Why the rosettes image may not be a map at all

[Wladimir D]

Consider the lower right rosette.

It is necessary to ask ourselves question. Why on the top "tent" are depicted the pillars, similar to hollow cleaved along tube? So do or make in the manufacture of bamboo spouts.
I believe that on this rosette are two methods of water collection shown.
1. It is conceivable that the lower tent has a concave profile and all the water is collected in a center,  where tube is located (middle a pillar in the figure), through which water flows downward and is fed to consumers.
2. The second method is illustrated in another circuit.