Evaporation of Pool Water - Further Reading

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Swimming Pool Evaporation: A Guide to Prevention

It comes as no surprise that pool leaks are a significant cause of water loss in your swimming pool. What you may be surprised at, though, is that while leaks are a major cause, they are not the number one cause. That honor goes to water evaporation.

You can lose 1/2" to 3/4" of water a day, or more, due to evaporation. The greatest evaporation rate can be in May and Sept/Oct in North America, when pool water can be in the 80's during the day and overnight air temperatures drop into the 70's.

According to the Department of Energy[1], swimming pools and spas lose up to 70% of water through evaporation. Leaks follow behind as the number two reason for water loss. For the purpose of water evaporation, let's examine possible reasons why this may be happening to your pool or spa.

Water loss through evaporation

We all know that evaporation is simply the process of liquid turning into vapor. When you see fog rising from your warm pool on cold mornings that is visible evaporation happening. So how does that make a difference for swimming pools? Well, evaporation occurs each time your pool is exposed to wind or air at the surface. In short, this is pretty much all the time unless the water is covered, in every pool. Water at the surface will form into a vapor. Eventually, this vapor is released into the air.

Interestingly enough, it isn't even day time when you can lose the most water through evaporation. Heated pools on cool nights will experience evaporation more rapidly. This explains why most of a heated pool's heat is lost overnight as the waters energy is used to change liquid into vapor as evaporation occurs. There are ways to fight evaporation, though.

Cover a Pool or Spa when not in use

The best way to prevent evaporation is as easy as using a pool or spa cover. Pool covers can reduce evaporation by up to 95%[2]. That alone should be enough proof to get you to consider using one if you haven't already. Aside from the water savings, there are other benefits to owning a pool cover.

Pool covers also keep and maintain pool warmth inside your pool. Remember earlier in the article; we mentioned losing heat in heated pools without a cover? Well, the cover will trap the heat inside the pool and reduce the energy usage of your heater. Your new pool cover can lower your Free Chlorine loss, reduce pool debris, and possibly let you reduce the amount of time your pump and filter needs to run.

The downside of a manual pool cover are the hassles in covering the pool when not in use, removing the cover before swimming, and storing the cover while the pool is in use. Some pool designs and shapes work better with pool covers then others.

Water features need downtime as well

Cool water features like the waterfalls and jets are great and make a beautiful accent to your pool, but turning them off will help prevent evaporation as well as reduce the pH rise they cause. Laminar jets and cascading waterfalls are the two big ones having the highest chance of contributing to water evaporation. It is recommended to turn them off when not in use. This will save you on water and chemicals anyway, and when you've got guests coming over, turn them on and have a great time.

Reduce your water temperature

You most likely have heard that warmer water evaporates faster than cooler water since the warmer water has more kinetic energy to change state from a liquid to a gas. Well, as air temperatures drop, such as in cold nights, the difference in energy between the cold air and the warm pool water is greater and the warm water evaporation will increase.

The closer the water temperature is to the air temperature the less evaporation you will have. Keep this in mind and lower the thermostat on the pool heater if your goal is to prevent evaporation and you want to save money on water and heating.

Add windbreaks around the pool

Surprisingly, wind on the surface of the water can cause more evaporation then the other factors discussed.

Some of the strongest and most intense wind in the country can be found in mountainous regions. The wind will undoubtedly have an effect on pool water, as wind can blow water particles and cause speedy evaporation. Other windy regions include the Great Plains, the Great Lakes and most coastal locations.

In windy areas the wind along the surface of the water can be causing the majority of the evaporation. A pool cover, landscaping, fences, or hedges can reduce the wind speed that hits the water surface.

Signs you may be losing water to evaporation

If your pool is only losing water in cooler months it is likely evaporation due to the temperature differential between the water and the air as discussed above. We often see members thinking their pool sprung a leak when evaporation increases as the seasons change.

One way to be sure to test if you're losing water to evaporation (or a leak actually as well) is to perform a water bucket test. Follow a structured approach to leak detection as described in the TFP Leak Detection article in Pool School. The bucket test as described in that article should be your first step to determine if water loss is from evaporation or a leak.

Evaporation Estimator

The Evaporation Estimator provides the estimated evaporation rate from these variables for a particular pool on a particular day.

You enter your zip code to automatically populate the current weather related information for your area. You can also enter weather related variables if you wish. Pool related variables are automatically populated with “typical” values although these also can be changed to best represent a particular pool.

Pool Water Evaporation Rates

This Water Evaporation spreadsheet will calculate the evaporation rate given air and water temperature, relative humidity, and wind. Note that this is the wind speed at the surface of the water which is usually quite a bit lower than general wind speed due to fences and the water being somewhat below ground level.[3]

To use the spreadsheet you enter Water Temperature in Celsius, Relative Humidity of Air, and Wind in MPH and the resulting evaporation in in/hr and in/day is calculated.

As described above, wind is a huge factor in pool water evaporation. Relative humidity is another factor, where you won't get evaporation when it's near 100%, while in dry low humidity climates such as desert areas you get more evaporation. Water temperature is also a factor.[4]

The following tables show how these factors come into play starting with a baseline of 85ºF water temperature, 80ºF air temperature, 50% relative humidity, and no wind at the pool's surface. The tables illustrate that evaporation increases as:

  • wind on the water surface increases
  • water temperature increases
  • relative humidity decreases
  • air temperature decreases.
Variation of evaporation with wind
0 MPH 0.21"/day
1 MPH 0.31"/day
2 MPH 0.40"/day
3 MPH 0.50"/day
5 MPH 0.69"/day
7 MPH 0.88"/day
10 MPH 1.17"/day
Variation of evaporation with water temperature
70ºF 0.07"/day
75ºF 0.11"/day
80ºF 0.16"/day
85ºF 0.21"/day
90ºF 0.28"/day
95ºF 0.35"/day
100ºF 0.44"/day
105ºF 0.54"/day
Variation of evaporation with relative humidity
0% 0.37"/day
25% 0.29"/day
50% 0.21"/day
75% 0.14"/day
90% 0.09"/day
100% 0.06"/day (evaporation still occurs in this case because air is cooler than water so is less than warm water vapor saturation; if the same temp then no evaporation)
Variation of evaporation with air temperature
60ºF 0.29"/day
70ºF 0.26"/day
80ºF 0.21"/day
90ºF 0.15"/day
100ºF 0.08"/day