Sue,
Just splash-out, rain overflow, backwashing and other methods of dilution reduce CYA. Evaporation does not reduce any chemicals in the pool as only water evaporates, concentrating the chemicals, and then fresh fill water just brings them back to their original concentration. If there is lots of evaporation, then whatever is in fill water gets added to the pool, so fill water high in TA or CH will increase those parameters.
CYA can get converted to ammonia by some types of bacteria and this usually occurs over the winter (if it occurs at all) when a pool is "let go" and chlorine is not added. There are some unexplained reports of CYA reduction even when chlorine was present, but most reductions appear to be when chlorine is not present (so that the bacteria can grow). Opening a pool that has had CYA converted to ammonia takes a lot of chlorine to get rid of the ammonia.
Richard
