A piezometric surface is the imaginary level to which water would rise in wells tapping an aquifer. It is also called the potentiometric surface.
The piezometric surface, also called the potentiometric surface, joins the levels to which water would rise in wells that tap a given aquifer. In a confined aquifer it lies above the top of the aquifer because the water is under pressure. In an unconfined aquifer it is the same as the water table. The height of water at one point is the piezometric head, or hydraulic head. It is the sum of the elevation above a datum and the pressure head, written h = z + p/γ, where z is elevation, p is water pressure and γ is the unit weight of water. Hydrogeologists map the surface by measuring levels in many wells at about the same time and drawing contours. Groundwater moves from higher head to lower head, and in uniform material it flows at right angles to the contours. Closely spaced contours show a steep gradient. Wells should all be open to the same aquifer, as mixing levels from different layers gives a map with no real meaning. Where the surface lies above the ground, wells flow without pumping.