When is water is flowing, it has energy related to the velocity of flow. This is the velocity head, measured in metres:
Velocity Head = 𝞫 v²/2g
v is the average velocity over the flow cross section (= Flow, Q / Area, A),
g is the acceleration due to gravity, and
𝞫 is a coefficient to account for the non-uniformity of velocity over the flow cross section. 𝞫 is close to 1, so it is usually ignored. Thus the equation reduces to:
Velocity Head = v² / 2 g
When flow enters a water conveyance system from a reservoir, some of the hydraulic head is converted to velocity head.
When flow leaves a turbine runner, the velocity head constitutes a considerable proportion of the flow energy. Turbine draft tubes are deigned to remove the swirl from the exit flow and to gradually reduce the flow velocity, enabling much off the velocity head to be recovered. When the flow leaves the draft tube, any remaining velocity head is lost. (This is one of the losses in the water conveyance system.)