The Subsurface Flow Module license is required to use this boundary condition (see http://www.comsol.com/products/specifications/).

The Pervious Layer node provides a boundary condition that describes a mass flux through a semi-pervious layer connected to an external fluid source at different pressure, pressure head, or hydraulic head. The model domain might connect to a larger body of water through the semi-pervious layer. This condition is represented with the following boundary condition:

In this equation, pb (SI unit: Pa) and Db (SI unit: m) are the pressure and the elevation of the distant fluid source, respectively, and Rb (SI unit: 1/s) is the conductance of materials between the source and the model domain (conductance to flow in the semi-pervious layer adjacent to the boundary). Typically Rb = K'/B', where K' is the hydraulic conductivity (SI unit: m/s) of the layer and B' (SI unit: m) is its thickness. Using logical relationships, it is possible to activate these expressions at different times or under various flow conditions.

When the pressure head Hp is specified instead of the pressure, the boundary condition is the following:

Hpb is the pressure head (SI unit: m) at the edge of the layer.

When the hydraulic head H is specified instead of the pressure head, the boundary condition becomes:

Hb is the hydraulic head (SI unit: m) at the edge of the layer.

K in Equation 8-7 and Equation 8-8 is the hydraulic conductivity.

At a free surface, such as a water table or seepage face, the pressure is atmospheric (here taken to be zero), so the total hydraulic potential equals gravitational potential, which is defined on D.

Select an External variable to specify — Pressure and elevation to specify the external pressure, Pressure head and elevation to specify the external pressure head, or Hydraulic head. For all selections, enter a value for the Conductance Rb (SI unit: 1/s).