Here is a graph showing how much the water capacity in Rome increased over time. The y-axis is in quinariae, and the x-axis is time:
...the above numbers were computed using Blackman and Hodge's figures. Notice that this graph is similar to the one I previous posted that tabulated totals in Excel. The big difference with this graph is the x-axis is appropriately scaled with respect to the aqueduct's construction date.
In the perfect world, I'd have enough data to correctly taper each of the horizontal lines, clip out sections where the aqueduct was incapacitated or otherwise non-functional, etc. But it isn't a perfect world, so I have to settle for the above. (which for all intents and purposes isn't that bad)
I recently reviewed another paper by H. Chanson, Hydraulics of Roman Aqueducts:
Steep Chutes, Cascades, and Dropshafts, which had a totally different approach to estimating water capacity. Chanson actually evaluated archaeological evidence to determine the hydraulic flow in ancient Roman aqueducts. His findings are quite interesting: "maximum" flow for the nine major aqueducts was 1,013,960 cubic meters per day. Adding in the Traiana and Alexandrina (I guess there's enough evidence to find their hydraulic potential), Rome was supplied with 1,148,960 cubic meters, per day. The graph for the data tabulated in Chanson's paper looks like so (in this graph, the y-axis is cubic meters per day):
Chanson's data also makes me think that Grimal's data is probably based on hydraulic potential rather than Frontinus' records. And this is probably fine. One interesting thing about these graphs is regardless of the magnitude of the units, they look pretty similar. As in, the overall progression of water flow into Rome is pretty obvious; the overall magnitude seems to be of contention, but not the fact that it A) increased, and B) increased by some proportional amount.
One important thing to note with Chanson's data is that this is the maximum capacity, so it gives us a well-defined upper bound for the aqueduct's delivery potential. However, I doubt the aqueducts were ever running at capacity, and it's probably safe to say that it was common for them to run at half-capacity (or less) during dry months (being spring fed means the water source wasn't constant). So the estimates around ~600,000 cubic meters per day still seem more reasonable to me.
I've also recently been reviewing sources on population; I'll be posting more about that very soon.