The hydrostatic pressure p is defined as p = p 0 − − ρ ρ g z , with p 0 some reference pressure, or when rearranged as head : ψ ψ = p ρ ρ g . The term ⁠ p / ρg ⁠ is also called the pressure head, expressed as a length measurement. It represents the internal energy of the fluid due to the pressure exerted on the container. The head due to the flow speed and the head due to static pressure combined with the elevation above a reference plane, a simple relationship useful for incompressible fluids using the velocity head, elevation head, and pressure head is obtained.