The alternative is to use an extended model which is founded on thermodynamic principles and is physically consistent. We present results of a numerical modelling study based on this extended model. In addition to the standard equations, the model uses a balance equation for specific interfacial area and a constitutive relationship for specific interfacial area as function of capillary pressure and saturation.We show that the extended model can capture additional physical processes compared to the standard model, such as hysteresis. New features of the extended model, such as a physically motivated description of mass transfer between phases are discussed."