The diagram depicts how a hydroelectric power station generated electricity and how it is contionously happened during day and night as a circular process to produce an electricity. This process also taken from the higher place that flows the water into the lower one.
During the day the dam is opened in order to flows a water collected in high-level of reservoir. After that the water flows to the generator in power station to generate electricity. While generator help the power station to produce power lines which then distributed to the national grid, the water then is kept in the low-level reservoir. It will be there until it pump-up again to the high-level reservoir in the night.
When the night comes, the generator reverse the turbin to send back the water into the high-level reservoir. Using the same route as it is happened during the day, the water is collected again in there. The dam is closed to help the water going down.
In short, this hydroelectric power station processes different purpose between day and night period. During the daytime, it is produced an electric energy, meanwhile in the nightime, it will keep the resources back to the place where it is belong.