Can Unsaturated Flow During Gravity Drainage Be Represented by Boulton's Formulation?

Despite recent advances in numerical methodology, the solution of the complete equations describing saturated‐unsaturated flow in an unconfined aquifer remains a difficult problem. Therefore in practical engineering calculations the formulation must be simplified. It is shown here that Boulton's equation (involving a time‐dependent drainable porosity) and also the linearized Boussinesq equation can provide results that are in agreement with certain aspects of the more realistic Richards solution, provided that an estimation of their parameters is possible. However, the performance of Boulton's formulation is only marginally superior to that based on the Boussinesq equation.