The diagrams compare two methods by which water is supplied to urban areas: The Roman aqueduct and modern syphon system.
In general, both methods transport water from a reservoir to the city through a conduit. However, the Roman system has used an aqueduct bridge structure, while modern system relies on an underground syphon. In addition, the modern syphon comprises a treatment plant which is not available in the roman aqueduct.
Regarding similarities in the diagrams, water is initially stored in a large reservoir before entering through the intake structure which acts as the entry point. It then flows through a long narrow conduit towards the urban area.
The main difference is that in the Roman system, water passes over an elevated, bridge-like aqueduct, which carries the water across the landscape. By contrast, in the modern system, the flowing water goes through underground syphon which forms a half circle, allowing it to descend and rise again. Moreover, in the modern syphon, before the water reaches the urban regions, it goes to the treatment plant, where the water is purified for use; whereas in the Roman system, the water goes directly to the city without any purification process. Consequently, in both processes, the used water flows into the sea, which serves as the final destination of the system.