The diagram indicates how electricity is generated in a hydroelectric power station.
Overall, it is clear that the power station can generate electricity by using the water flow from High-level reservoir to low-level reservoir. It works differently regarding its operating time.
Looking at the diagram more closely, one can see that in day time the water in the high-level reservoir pass through the intake under the dam to power station to generate energy. Intake opens only in day time.
The water, which is flew down to the power station, operates generator by using the reversible turbines. The turbines convert the flow of water into the mechanical power. The generated energy moves to national grid through the power lines.
In contrast, during night time the water flows to the opposite direction. The reversible turbines pump up the water back into the high-level reservoir. Also at the same time the power station uses the generated energy. This water pumping process is required procedure to produce more energy for the next day.