text: In fluid dynamics, head is a concept that relates the energy in an incompressible fluid to the height of an equivalent static column of that fluid. From Bernoullis principle, the total energy at a given point in a fluid is the kinetic energy associated with the speed of flow of the fluid, plus energy from static pressure in the fluid, plus energy from the height of the fluid relative to an arbitrary datum. Head is expressed in units of distance such as meters or feet. The force per unit volume on a fluid in a gravitational field is equal to &rho;g where &rho; is the density of the fluid, and g' is the gravitational acceleration. On Earth, additional height of fresh water adds a static pressure of about 9.8 kPa per meter (0.098 bar/m) or 0.433 psi per foot of water column height.
  The static head of a pump is the maximum height (pressure) it can deliver. The capability of the pump at a certain RPM can be read from its Q-H curve (flow vs. height).
  Head is useful in specifying centrifugal pumps because their pumping characteristics tend to be independent of the fluid's density.
  There are generally four types of head:
  #Velocity head is due to the bulk motion (kinetic energy) of a fluid. Note that  is equal to the dynamic pressure for irrotational flow.
  #Elevation head is due to the fluids weight, the gravitational force acting on a column 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 fluids motion by the container. For a continuous medium, this is described by Darcys 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 Bernoulli’s equation.