The Unsaturated Porous Medium node adds the Richards’ equation (Equation 4-11) for flow in variably saturated porous media.

The Unsaturated Porous Medium feature has two default subfeatures — the Fluid subfeature where the fluid density ρ (SI unit: kg/m3) and viscosity μ (SI unit: Pa·s) are defined and the Porous Matrix subfeature where the permeability κ (SI unit: m2) and porosity εp (dimensionless) are specified as well as the retention model.

Select a coordinate system from the Coordinate system list for the interpretation of directions in anisotropic material properties. The default is the Global coordinate system, and the list contains any additional orthonormal coordinate system (except boundary coordinate systems) added under the Definitions node.

The subnodes inherit these coordinate system settings. In particular, the Permeability or Hydraulic conductivity (in the Porous Matrix subnode) should be set according to the coordinate system selected in this section.

With this option, define the first term in Equation 4-11. Therefore, choose from a drop-down menu one of the following formulations:

If From retention model is chosen, the first term of Equation 4-11 accounts only for the specific moisture capacity Cm which is defined by the Retention Model in the Porous Matrix subnode:

For the option Linearized storage the storage coefficient Sp in Equation 4-11 is defined as

From liquid content enforces the first term of Equation 4-11 to be defined in terms of liquid volume fraction as

and the User defined option allows to define the storage coefficient Sp in Equation 4-11 directly.