In this equation g is the vector of gravitational acceleration. The hydraulic potential in the equation comes from the pore pressure,
p, and gravity,
ρ g.
Furthermore, in the Subsurface Flow Module,
g can be defined as using the elevation
D according to
where g is the magnitude of gravitational acceleration. The direction of the elevation is defined as
If a reference position rref is specified the elevation is determined using
The choice of D has a significant impact on results and the physics involved. For example, if
D is the vertical coordinate
z and if the flow is entirely horizontal within the
x-y-plane, then the gradient in
D vanishes and the driving force is caused by pressure gradients alone.