18 Aug 2008

How Many People per AF?

One of the most widely used measures of water is the acre-foot (AF), i.e., enough water to cover one acre (0.4ha) on foot (30cm) deep. An acre-foot contains 325,851 gallons (1,234 m^3) of water.

That's a lot of water.

So much, in fact, that water managers, reporters, et al. try to translate the figure into something easier to grasp. Equivalents in bathtubs, swimming pools, etc., are vague measures that mean different things to different people ("my pool is bigger than your pool"), but the worst equivalent is "the amount of water the average family uses in a year."

Why? Because use varies all over the place (does the family have a yard? live in Arizona? etc.) and various groups like to inflate/deflate the "number of families served" according to their agenda.

A water manager seeking funding for a new project has an easier time if he says the project will serve "5,000 families" instead of "4,000 families" even though the project produces the same number of AF. Someone who opposes the project can say (with a straight face) that the same project will only produce enough water for 3,000 families, etc.

So how many people can use an acre-foot of water?

326,000 gallons works out to about 900 gallons/day. In my forbes piece, I cited average use of 350 gallons/day for an LA family of three (117 gal/each), and proposed that the first 75 gallons/person be free. In this post, a researcher claims a "personal-use minimum" of 85 liters (22 gallons) per person per day. Many commentators on this blog have remarked that they use far less than the LA average but more than 22 gallons.

So, at "LA rates" of consumption, an AF is enough to support about 8 people; at "minimum" consumption, an AF can support about 40 people. That's a pretty big range.

Lloyd Carter adds his two bits:
Metropolitan Water District says 2 families of five, or 10 people, use an acre-foot a year, for domestic purposes. Apartment dwellers obviously use less (for landscaping) than do homeowners with large lot. A 10-member Hmong family living in a two-bedroom apartment would use less than 10 yuppies all living individually in beachfront condos. In Fresno... one family of five uses an acre-foot of water a year. Depends what you include in domestic use, amount of landscaping, etc.
Bottom Line: When you're talking water policy and water projects, just use acre-feet (or cubic meters) -- don't bother with "number of families per year," since that number means different things to different people and can be manipulated to serve particular interests.