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Name: Lawrence
Status: other
Age: 60s
Location: N/A
Country: N/A
Date: 1999-2001

Does water evaporate faster(inchs per day) from a smaller surface area ie. a spa vs a larger surface area ie. a swimming pool? My spa (7 ft dia) loses 1 1/2 in. per day while my swimming pool (22' x 42') loses 1/2 in. per day. My pool builder says that is normal!

No, water doesn't evaporate from a smaller surface area than a larger one. There may be a couple different reasons why the spa loses more than the pool though!

1. If the spa is heated it will evaporate faster.

2. If the bubble jets are on (if the spa is equipped with them, I am guessing it is like a hot tub?)

3. If the spa is smaller than the pool and is painted with a dark color, the water will warm faster in the spa than the pool water would warm, this would just be the sunlight heating two different volumes of water.(this would occur regardless of color but would be more apparent if the paint was dark) The only thing I could suggest is leave a thermometer in both and compare the temperatures throughout the day and see if they vary by much.

Hope this helps

Dr. Baldwin

Isn't your spa warmer than your pool? If so, this is the reason its water evaporates faster.

Tim Mooney


I'm going to guess that in general you keep your spa warmer than your pool. Water evaporation is related to temperature, so that would explain a lot of it.

If they're being used by the same number of people each day, the spa will also lose water faster, as the water absorbing in the suits and hair and skin of each user will be the same, but since the spa is smaller, each bit lost represents a larger percentage of the total volume.

Donald Yee Ph.D.
San Francisco Estuary Institute
180 Richmond Field Station,
1325 South 46th St. Richmond, CA 94804

Dear Lawrence-

There are several factors affecting the rate of evaporation of a liquid. Probably the most important two factors are the temperature of the liquid that is evaporating, and the humidity of the air into which the water is evaporating.

If the air is "dry," the water will evaporate faster than if the air is very humid. And warmer water evaporates faster than cold water. Probably the temperature of the spa water is warmer than that of the swimming pool. And possibly the air around the spa is drier than that over the swimming pool.

Other factors being equal, the size of the container of the water should not matter.

Wendell Bechtold, meteorologist
Forecaster, National Weather Service
Weather Forecast Office, St. Louis, MO


There are many factor the can affect the rate of evaporation from a pool: volume, surface area, wind, location, chemistry of water, air temperature and moisture, color of the pool walls (different colors absorb at different rates), etc.

Water is heated up by the sun and by the warm walls of the pool. Infrared radiation is absorbed by water, mostly in the top 0.1 mm or so, while the visible light goes deeper in water. Warm water -having lower density- moves to the surface when it is further heated and evaporated. The mechanism is complex, however, it is not difficult to understand why the SPA loses more water than a pool - the question you posed.

SPA has a smaller volume of water than the pool. As such, for the same amount of sunlight impinging on the surfaces of the pool and the SPA, the water in the SPA is warmer, leading to more evaporation. Warm ground may make additional heat contribution. In fact, you can measure the water temperature at regular interval in the two reservoirs and correlate the results with evaporation rates.


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