Does Pool Water Evaporate While Covered?

[Rowan555]

My father is convinced my 24' above ground vinyl pool has a small leak because it's lost about 3" of water in the past few months. We closed the pool in October, and it didn't appear to start losing water until late February/early March. Of course this is when the temperature went up (we're in New Orleans) - and we've had some downright hot/sunny days lately. Windy, but very sunny. Our cover is on the pool pretty securely, but of course it's not airtight - so I'm thinking maybe the water is evaporating and escaping through the "loose" areas where the cover overhangs the pool but isn't snug against the pool. So who's right? Obviously I would like to think it's me who is right, as that means we don't have a leak! Our first two liners had leaks (we had the pool installed July 2004) so I'm a little paranoid when it comes to problems with the liner. Our pool survived Katrina, which was a miracle in itself - so I'm hoping it will survive this summer! I want to open it this week, but first I'd like to know whether the water level dropping a bit id indicative of a problem. Thanks - I lurked here for months before and after getting the pool, and the posts here were invaluable to me.

Susan

[duraleigh]

Hi, Susan,

it's lost about 3" of water in the past few months. We closed the pool in October, and it didn't appear to start losing water until late February/early March.

That much water in two months seems somewhat excessive. I assume it's not a solar cover so there would be some evaporation but maybe not that much.

However, there are sooo many variables. What about rainwater? Direct Sun or shade? Since it's an AG pool, does it look wet anywhere? If you decide it's a leak, are you going to drain and fix it? I think with a little detective work, you can probably answer your own questions.

Wish I could be more definitive, but that much loss perhaps could be from either cause (or maybe both!!! Who wins? )

Dave S.

[Rowan555]

It's a black winter cover, and the area gets sunlight for many hours each day - no trees, no really tall houses around it - when I reach my hand into the skimmer the water at the top feels HOT.

When I had the leak, there was one spot outside the pool that was REALLY wet - but when I checked last week, the ground is damp all the way around the pool - not really wet in any particular spot - but damp everywhere. That wouldn't IMO correspond with a leak.

I'll probably open the pool this week and hope for the best. I was just thinking that the sun beating down on the black cover might have caused the water to begin evaporating at a faster pace since it's warmed up these past two months - and since it's not a solar blanket, but a cover that overhangs the pool, it seems possible that water could indeed evaporate and escape. But who knows!

[markphin]

If your winter cover actually lays on the water, your cover may have some small holes or be worn enough to allow water to pass through. The weight of the cover itself on the pool coupled with any rain water on top of it will allow water to pass through any holes on the cover until there is equal foces on top of the cover as underneath. as the water on top of the cover evaporates the cover will continue to allow water to pass through until all is in equalibrium again. The best way to tell is to observe during a prolonged dry spell and see if your cover has any free standing water on it. If so, your cover is leaking.
Hope this helps.

[chemicalbalance]

3" in 2 months. I wouldnt lose any sleep over it. Open the pool and try to measure a loss in 1 week.

You may be surprised to know that your plastic winter cover will actually allow water through on a micro basis given enough time.

If you have rain water lying on your cover, it will exert pressure on your cover and you actually may have a little water seep up through and evaporate.

I'd turn on the hose for a few hours to gain the lost water and start swimming.

[docsobeck]

i can attest to the "water through the cover" theory. i used to panic about how much my water level went down over the winter (inground pool, solid cover). i would leave the water level up at closing (late september), and it would be down a foot and a half at opening (late may). i kept thinking that there just HAD to be a leak, but i wasn't losing water during the summer. since i pump water off the cover several times a winter, someone finally pointed out that i was probably pumping out pool water since there was some "pass through equilibrium" going on.

sure enough, my old cover was starting to get ratty so i put a brand new one on last fall, and i still had water loss, but not nearly as much.

if you take a look at how those covers are really made, it's not all that surprising. they're basically woven from strips of plastic, and the weave allows water to pass through very, very slowly.

not to worry... you could EASILY have lost 3" from pass-through.

[waste]

Susan, it seems to me that water's first two priorities are; 1) to seek it's own level or the lowest place it can get and, 2) Evaporate, ie - leave the place it's worked so hard to get (somewhere further down it's agenda is to give life, esp. to algae) What I've noticed on the many pools I've worked on is that a hole in the cover is the prime suspect, if you have one and try to pump the water off your cover, you will lose pool water (however, the faster you pump off the cover, the less pool water you'll lose). I can't question that some water may make it through a 'holeless' cover, but I think the amount would need to be measured with a well calibrated micrometer.
So I offer this other scenario:
When you get water from precipitation on top of the cover the overall level of the pool water is raised (both above and below the cover). Water in a pool freezes from the top down. Only the top ~10% of an iceburg is above water. If the water on top of the cover freezes, it will try to 'sink' into the pool, forcing the still liquid water up and possibly over the top edge of the pool. Since water expands when it freezes, when it melts you will finally notice that you've lost some water. (This isn't as elloquent as I might like, but should do for starters) In short, don't worry too much about the 3 missing " of H2O, until you notice a 'larger than expected' drop in the pool water after running it.