In EPS, a 75-ft diameter tank drops 40 feet in an hour, but the flow out would result in less than a 3-foot drop. The physical settings appear to properly define a 75-ft diameter tank with a total capacity of 1.56 MG. What could be causing this anomaly?
The tank level remains steady until the booster station feeding the zone shuts down due to dropping suction pressure when a fire demand is applied in the suction-side pressure zone. The flow out of the zone through PRVs and from demands matches the flow out of the tank, once the PS shuts off. But the tank level drops rapidly without obvious cause.
In addition to Craig's response, what is the operating range you have specified for tank operation? Along with the "reporting time" it would be good to check the tank levels you have specified.
Here is a reference article which can help you understand the tank operating range;
Understanding Tank Operating Range
Hope this helps.
Bentley Technical Support