Sherra, what an intriguing idea!

As I first envisioned this, one would introduce fresh water onto the membrane (tarp) which has been spread over the pool. As the water accumulates on the top side of the tarp it would sink and gradually displace the old pool water while complying with the sides and bottom of the pool.

Can this be? I am not a physicist or an engineer, but both the old and the new water would be essentially equal in mass, so it wouldn't seem that one could easily displace the other. Perhaps the old water is pumped out at close to the same rate fresh water is introduced to the tarp.

If this all worked out, how in the world would one remove the tarp once the pool is refilled? Have I correctly understood what you were suggesting?

If there is a tried and true procedure, I'd appreciate reading about it.

-Jim