Significant Figures by Peter Gleick

Water policy and water problems always seem to be someone else’s responsibility. Those farmers who use all the water; the guy down the street who lets his sprinklers run all over the sidewalk; the Central Valley cities that don’t even have water meters; the environmentalists who are demanding water for some inconsequential fish we can’t even eat; those swimming pool owners in hot Los Angeles.

The reality, of course, is that water problems belong to all of us. We all contribute in various ways through our choices of appliances, or diets, or Congressional representatives, or gardens. And every little thing adds up to stress our limited freshwater, or contributes just a bit more to water pollution that has to be treated or ends up contaminating a local waterway.

Just recently, however, an interesting piece of information became available that let me actually calculate something I’ve always wondered. How much water do those Los Angeles pools actually contain, and how much is lost every year to evaporation? Could we solve California’s water problems if we just addressed the “swimming pool” problem?

Here are the pieces of the puzzle:

How many pools are there? A new “Big Atlas of L.A. Pools” has just been released. It is a digital analysis of every swimming pool in the Los Angeles basin between San Pedro and the Hollywood Hills, and from Santa Monica to Alhambra. This analysis, by Joseph Lee and his research partner Benedikt Gross, identified 43,123 swimming pools.

[Yes, parts of LA have lots of pools!]

[Yes, parts of LA have lots of pools!]

Figure 1: Pools in a single block of Los Angeles, via Google Maps.

How big are these pools? The Lee and Gross analysis also provided an estimate of the average size of the pools in LA. According to them, the typical swimming pool in Los Angeles is oval-shaped and measures 16 feet, 4 inches (width) by 33 feet, 6 inches (length). Other research suggests that the average depth of a pool is 5 and a half feet. Using these data, the average surface area of a pool is around 430 square feet (the area of an oval is π*a*b, where “a” and “b” are the semi-major axis (half of the length and width)) and the average volume is 2,400 cubic feet, or around 18,000 gallons.

The area of an oval is π*a*b.

The area of an oval is π*a*b.

Figure 2. Calculating the area of an oval.

This suggests that the total amount of water stored in Los Angeles’ swimming pools at any one time is around 760 million gallons, or 2,300 acre-feet of water.

How much water is lost every year to evaporation? The Los Angeles Airport experiences an estimated 65.5 inches per year of “pan” evaporation – the amount of water that evaporates from a standard measuring device used by hydrologists – a four-foot diameter Class A evaporation pan. This rate of water loss is approximately (perhaps 25% higher than) what would occur off of the surface of a pool. Assuming there are no pool covers, this means that the surface area of all Los Angeles swimming pools would lose roughly 2,000 acre-feet of water per year to evaporation. [It is a coincidence that the total amount of water stored in LA pools is close to the same amount as lost to evaporation annually. That is due to the fact that the annual evaporation losses are about equal to the average depth of a pool.] Swimming pool covers can save an estimated 30 to 50 percent of this water, so actual losses are likely to be somewhat less (depending on how many pool covers there are and how they are used). By the way, as the climate continues to warm, evaporation from pools is expected to incrementally increase over time.

To put this in perspective, total water use in the LA Basin is around 600,000 acre-feet per year.

The bottom line: water use and losses from swimming pools in LA are relatively low compared to total water use. Certainly, pool covers should be used everywhere, and leaky pools should be repaired. And pool owners should certainly be paying the full cost of providing, treating, and using this water. But taking away those amenities is not the solution to the state’s water challenges.

Lawns, on the other hand….

(Stay tuned.)

Comments

  1. #1 G
    California USA
    December 4, 2013

    Interesting about those pools not being quite so harmful as one might first assume. There’s nothing like a decent empirical result to put something into perspective.

    Mandatory pool covers would keep some local businesses going, making them and installing them.

    Better yet, automatic pool covers that extend or retract at the press of a button, and that automatically extend over the pool when motion sensors detect no motion around the pool for 30 minutes or longer.

    Hot damn!, new invention! (assuming nobody else has patented that yet): OK, Peter, you can have that one if you like, in which case feel free to delete this post so it’s not “public disclosure of an invention.”

