the current degree of saturation, either the moisture content or the pressure head is used as primary variable when solving the discrete governing equations iteratively. In this paper, an alternative to these techniques is proposed. Although, from a mathematical point of view, the resulting procedure bears some resemblance to the standard primary variable switching procedure, it is much simpler to implement and involves only slight modification of existing codes making use of the mixed formulation with pressure head as primary variable. Representative examples are given to demonstrate the favourable performance of the new procedure.