How many homes in Arizona, on average, share an acre-foot of water each year?
Item: A July 2020, news story headlined, “Central Arizona housing boom tees up opportunity for water investors,” included the following: “An acre-foot is roughly what a family of four uses in a year.”
Item: A March 2021 news account headlined “Western states chart diverging paths as water shortages loom,” included the following: “An acre-foot is enough to serve one to two average households a year.”
Item: An April 2021 story headlined “Southern Cal water giant wants Sacramento Valley water,” included the following: “An acre-foot (of water) is 326,000 gallons, enough for a year’s worth of use for one to two California households.”
Item: Another April story headlined “Water restrictions headed for Santa Cruz in May,” included the following: “An acre-foot of water is about 326,000 gallons, enough to supply two average households for a year.”
The question is officially begged: How many households does an acre-foot of water serve annually?
Household water use is a timely subject. The entire Southwest is being wracked by chronic drought. Just this week, the managers of the Colorado River system – the federal Bureau of Reclamation – released data indicating that a first-time-ever “Tier 1” shortage may be declared for 2022, which will curtail Arizona’s Colorado River allocation.
As a topical matter, the need for communities to conserve water is a story that is gaining resonance throughout the western U.S. From the decades-long effort in Arizona to reduce per-capita consumption to the recent campaign in Las Vegas to eliminate ornamental lawns, water conservation is in the news.
And a key feature of many of those stories? A generalized statement about the typical amount of water that households use.
But what amount is that, exactly? An acre-foot of water is just what it appears to be: enough water to cover an acre of land a foot deep, which provides a far clearer mental image than the “326,000 gallons” alternative.
So, how many urban households in Arizona will that amount of water supply over a year’s time?
Is it… one acre-foot per household? One to two detached single-family homes? Two of them? Or is it more?
ADWR researchers are finding that the average acre-foot of water delivered in the Phoenix area supplies not one… not two… but three single-family households, on average.
The average number of homes served annually by an acre-foot of water actually increases to 3.5 when all of Arizona’s five “Active Management Areas” are taken into account.
While it is certainly fair to credit conservation-conscious residents for their water-saving habits, there are a lot of variables that come into play when averaging out water use.
Climate, obviously, plays a role. Homeowners in metro Phoenix are destined to use some measure of water greater than homeowners in mile-high Prescott.
The prevalence of multi-family housing has a big impact on average water use. The more family units built onto a residential lot, the less water each of those units uses, on average.
But the still rising number of urban residences relying on that single acre-foot of water is not entirely a function of better plumbing and drip-irrigation timers. Water-saving attitudes matter, too.
From 2001 to 2013, the period of a major landscape study by the city of Phoenix, residential water use actually declined 30,000 acre-feet, even as the population increased by more than 151,000, the size of a good-size city.
So what is the correct average number for households in Arizona that are able to satisfy their water needs with a single acre-foot?
Well, the “correct” figure is not absolute. Variables matter. But, in Arizona Active Management Areas, the average is at least three. And it sure ain’t just “two” anymore.