Hydraulic head pressure or piezometric head is a specific measurement of liquid pressure above a geodetic datum. It is usually measured as a liquid surface elevation, expressed in units of length, at the entrance (or bottom) of a piezometer. In an aquifer, it can be calculated from the depth to water in a piezometric well (a specialized water well), and given information of the piezometer's elevation and screen depth. Hydraulic head can similarly be measured in a column of water using a standpipe piezometer by measuring the height of the water surface in the tube relative to a common datum. The hydraulic head can be used to determine a hydraulic gradient between two or more points

 

Fluid flows from the tank at the top to the basin at the bottom under the pressure of the hydraulic head.

Fluid Dynamics Head - 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 Bernoulli's Principle, the total energy at a given point in a fluid is the energy associated with the movement of the fluid, plus energy from pressure in the fluid, plus energy from the height of the fluid relative to an arbitrary datum. Head is expressed in units of height such as meters or feet.

 

The static head of a pump is the maximum height (pressure) it can deliver. The capability of the pump can be read from its Q-H curve (flow vs. height).

 

Head is equal to the fluid's energy per unit weight. Head is useful in specifying centrifugal pumps because their pumping characteristics tend to be independent of the fluid's density.

 

There are four types of head used to calculate the total head in and out of a pump:

Velocity head is due to the bulk motion of a fluid (kinetic energy).

Elevation head is due to the fluid's weight, the gravitational force acting on a column of fluid.
Pressure head is due to the static pressure, the internal molecular motion of a fluid that exerts a force on its container.

Resistance head (or friction head or Head Loss) is due to the frictional forces acting against a fluid's motion by the container.

 

 

A mass free falling from an elevation z >0 (in a vacuum) will reach a speed:

 

 when arriving at elevation z=0, or h = v2/2g

 

Where g = acceleration due to gravity

 

v2/2g 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:

 h = Hydraulic head (Length in m or ft), also known as the piezometric head.

 = Pressure head, in terms of the elevation difference of the water column relative to the piezometer bottom (Length in m or ft), and

 z = 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, � = 300 m, and h = 900 m.

 

The pressure head can be expressed as:

 

Where:

 

P =  Gauge pressure (Force per unit area, often Pa or psi)
  = Unit weight of water (Force per unit volume, typically N�m−3 or lbf/ft�)
 p = Density of the water (Mass per unit volume, frequently kg�m−3)
 g = Gravitational acceleration (velocity change per unit time, often m�s−2)

 

 

The pressure head is dependent on the density of water, which can vary depending on both the temperature and chemical composition (salinity, in particular). This means that the hydraulic head calculation is dependent on the density of the water within the piezometer. If one or more hydraulic head measurements are to be compared, they need to be standardized, usually to their fresh water head, which can be calculated as:

 

 

hfw = Fresh water head (Length, measured in m or ft), and
 = is the density of fresh water (Mass per unit volume, typically in kg�m−3)