I've ascertained that my 20,000 gal. in ground (which has had a solid cover on it since last September) is losing 1.0 to 1.5 inches of water a month. I discovered it when I went to turn the pump on in mid-winter because the temps were going below 32 degrees and it couldn't get prime because the water had dropped below the skimmer. I've been monitoring it since. (I live in the south.)

I called a leak detection guy who in the past has come out with his drysuit and tanks and dived under the cover to patch a leak in winter. He said he doesn't really service my area any longer and that an inch or a little more a month probably isn't a leak, but rather evaporation. I expressed my doubts as to this (I just don't recall having to add water routinely when it's covered) and he told me to put out a bucket of water, cover it with a tarp, and I would see it evaporate over time. By the way, I have continuous water tubes holding the cover down with no gaps in between them whatsoever.
I called a local pool place and they want--gasp--$250 to dive under and patch a leak. If an inch to an inch and a half of water loss behind the liner (figure on a monthly basis for this analysis) will not damage my aging liner, I'm thinking of waiting until I can get in the pool myself with my wetsuit in early May and try my luck at finding the leak (assuming there is one) with the food coloring technique.

So with the jury out as to whether it's a leak or not, I guess my question is, will this much water going behind the liner cause irreparable harm to the liner? And, I still don't think it's normal to lose this much water through evaporation each month with a solid cover.

Thanks for any ideas, comments.