of fluid. The elevation head is simply the elevation (h) of the fluid above an arbitrarily designated zero point: Pressure head is due to the static pressure, the internal molecular motion of a fluid that exerts a force on its container. It is equal to the pressure divided by the force/volume of the fluid in a gravitational field: Resistance head (or friction head or Head Loss) is due to the frictional forces acting against a fluid's motion by the container. For a continuous medium, this is described by Darcy's law which relates volume flow rate (q) to the gradient of the hydraulic head through the hydraulic conductivity K: while in a piped system head losses are described by the Hagen–Poiseuille equation and the Bernoulli Equation. Components of hydraulic head After free falling through a height in a vacuum from an initial velocity of 0, a mass will have reached a speed where is the acceleration due to gravity. Rearranged as a head: The term is called the velocity head, expressed as a length measurement. In a flowing fluid, it represents the energy of the fluid due to its bulk motion. The total hydraulic head of a fluid is composed of pressure head and elevation head. The pressure head is the equivalent gauge pressure of a column of water at the base of the piezometer, and the elevation head is the relative potential energy in terms of an elevation. The head equation, a simplified form of the Bernoulli principle for incompressible fluids, can be expressed as: where is the hydraulic head (Length in m or ft), also known as the piezometric head. is the pressure head, in terms of the elevation difference of the water column relative to the piezometer bottom (Length in m or ft), and is the elevation at the piezometer bottom (Length in m or ft) In an example with a 400 m deep piezometer, with an elevation of 1000 m, and a depth to water of 100 m: z = 600 m, ψ = 300 m, and h = 900 m. The pressure head can be expressed as: where is the gauge pressure (Force per unit area, often Pa or psi), is the unit weight of the liquid (Force per unit volume, typically N·m−3 or lbf/ft3), is the density of the liquid (Mass per unit volume, frequently kg·m−3), and is the gravitational acceleration (velocity change per unit