Head (hydraulic)

Hydraulic head is a specific measurement of water pressure or total energy per unit weight above a datum.

It is usually measured as a water surface elevation, expressed in units of length, but represents the energy 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.

 Atmospheric pressure 
Even though it is conventional to use gauge pressure in the calculation of hydraulic head, it is more correct to use total pressure (gauge pressure + atmospheric pressure), since this is truly what drives groundwater flow. Often detailed observations of barometric pressure are not available at each well through time, so this is often disregarded (contributing to large errors at locations where hydraulic gradients are low or the angle between wells is acute.)

 Analogs to other fields 

Hydraulic head is a measure of energy, and has many analogs in physics and chemistry, where the same mathematical principles and rules apply:
Hydraulic head is analogous to:
magnetic monopole
electric charge
heat (i.e., temperature)
concentration
A continuous "field" of hydraulic heads is analogous to:
an electric field
a magnetic field
Similar differential operators can be applied to the fields, to find:
the gradient, or the direction of flow
the divergence of flow
the curl, or if the field is rotating

Related pages
fluid dynamics
Darcy's law

 References 
 Bear, J. 1972. Dynamics of Fluids in Porous Media, Dover. .
 for other references which discuss hydraulic head in the context of hydrogeology, see that page's further reading section

Category:Fluid mechanics