In a hydropower station, a second reservoir is constructed near the region of water outflow
to control the flow of water as per our requirement of electricity.
In water storage plant, a second reservoir is constructed near the water outflow. When the demand for electricity is low, the surplus water is used to pump the water from the lower reservoir into the main reservoir. This is to ensure that a sufficient amount of water is available in the main reservoir to fulfil the peak loads i.e. when there is more demand of electricity.