I decided to look a bit more into the water quantity figures Hodge listed in his 2002 work, Roman Aqueducts and Water Supply. It turns out that his figures are derived from the work of a previous author, Grimal. I know little about Grimal and some google searching didn't yield much information beyond basic facts: he's French, and most of his publications seems to have occurred in the 60s and 70s.

Purportedly, his figures are based on the records of Frontinus, but I don't possibly see how that could be: Grimal's cumulative total of the nine aqueducts for which Frontinus supplied measurements is fully double the total of other authors, including Hodge's figures in his collaborative work with Blackman. Almost all estimates are based on the figured supplied by Frontinus, and tend to be in the range of 12,000 to 15,000 quinariae. Grimal is hovering around 25,000.

The generally accepted conversion from a quinaria to some unit of volume over time appears to be about 40 m³/day. As always, estimates vary; some seem to have a more conservative figure of 20 m³/day, and I saw a few estimates above 40 as well. To get a sense of what 40 cubic meters of water actually looks like, think of it as being a cube of water ~3.41 meters on all sides, or about ~10,000 gallons of water. A swimming pool 24 feet round and four feet deep has about 12,000 gallons in it.

Knowing this, Grimal's total for the water supply of Rome is 24,805 quinaria per day, which Hodge seems to happily accept and convert to cubic meters using the 40 m³ estimate: 992,200 m³ per day. To get a sense of how much water this is, this would be a cube of water 99.74 meters on all three sides. This is 262 million gallons of water. Per day. Assuming Rome contained a million inhabitants (which seems to be the generally accepted estimate), this means that the water supplied 262 gallons of water for each person in Rome, per day.

Personally, I find Grimal's figures wildly suspect. For starters, Grimal arrives at totals double of other estimates. An "alternative estimate," according to Hodge, was H. Fahlbusch's proposed range of 520,000 to 635,000 m³. Other estimates seem to fall in this range as well.

Second, looking at what typical US cities used, it seems really unlikely to me. In a paper by Morgan, he gives the following table for U.S. cities in 1900:

...assuming the above figures are correct (they seem reasonable, but I haven't verified them myself), Hodge/Grimal are stating that Rome supplied more water per person than most major cities in the U.S. did by the start of the 20th century. I find that a very hard pill to swallow. I'm not alone on this: in Morgan's paper, his estimates are based on Frontinus' figures, and he arrives at a more typical ~14,000 quinaria, which using the 40 m³ conversion would end up being 560,000 m³ of water per day. However, Morgan settles on a more conservative figure of 6000 gallons/day per quinaria (22.7 m³/day), which makes his estimate 318,000 m³ per day. This is fully one-third of Hodge/Grimal's estimates.

Worth note is that when the Aqua Traiana and Aqua Alexandrina are added in (which Grimal estimates at 113,920 m³ and 21,160 m³ per day, respectively, although one what basis these estimates were made is unknown), it pushes the Rome total to 1.127 million m³ per day. Considering most other authors don't even bother supplying figures for the Traiana and Alexandrina, I wonder A) where Grimal got this information, and B) on the basis of what estimations. It certainly wasn't Frontinus' estimates.

What do modern aqueducts supply? The Catskill Aqueduct, which begins at the Ashokan Reservoir in Olivebridge, New York, in Ulster County and runs 120 miles to New York City, has an operational capacity of about 580 million gallons (219,240 m³) per day. It typically carries less than this, with flows averaging around 350 - 400 million gallons (132,200 - 151, 200 m³) per day.

Another example--the Colorado River Aqueduct--runs 242 miles from the Arizona-California border. Its capacity is 1.3 million acre-feet (1.6 km³) per year, or 1.6 billion cubic meters per year, or ~4.4 million cubic meters per day. Average flow is probably less than this, but it's a staggering number to think about. The construction project lasted eight years, and employed a total of 30,000 people.

I won't even list the figures for the California Aqueduct. It'd make your head explode. :)

Perhaps the most appropriate comparison, however, is with another Roman Aqueduct: Pont Du Gard. The water conduit, which is 1.8 meters (6 feet) high and 1.2 meters (4 feet) wide and has an average gradient of 0.4 percent, supplied the Roman city of Nemausus (now Nîmes) with an estimated 20,000 cubic meters of water per day (note: this is an uncited figure from wikipedia; I'll have to look into how that number was arrived at). This is also 500 quinaria, using the typical 40 cubic meter conversion rate.

Using the cube analogy, Pont Du Gard supplied Nemausus with a daily 27.1 meter cube, or 5.3 million gallons a day. Estimates for the population of Nemausus were around 50,000 inhabitants, so that would be roughly 106 gallons per person, per day, which is a pretty far cry from Hodge/Grimal's estimate of 262 gallons person/day in Rome.

Aqueducts

I'm currently taking a class on Roman aqueducts. We discuss and research the aqueducts that supplied Rome (and other portions of the Roman empire) from a number of different perspectives, including historical, hydrological, cultural, economic, logistical, etc. Basically, it's a class covering a broad range of topics pertaining to Rome's water infrastructure, with the aqueducts at the core of discussion.

