The depicted diagram illustrates the process of electricity generation in a hydroelectric power station. Overall, the picture shows that the station operates back and forth, between a high-level reservoir and its low-level counterpart, in a cycle using river water during both the daytime and at night.
At the beginning, water from a river flows into a high-level reservoir and is retained by a dam, whose intake opens during the daytime to allow the river water to flow through a water tube into the power station. There, the reversible turbines operate the generator to produce electricity.
The generated electricity is then transported by power lines to the national grid, while a certain amount of water flows into a low-level reservoir. At night, this stored water is used to power the reversible turbines, which pump the water back up to the high-level reservoir, and the intake remains closed to hold back the water until the next daytime cycle.