The total energy per unit weight of water at a specific point in a groundwater system, equal to the sum of elevation head and pressure head. It determines the direction of groundwater flow.
Hydraulic head, also called piezometric head or total head, is a measure of the total mechanical energy per unit weight of water at a given point in a groundwater system. It is composed of two components: the elevation head (z), representing the gravitational potential energy relative to a datum, and the pressure head (h_p = p/ρg), representing the energy due to fluid pressure. In groundwater systems, the velocity head component is typically negligible due to slow flow velocities. Hydraulic head is measured directly using piezometers or observation wells, where it corresponds to the water level elevation in the well. Water flows from points of higher hydraulic head to points of lower hydraulic head, and the spatial distribution of head values defines the potentiometric surface that governs flow direction and rate. Contour maps of hydraulic head (potentiometric surface maps) are fundamental tools in hydrogeology for visualizing groundwater flow patterns, identifying recharge and discharge areas, and planning groundwater development. The concept of hydraulic head is the foundation of Darcy's law and all quantitative groundwater flow analysis.