# HydroGeoSphere/Variably-Saturated Subsurface Flow

As discussed in Section 2.1, one of the requirements for simulating variably-saturated flow is that we define the constitutive relationships that govern the relation between the pressure head, saturation and relative permeability. These relationships must be defined for the porous medium, discrete fractures and the dual continuum and, for each of these types of media, the choice is to use either pseudo-soil, functional or tabular relationships. Wells are a special case and are handled automatically by **HydroGeoSphere**.

Media specific instructions and default values for porous media, discrete fractures and dual continua are given in Section 5.8.3.1, Section 5.8.3.2 and Section 5.8.3.3 respectively.

General instructions for modifying the default functional and tabular relationships are given in Section 5.8.3.4 and Section 5.8.3.5 respectively.