How much water do you lose via evaporation?

[Fuzz1]

I have a fairly standard in-ground Gunnite/plaster pool of about 25k gallons. It has been losing 1/2" water per 24 hours. Daytime temps are 95 degrees with around 15-20% humidity. Nights are about 70 degrees with similar humidity.

Is that excessive water loss under these conditions?

[Fuzz1]

Found references to "The Bucket Test" on the net, and will implement it today.

[chem geek]

The amount of evaporation also depends on your water temperature. If I assume 85 degrees for that, then I calculate 0.29" per day using the 95 degree daytime temperature and 0.33" per day using the 70 degree nighttime temperature, both with no wind. However, usually humidity rises at night as the air cools since the water in the air doesn't simply disappear so your night humidity at 70 degrees temperature is probably around 40% making the evaporation rate the same as during the day, or 0.29".

If you have just a 2 MPH wind at the surface of the pool, the evaporation rate goes up to 0.54" so your 1/2" is certainly plausible.

The bucket test will tell you definitively if you have a leak. Make sure the bucket is sitting in the pool water since you want the temperature in the bucket to be the same as that of the pool. Ideally, the bucket is a shallow pan not much taller than the water in it. If you don't have that, then try and fill the bucket to near the top with water and put the bucket mostly in the pool, but not so much that pool water would splash into it.

[Fuzz1]

After performing the bucket test I'm confident in saying that there's a leak somewhere. The bucket lost about 1/2" while the pool lost closer to 3/4". I had suspected an issue because when I turn on the pool filter after it has been off for a long time it takes a while to clear out the air. The return jets blow lots of bubbles and it takes the better part of a minute before all the air is gone. It's an older pool (1980's?) with a single bottom drain. I believe that bottom drain is piped to the skimmer, where there's a floating valve that prevents the skimmer from ingesting air when the water level is below the skimmer.

Would the next step possibly be to put a cork in the bottom drain and skimmer holes, and perform the bucket test again to fully confirm if this is a plumbing issue vs a shell issue?

Is it possible to fix if there is indeed a leak deep underground in the pipes? I see no signs of a leak anywhere on the surface of the yard.