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     Standpipes or water towers are used to monitor the flow rates in a water supply system.  As  demand decreases the level in the tank increases and large pumps are cycled off.

Water towers are not used to monitor the flow rate. They maintain a certain level, usually 90%, so that there is a constant pressure in the distribution system. Water treatment plants use multiple pumps that are used to fill the tower to maintain that level. If the demand is low, one pump would be used.At higher demand rates, multiple pumps would be run. (I've done instrumentation work for the water treatment plants for many years).

I do agree with the containers. If they are not water containers they could be for emergency storage.