# water evaporation rate formula ??

#### Jaxwaterboy

Anyone ever heard or know of an acurate formula for calculating evaporation rate of a pool ?
I am certain it depends on air temp, water temp, RH, agitation, air movement, disolved solids in water, etc, so I am sure it isn't an easy one, but this has really peaked my curiosity, especially here in NE FL with the drought conditions (RH between 20 and 25% with highs at 88 to 90) I have watched the water level drop almost 2 inches in less than a week. I do not have any leaks, and haven't backwashed (used the bucket on the ladder trick to verify)
any thoughts ?
figured this was a "geeky" question so I posted it here in the chem side...

#### medvampire

I think a that would be hard to predict and the math would change day by day. You may want to think about using a cover of some type to prevent evaporation.
Steve

#### JohnT

TFP Expert
Pretty tough one to calculate. You'd have to consider humidity, wind speed, air temp, water temp, cloud cover, maybe even sun angle among other things. You'd have to update the calculation as conditions changed.

#### Jaxwaterboy

there in lay the challenge !!!

I have found a few sites that discuss this, but I am not a scientist, nor chemist, so I mainly pose this question for theory and discussion. I am aware the only way to prevent, or at least slow down evap is to cover, I was just pondering if anyone had seen or developed a formula that took to count the above mentioned variables, and a few we may not be considering. (i.e. disolved solids, ph, etc.)

#### chem geek

TFP Expert
MarkS at the Pool Forum posted this link that gives evaporation rates in different areas of the country. These are annual rates so the rate in summer is likely to be much higher than that in winter, but at least it's something -- it seems that the Class A evaporation values are for May through October so perhaps this is closer to a summer season rate. It should be noted that the evaporation for water in a swimming pool will probably be less than the pan evaporation rate since the pan rate is high due to the metal pan getting hot from the sun. This is why the evaporation rate for lakes is about 75% of the pan evaporation rate. On the other hand, pool water is typically heated and may be warmer than air temperature or certainly warmer than it would be if kept in a pan for 24 hours a day. So maybe the pool evaporation rate is close to the pan evaporation rate.

#### KurtV

chem geek said:
MarkS at the Pool Forum posted this link that gives evaporation rates in different areas of the country. These are annual rates so the rate in summer is likely to be much higher than that in winter, but at least it's something -- it seems that the Class A evaporation values are for May through October so perhaps this is closer to a summer season rate. It should be noted that the evaporation for water in a swimming pool will probably be less than the pan evaporation rate since the pan rate is high due to the metal pan getting hot from the sun. This is why the evaporation rate for lakes is about 75% of the pan evaporation rate. On the other hand, pool water is typically heated and may be warmer than air temperature or certainly warmer than it would be if kept in a pan for 24 hours a day. So maybe the pool evaporation rate is close to the pan evaporation rate.
Interesting information, Richard. I'll bet that in my area of the country (Gulf Coast) the rate doesn't vary much from Summer to Winter as the temperature and humidity tend to track together (e.g. cooler but drier air in the Winter vs. warmer, wetter air in the Summer).

That site also makes it easy for me to convert the water level drop in my neagtive edge trough to the equivalent water levl drop in the pool. Thanks.