Roman aqueducts brought massive volumes of fresh water to the city during the empire. (The Aqua Appia -- built in 312BC -- traveled 16 km, dropped 10 m in height and brought 73,000 m^3 of water per day to Rome.)
Two aqueducts met at the Porta Maggiore. Even today, the area is a "utility hub," with power lines and transport rails cross-crossing in front and behind the Porta.
What happened, I think, is that most of this water just flowed through Rome. It was not stored on a large scale. People used it in the bath houses, from where it went into sewers, and for drinking and bathing from "always on" fountains and taps like this (Anne was the leg model :)
What's interesting to my modern eyes (and end of abundance perspective) is that ALL of this water was "non revenue," not just because people got water for free, but because the system was 100 percent "leaking."
That was not a problem when water was abundant, but it is (increasingly) today.
Bottom Line: We will know that the Romans have water scarcity (and are seriously addressing it) when they put close valves on their always-on taps.