Ringside: Tips to Understand Our Convoluted Yet Obligatory Units of Water

Those of us following water politics and the water industry have become familiar with the most common units of water volume and water flow. Professionals in the industry make constant use of terms, often reduced to acronyms, forgetting that the rest of us may have no idea what they’re talking about.

When it comes to encouraging meaningful discussions over water policy, understanding these terms is mandatory. But whether it’s politicians who rely on staff members who are themselves usually spread too thin to become expert anyway, or journalists who often just grab a quote with a number in it to give their story a whiff of verisimilitude, water numeracy is unusual.

The situation is compounded by the fact that unlike the rest of the world, where units of water are divided into neat levels of magnitude according to a decimal system – 1,000 liters of water is a cubic meter (weighing a metric ton), and 1 billion cubic meters is a cubic kilometer (weighing a metric gigaton) – American units of water volume are, to put it charitably, convoluted.

Rather than measure gradations of volume in increments of ten, we have three primary units of volume (notwithstanding the Byzantine assortment of smaller measures – teaspoon, tablespoon, ounce, cup, pint, quart), they are the gallon, the cubic foot, and the acre foot. Here’s how those three relate to each other:

One acre foot is equal to 43,560 cubic feet, and one acre foot is equal to 325,851 gallons. In turn, one cubic foot is equal to 7.480519 gallons. Simple. Right? And before moving on, note that most retail water bills will report your monthly consumption in “CCF” units, which stands for “Centum Cubic Feet,” or, for us mortals, one hundred cubic feet (748 gallons). And just to torture you further, 436 CCF is equal to an acre foot.

To reduce this complexity to units of volume most often used in discussions of water policy, perhaps the most common units are multiples of an acre foot: one acre foot (AF), one thousand acre feet (TAF), and one million acre feet (MAF). Typically when measuring the storage capacity of large reservoirs, we use thousand acre feet units. For example, as seen on this report originating from the California Department of Water Resources, Diamond Valley, an important off-stream reservoir in Southern California, has a capacity of 810 TAF. When discussing statewide water consumption, we use millions of acre feet. For example, on average California receives about 200 million acre-feet (MAF) of rain each year, and California’s water withdrawals for agriculture generally total around 30 MAF/year, and for urban use around 9 MAF/year.

That’s the easy part.

Understanding this convoluted system gets even more convoluted when measuring flow. To simplify matters, however, there are only a few ways in common use by which rates of flow are measured. Gallons per minute (GPM), cubic feet per second (CFS), gallons per day (GPD), acre feet per day (AF/day), and acre feet per year (AF/year). So here goes:

One acre foot per day (AF/day) equals 325,851 gallons per day (GPD), 226 gallons per minute (GPM), and almost exactly one-half (0.50417) a cubic foot per second (CFS). Let’s repeat that last comparison: One AF/day is a rate of flow that is almost exactly equal to a 1/2 CFS rate of flow.

The serendipitous utility of this coincidence is well understood among water watchers. For example, one of the most consequential water policy indicators available in California is the daily data on how much of the Sacramento-Joaquin Delta flow goes out to the San Pablo Bay and thence out to the ocean (reported in CFS), how much is pumped into the federally owned Delta Mendota Canal (reported in CFS), and how much is pumped into the state owned California Aqueduct (reported in AF/day). How these allocations are made affect ecosystem health, farm production, and the quality of life in our cities.

The good news? If you’re watching this data, you always know that to within less than one percent, CFS x two = AF/day, and AF/day x .5 = CFS.

While to the uninitiated this may seem like an excursion into Nerdvana, that perception ignores the weight of these numbers. Around the state, most significantly in the delta, but all over the place, from withdrawals from rivers to releases from reservoirs, we get reports denominated in CFS. But for the farmer in the field who watches upstream CFS reports because that will affect how many acre feet in downstream water allocations he’s going to get, or the analyst trying to make sense out of it all by normalizing data to acre feet, the lucky fact that there is an intuitive connection between CFS and acre feet is helpful.

Another common unit of flow is gallons per day (GPD), which happily has an almost as directly intuitive connection to AF/year. For example, the Carlsbad desalination plant produces 50 million gallons per day of fresh water, which happens to equal 56,045 acre feet per year. That’s not a bad in-your-head calculation. To convert GPD to AF/year, just bump the GPD number by 10 percent and drop three zeros, and you have a roughly accurate estimate of AF/year.

Unless you like numbers, the foregoing are tedious paragraphs indeed. But such is the price we pay for not using the metric system. If you wish to delve into global water use – if you care about water in California, just to develop a sense of perspective, you should – here’s a handy thing to know: 1 million acre feet (MAF) equals 1.23 cubic kilometers (km3), and 1 cubic kilometer equals 0.81 MAF.

Do not despair. If you can calculate a 20 percent restaurant tip in your head, you can convert MAF to km3.

For those of you who value the ability to understand, convert, normalize, and put into meaningful context our units of water volume and water flow, but don’t want to keep all these ratios in your head, you can download this spreadsheet. It will automatically convert any common unit of water volume (tab one) or water flow (tab two) you choose into any other common unit. It is the product of sustained brow beating, and we are happy to share it with the world. Enjoy.