The diagram shows the components of a hydroelectric power station and what happens there to generate electricity for the grid.

At the 'beginning' of the diagram, we can clearly see that the high-level reservoir with main dam is situated and fed by a river. At the base of the dam is where the intake is located and it indicates that it is 'open' during the day. During the day, with the intake open, the flow of water down will be able to move the turbines that are attached to the generator that is, in turn, attached to power lines and the national grid. The water is stored in a low-level reservoir at this time.

During the night, this process is reversed. The turbines are labelled as reversible, which means that they are used at night to reverse the direction of the water. It is unclear from the diagram exactly where the water goes after it is pumped up towards the intake and high-level reservoir, as the intake is labelled as 'closed' at night, but it is assumed that there is some mechanism allowing the water to be re-added.

In summary, the flow of water shown shows a bi-directional pattern that allows water to be re-used in the creation of electricity for the national grid.

(215 words)