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.).