What it means
An acre-foot is the volume of water it takes to cover one acre of flat ground one foot deep: 325,851 gallons, or 43,560 cubic feet. It is the unit Arizona uses for every serious conversation about water, and in Scottsdale you will hear it at council meetings, in Water Department reports, in the news every August when the federal government announces next year’s Colorado River cuts, and from the neighbor who follows all of that. “The city’s allocation is about 81,000 acre-feet.” “The Tier 1 cut is 512,000 acre-feet.” Without the word, neither sentence means anything; with it, they are the whole debate.
The useful conversion is households. The Arizona Department of Water Resources and the Valley’s municipal water association both put one acre-foot at about three single-family homes for a year. So Scottsdale’s CAP allocation is water for roughly 240,000 homes, more than the city has; the Scottsdale Water guide explains why the city built a supply bigger than its demand.
Where you’ll meet it
In the Arizona water law guide and every state document behind it; in the city’s drought and water-supply pages; in golf-course talk about recycled water on public courses; and on the signs at the Xeriscape Garden, where conservation is sold in gallons because acre-feet are too big for a front yard.
Where the word came from
The unit is older than Arizona and nobody here claims it. It is an irrigator’s measure: a farmer applies water to land by the acre and by the depth, so “one foot over one acre” was the natural quantity long before cities bought water. The number falls out of the arithmetic, since an acre is 43,560 square feet and a foot of water over it is 43,560 cubic feet, which is 325,851 gallons. When the Salt River Valley’s farmers formed their water users’ association in 1903 and the federal government built Roosevelt Dam, the water was accounted for in acre-feet, and the state’s water law, from the 1980 Groundwater Management Act to the Colorado River compacts, kept the unit. Cities inherited it when they inherited the water.
The household conversion has moved. For years the rule of thumb was two families per acre-foot; the Arizona Municipal Water Users Association put it at three Phoenix-area households in 2017, and the state agency uses the same figure, because Valley homes use far less water than they did in the 1980s. The other comparisons AMWUA offers are a football field under about a foot of water and two acre-feet to fill an Olympic swimming pool.
The numbers that matter to Scottsdale are all in this unit. The CAP canal delivers about 1.5 million acre-feet of Colorado River water a year. Scottsdale’s share, its municipal allocation, was about 81,000 acre-feet in 2015 by the city’s own count, roughly two-thirds of its drinking water. Its normal-year supply from the Salt River Project is 16,894 acre-feet. In 2014 the city put more than 29,000 acre-feet of treated water back into the aquifer. And the federal shortage tiers on Lake Mead cut Arizona by 512,000 acre-feet at Tier 1 and 592,000 at Tier 2a, cuts that so far have landed on farms with lower-priority contracts rather than on cities.
What it is not
An acre-foot is not a measure of flow. Flow is cubic feet per second, which is how the canals and rivers are gauged; acre-feet are the total volume delivered or stored over a year. And “acre-feet per year” is already implied in most Arizona sentences, so “acre-feet” and “AF” alone usually mean the annual amount.
The fine print
The unit hides scale. A Tier 1 cut of 512,000 acre-feet is about 167 billion gallons; the CAP’s 1.5 million acre-feet is about 489 billion. The household conversion is how reporters bring such numbers down to earth. Three homes per acre-foot is a Valley average; a Scottsdale lot with a pool and a lawn will use more, a condo far less.
Scottsdale’s figures carry dates for a reason. The 81,000-acre-foot CAP allocation is the city’s 2015 statement and includes water the city leases as well as its base contract; allocations shift with leases and settlements, and the city does not publish a single running total. The SRP figure, 16,894 acre-feet in a normal year, applies only to the older, southern parts of the city inside SRP’s service area, which is why north Scottsdale is almost entirely a CAP and recycled-water place. The recharge figure is from 2014 and the city says it has maintained safe yield, putting back as much groundwater as it pumps, since 2006.
The smallest number in the local vocabulary is the one that made the news: about 150 acre-feet a year, the amount set aside for the Rio Verde Foothills filling station that opened on January 1, 2026, after the city cut off hauled-water sales to the unincorporated community in 2023. That is water for perhaps 450 homes. The Colorado River entry explains the tiers; the CAP canal entry explains the ditch the acre-feet arrive in.