> The only way to know for sure whether permanent compaction has occurred is through expensive core samples or other measurements at depth

Wouldn't it be for rain to fall and the aquifer's levels to rebound less than expected?

That is what the one expert quoted said but others thought it might still come back over a longer time period - I interpreted it to mean that method as being more accurate than the one season refill rate.

Yes, looks like they are acting like a sponge whose efficiency to retain water decreases as time passes.