Questions and Answers

Water Evaporation from a Pool

The evaporation rate depends on the surface area of the pool, the temperature, the relative humidity and the wind. I’m sure an “equation” containing the variables is known. Perhaps a civil or mechanical engineer would have a specific equation. “Splash-out” water is probably a much more significant source of water loss.

Here are more opinions and answers:

In my 14 years of experience, I found that there are a lot of circumstances involved in how much water is lost during evaporation. The best way to find out how much water your pool is losing is the following: Take a 5-gallon bucket and make a mark with a permanent marker at about the halfway point. Fill the bucket with water up to that point, and set the bucket right beside your swimming pool. Simultaneously, use a pencil to mark the water level on your pool tile. In that way, you can determine how much water has evaporated from the bucket and from the pool.
The evaporation rate of water from a swimming pool is fairly easy to calculate; given you have access to a psychometric chart or water vapor tables. W=(A (69.4+30.8 (V)) / Y )(Pw-Pa) W = lb / hr of evaporation. A = surface area of the swimming pool. V = mean wind velocity (mph). Y= Latent heat (approx. 1000). Pw – Sat. Vapor Press at water temp (in Hg). Pa – Sat. Vapor Press at room air temp (in Hg)
Wow! Who has enough time on their hands to run complicated math formulas? A simple rule of thumb in the pool industry is that 1/2″ to 1″ per day is normal evaporation. Evaporation more than 1″ per day would indicate a considerable amount of splash-out or a water leak.
In different areas the rate of loss varies, but in Houston, Texas, we figure on evaporation of 1″ to 1.5″ per week during the summer.
Some people don’t realize that water evaporation is much greater in the fall than in the summer — when water temperatures are high and air temperatures are cold (especially at night). This will just suck the water right out of the pool. I am sure there are a lot of variables regarding geographic location and humidity levels. In the Mid-Atlantic, I would consider anything over 1/2″ per day the mark to start looking for problems (unless you have a heated spa or waterfall, etc.).