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….