Primary unknown is the groundflow_pressure p. Further notation:
group_groundflow_density; co group_groundflow_compressibility;
kp group_groundflow_permeability; xi space coordinate;
vi material velocity (if present);
f force_element_volume (pressure source).
The equation is given for space coordinates following material
velocities vi (if present).
The groundflow velocities (see groundflow_velocity) follow from:
In case of gravity, the initial nodal pressures should be initialized in the pressure part of the node_dof records, and the gravity force should be included in the pressure part of the force_element_volume record.