Pool Evaporation

Shaf77

Well-known member
Jun 10, 2010
242
Toronto, ON. Canada
How much is a realistic amount of water to lose due to evaporation? Just wondering if we have a small leak or just losing water due to evaporation. Losing about an inch a week.
 

DaninFLA

Well-known member
Apr 29, 2015
1,011
Sarasota, FL
I am in Florida, so a bit hotter and I lose probably around 1/4" a day during very hot and windy days. the wind is a big factor in evaporation.
 

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
12,083
San Rafael, CA USA
As was noted, wind is a huge factor. 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. The following shows 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.

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 temperture
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

Based on this link for Toronto, I'll assume during the day 80ºF air temperature with 55% relative humidity and nighttime 65ºF air temperature with 90% relative humidity. If you are gas heating the pool to 80ºF, then with no wind the evaporation would be 0.14"/day during the day (0.98"/week) and 0.15"/day at night (this is typical where day and night evaporation is the same unless humidity gets to 100% at night and water comes out of the air as dew). With 1 MPH wind, it's 0.21"/day, with 2 MPH wind it's 0.27"/day, with 3 MPH wind it's 0.33"/day, with 5 MPH wind it's 0.46"/day. However, if you are using a pool cover, then there should be no evaporation.
 

Shaf77

Well-known member
Jun 10, 2010
242
Toronto, ON. Canada
Wow!! Thanks for all the info! Will read up and try the bucket test.

If using a solar cover, should I leave it off for 24 hours to test with the bucket or how do I do it?
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
17,445
Tucson, AZ
Pool Size
16000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
Is there a good calculator for evaporation rate as a function of the various parameters mentioned?

I was in another thread where the OP was asking about salinity increase due to evaporation and high salinity fill water. It would be nice to be able to point folks towards a calculator if one is available.


Sent from my iPhone using Tapatalk
 

Divin Dave

TFP Expert
LifeTime Supporter
Oct 2, 2013
5,681
Longview, Texas
I would leave the pool and bucket uncovered for the day, so that temp, wind, humidity, etc is all constant.

Wow!! Thanks for all the info! Will read up and try the bucket test.

If using a solar cover, should I leave it off for 24 hours to test with the bucket or how do I do it?
 

DaninFLA

Well-known member
Apr 29, 2015
1,011
Sarasota, FL
Is there a good calculator for evaporation rate as a function of the various parameters mentioned?

I was in another thread where the OP was asking about salinity increase due to evaporation and high salinity fill water. It would be nice to be able to point folks towards a calculator if one is available.


Sent from my iPhone using Tapatalk
not sure why that would really be helpful? the main issues with evaporation, especially fast evaporation is people being concerned about having a leak. that is easily confirmed with the bucket test. as far as a calculator, not sure how helpful that would be, being that humidity, air temperature and wind speeds are so variably over a 24 hour day. you just wouldn't get great results. if your bucket tests confirms 1/4" per day, etc, that what it is. it really doesn't matter if you use a calculator and determine you "should" be getting 1" of evaporation and really get 1.5". if there is no leak, then you are losing 1.5" to evaporation :) no biggie.


as far as figuring out salinity increases, that's pretty simply. if you have an average depth of 4 feet in your pool, and you lose 1" that's 2% of volume decrease. so everything increases by 2%. 3500ppm salt becomes 3570ppm. negligible, and not within the range of testing. its the same issue with rain , the increases in total volume of the pool is so small its negligible

as far as someone filling up the pool with high salinity water, not sure on that thread, but that can be calculated pretty easily too. all you need to know is how much each inch of pool water is on a percent of the pool volume. if you know the salinity of the water being added, you can calculate the total increase in ppm per inch of water refilled. evaporation really isn't a factor in determining that, just the total volume added each time and keeping a running tally of salt ppm. but again, I don't think that is a very common practice at all.
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
17,445
Tucson, AZ
Pool Size
16000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
not sure why that would really be helpful? the main issues with evaporation, especially fast evaporation is people being concerned about having a leak. that is easily confirmed with the bucket test. as far as a calculator, not sure how helpful that would be, being that humidity, air temperature and wind speeds are so variably over a 24 hour day. you just wouldn't get great results. if your bucket tests confirms 1/4" per day, etc, that what it is. it really doesn't matter if you use a calculator and determine you "should" be getting 1" of evaporation and really get 1.5". if there is no leak, then you are losing 1.5" to evaporation :) no biggie.


as far as figuring out salinity increases, that's pretty simply. if you have an average depth of 4 feet in your pool, and you lose 1" that's 2% of volume decrease. so everything increases by 2%. 3500ppm salt becomes 3570ppm. negligible, and not within the range of testing. its the same issue with rain , the increases in total volume of the pool is so small its negligible

as far as someone filling up the pool with high salinity water, not sure on that thread, but that can be calculated pretty easily too. all you need to know is how much each inch of pool water is on a percent of the pool volume. if you know the salinity of the water being added, you can calculate the total increase in ppm per inch of water refilled. evaporation really isn't a factor in determining that, just the total volume added each time and keeping a running tally of salt ppm. but again, I don't think that is a very common practice at all.
Thanks for the post. I agree that there are certainly rough, ball park figures you can calculate but my original question had a different intent. Perhaps I will ask it over in "The Deep End" forum as opposed to cluttering up this thread.
 

carveitup

Well-known member
Jun 13, 2014
59
Burlington, Ontario
For what it's worth, I'm in Burlington and use a solar cover whenever we aren't swimming.. Rain typically more than makes up for evaporation in my pool an I end up having to drain off some water at least twice a summer.