A pool can pop up when it weighs less than the water it is displacing, just like a boat. If it's full of water, then it won't float. If it's empty of water, it most certainly will weigh less than water surrounding it. That water would be ground water (your water table) that may or may not be higher than the bottom of the pool. With me so far? If your water table is below the bottom of the pool, your pool won't pop up. It the water table is very near the surface of the surrounding dirt, it most certainly can pop up. If the water table is somewhere in between, you run the risk of it popping up.
None of the takes months, necessarily. It's not a process. If the water table is high, the pool will pop up pretty much right after you empty it. If the water table is below the bottom, and your pool is not full, and then the water table later rises high enough, then it'll pop up at that point. So that could take a day, or a year+.
You have to empty a pool to replaster it. That's the "high-risk" period that is unavoidable. You want that done when there is no possibility of a high water table (like end of summer perhaps). If you empty it before you absolutely need to, your risk increases. If you empty it when it might rain, or just after winter, your risk increases.
Now if you live in an area where there is never a water table anywhere near your pool, then you'll be OK. But you have to determine that before you make decisions about how low you let your pool get, and when.
sgma.water.ca.gov
