The difference between the reservoir level and the river level at the reservoir outlet, which determines the amount of mechanical energy contained in the water flowing out of the reservoir. In addition to flow, hydraulic head is an important parameter that determines how much energy can be extracted from a hydropower plant by a turbine. The size of the hydraulic head and the flow are used to select the most suitable type of turbine. For example, a Pelton turbine is better suited to higher heads, whereas a Kaplan turbine is better suited to lower heads.