    Looking forward to what you have to say about lawns. IMHO, require graywater irrigation with shower/laundry graywater that is surplus beyond what’s needed to flush the toilets.

  2. #2 Daniel
    North America
    December 4, 2013

    Swimming pools? Let’s talk about how much water FRACKING uses, and how that water is permanently poisoned and can never be recovered.

    A fracked well can use up to 2 Million gallons of fresh water PER FRACKING INCIDENT, and each well can be fracked up to 18 times.

    We have about 36,000 fracking sites in the U.S., many of which sit on BLM land in the Western U.S.

    Do the math, it’s a lot of water we’re poisoning and pumping underground.

  3. #3 Lou Grinzo
    December 4, 2013

    Terrific post, Peter. This nicely points out how the human mind can leap to the wrong conclusion in a stock vs. flow situation — “Golly, there’s a LOT of water in all those pools, so it must be a Really Big Deal for water consumption!”

  4. #4 Thomas Meixner
    United States
    December 4, 2013

    Back of the envelope calculations are always a good way to go. I do sense that LAX evaporation estimates are probably low for LA as a whole given coastal location of the airport. I sense that overall value of ET might be slightly higher but not by much. Word of mouth at Tucson Water is that lawns use as much water per sq foot as pools.

  5. #5 Mike
    Orange County CA
    December 4, 2013

    Per exposed sq ft a pool uses 20 to 30% more water than the same area of a properly irrigated lawn

  6. #6 Thomas Meixner
    December 4, 2013

    Mike you are right about a properly irrigated lawn. Problem is most lawns are not properly irrigated. Folks regularly over water, water at wrong time of day, fail to adjust timing for seasons or allow some water to run down the street.

  7. #7 Chet
    Redlands, CA
    December 4, 2013

    I have a lawn of 4,000 sf and a pool of 450 sf. I killed off my lawn 5 years ago to avoid wasting 60% of my water on grass, and have left my pool empty the same time for the same reason. Apparently I was being irrational about my pool; my neighbors won’t appreciate the distinction. Grass is the enemy of California.

  8. #8 Water Dog
    ventura county
    December 4, 2013

    i realize Thousand Oaks is upscale and likely not typical, but there have been 10,000 swimming pool permits obtained from the City (population 131,000). Additionally, its estimated that more than 500 are ‘salt-water’ chemistry, which is also water that is ‘lost’ for re-use and generally contaminates a larger volume when it is discharged…

    • #9 Peter Gleick
      December 4, 2013

      Interesting. This also raises a new water challenge. When those pools are drained, presumably into the sewer/wastewater system, they impose an extra serious burden of salt on water treatment plants. I wonder if anyone has written about this issue, and whether it is a problem…

  9. […] So just how much water DO those oft-mentioned Southern California swimming pools use?  Peter Gleick works it all out at the Significant Figures blog:  “Water policy and water problems always seem to be someone else’s responsibility. Those farmers who use all the water; the guy down the street who lets his sprinklers run all over the sidewalk; the Central Valley cities that don’t even have water meters; the environmentalists who are demanding water for some inconsequential fish we can’t even eat; those swimming pool owners in hot Los Angeles. The reality, of course, is that water problems belong to all of us. … Just recently, however, an interesting piece of information became available that let me actually calculate something I’ve always wondered. How much water do those Los Angeles pools actually contain, and how much is lost every year to evaporation? Could we solve California’s water problems if we just addressed the “swimming pool” problem? … “  Read more from Peter Gleick at the Significant Figures blog here: Water Policy: What about All Those Swimming Pools in Los Angeles? […]

  10. #11 Jerry Cadagan
    Tuolumne County
    December 10, 2013

    As a general rule crops in the San Joaquin Valley such as cotton, pistachios, almonds use about 3 acre feet per acre per year. I’m horrible at math, but I think all the SoCal pool evaporation equals about 700 acres of cotton, etc. In the Westlands Water District alone there are about 600,000 acres in crops of one sort or the other.