As part of the class, we have to do research on a topic of our choice. Originally, I wanted to do research on how the aqueducts reduced the instance of water-borne disease, but my preliminary research revealed a rudely obvious fact: getting data on that subject was going to be close to impossible. I'd also made some crude assumptions, such as assuming that water-borne diseases like Cholera that produced epidemics during the 19th century were present (they were not--Cholera was endemic to the Indian subcontinent until the 18th and 19th centuries). Basically, it was doomed from the get-go.

With that fresh defeat behind me, I got to thinking--how did the water supply effect the population of Rome? How did the population of Rome effect the water supply? The aqueducts were built over a span of 500 years, so one thing I plan on examining is how the water supply grew in relationship to the population growth of Rome during 312 B.C. and A.D. 200. I'd like to attempt to answer whether or not new capacity was added in response to demand, and if the added water in turn sparked increased population growth.

Admittedly, this task is also fraught with peril: gigantic discrepancies in the capacity of the aqueducts exist in current research. Archaeological evidence and historical accounts of their capacity are obviously suspect, because stuff generally isn't built to last 2000+ years (even the Romans, notorious for their over-engineering, aren't up to snuff), and historical records are sparse and sometimes cryptic.

Another huge stumbling block is the actual population of Rome--again, estimates vary greatly, from a peak population of 200,000 inhabitants (absurdly low) to 2,000,000+ (absurdly high). The general consensus among reading seems to be around a million, but it's an exceptionally wide range. Also unknown is the rate of growth, epidemics that may have caused lapses in growth, immigration, the effects of being sacked seven or eight times, famines, etc. The demographic information leaves much to be desired.

Still, the estimates are good enough to attempt to draw some conclusions. And the idea sounds entertaining to me, so...without further ado:

...the above is a chronological listing of Rome's aqueducts. I have the dates as negative numbers because I wanted to do some basic graphing in Excel. Anyways, this chart shows their rough completion date, overall length, altitude of water source, altitude of the water level in Rome, and the total portion of the aqueduct that was above ground. Much of this information is from the meticulous records of a model public servant by the name of Sextus Julius Frontinus, from which most of the "hard" data today is derived. Because Frontinus died before the completion of the Aqua Traiana and the Aqua Alexandrina, little is known about those aqueducts other than some scant archaeological evidence.

Here is another chart, showing the "output" (in quotes for a damn good reason--more on this in a bit) of each aqueduct in quinariae, according to several sources:

Overall B&H is "Blackman and Hodge," who had estimates based around modified figures derived from Frontinus' records. Hodge later tabulated greatly increased values in a subsequent publication, hence the separate column. The last column are figures from Gerda de Kleijn, who also made estimates based on modified figures from Frontinus.

The first thing to notice about this table is the vast discrepancy between the last four columns. All four columns represent estimates by published sources. I'm going to do more research into this discrepancy, but for now, I'll let it be. The second thing to notice is that these units are all in quinariae.

Much of the controversy over the total water "output" supplied by the aqueducts stems around the rather nebulous nature of a quinaria. A quinaria was a Roman unit of area, roughly equal to 4.2 square centimeters. Its primary use was to measure the cross-sectional area of pipes in Roman water distribution systems. Even in Roman times, there was controversy over the value of a quinaria--according to Frontinus:

Those who refer (the quinaria) to Vitruvius and the plumbers, declare that it was so named from the fact that a flat sheet of lead 5 digits wide, made up into a round pipe, forms this ajutage. But this is indefinite, because the plate, when made up into a round shape, will be extended on the exterior surface and contracted on the interior surface. The most probable explanation is that the quinaria received its name from having a diameter of 5/4 of a digit...

...in other words, Frontinus disagreed with Vitruvius about the actual value of a quinaria (primarily on the basis of Vitruvius' estimate being "imprecise" with respect to inner and outer circumference). Regardless of the Romans haggling over the value of their own units, one thing is certain: using quinariae to measure flow would be somewhat like using feet to measure acres, or kilograms to express newtons. The Romans did have units of volume, but the issue with the water flow is that it's a measurement of volume over time, which the Romans had no accurate or practical means of measuring. Culleus per elapsed sundial just doesn't roll of the tongue quite like cubic meters per second. Considering they had no accurate temporal measuring devices, it's debatable if the Romans even grasped the concept of units expressed over time.

To get a better sense of how the flow of water changed over time, I did some graphing in Excel:

The x-axis is time, and the y-axis is cumulative water flow in quinariae. The different colors correspond with a specific aqueduct. This is my first stab at graphing some of these data, so I plan on doing much more here; I want the temporal axis to be to scale, rather than the regular intervals in the above graph, but Excel seems suited only for the most basic charts. I suspect I'm going to have to draw much of these by hand.

Here's another interesting graph that shows how dependent Rome was on each aqueduct over time:


...considering the scale of aqueduct construction, new projects were not to be taken lightly. So there must have been a practical reason to add new capacity--could it be population? Who knows. :)