The hydraulic pressure head follows from the storage equation:

Primary unknown is the hydraulic pressure head **groundflow_pressure**. Further notation:
**group_groundflow_capacity**;
**group_groundflow_permeability** in -direction;
space coordinate;
material velocity (if present);
**force_element_volume** (hydraulic pressure source).
The equation is given for space coordinates following material
velocities (if present).

**Groundflow velocities**

The groundflow velocities, after initializing **groundflow_velocity**, follow from:

**Total groundwater pressure**

The total groundwater pressure is by example needed to calculate the total stresses in soils (total soil stress = effective soil stress + total groundwater pressure). The total groundwater pressure follows from:

where is the gravitational acceleration,
and is the **groundflow_density**
(Please notice that typically is a negative number).

**Static groundwater pressure**

The static pressure due to gravity is:

where the is the distance to the groundwater level, the phreatic level. The phreatic level needs to be specified with the

**Dynamic groundwater pressure**

The dynamic groundwater pressure follows from

**Boundary conditions**

If the groundwater velocity is 0 normal to an edge (say at the interface with a rock layer it is zero), then you should prescribe nothing on that edge (Tochnog will then take care of that boundary condition for you).

At the phreatic level where the groundflow meets free air the hydraulic pressure head
should become .
You can either set this yourself by using **bounda_unknown** combined with
**bounda_time** or else demand that Tochnog automatically does it for you
by activating the option **groundflow_phreaticlevel_bounda**.

At edges where you have some other hydraulic head you need to
specify that head yourself with **bounda_unknown** and **bounda_time** records.

If gravity is not of importance, e.g. in biomechanics where the storage
equation is used to model fluid transport in soft tissues,
the hydraulic pressure head is equal to the total pressure,
and thus is zero at edges where the water meets the free air.
In this case, set to zero by using **bounda_unknown** combined with
**bounda_time**.

**Postprocessing**

For all printing, plotting etc. you normally get the hydraulic pressure head
since it is the primary unknown solved in the storage equation.
The total pressure, static pressure and dynamic pressure
you can get by the **post_calcul** option.

**Naming conventions**

To connect better to conventional naming, we rematk that the capacity depends on the porosity and water compressibility :

and for the permeability:

where is the permeability in -direction.

**Consolidation analysis**

Look in the 'Consolidation' section of the 'Interaction analyzes and advanced analyzes' chapter in the end of this manual on how to perform consolidation analyzes (combined groundwater flow with soil stress analyzes).