[LonePalm]

If you want a reliable measure of population, determine the amount of fresh water coming into the city on a daily basis.

Garde la Foi, mes amis! Nous nous sommes les sauveurs de la République! Maintenant et Toujours!
(Keep the Faith, my friends! We are the saviors of the Republic! Now and Forever!)
LonePalm, le Républicain du verre cassé (The Broken Glass Republican)

[decimon]

If you want a reliable measure of population, determine the amount of fresh water coming into the city on a daily basis.

That would be the population of just the city of Rome, itself a disputed figure.

[Tallguy]

I think that the water flow of the Roman aquaduct system would only set an upper limit for the population. Probably not useful for predicting actual population. A good bit of that water would have flushed directly into the sewage system anyway.