Thanks for the update. In my own pool, I kept the winter rains out (the pool has an automated opaque safety cover and I had the cover pump take the water into a drain, not into the pool. This winter, the CYA dropped from around 20 ppm to perhaps 10 ppm (it's hard to measure that low) and I maintained chlorine levels the entire time. That sort of change isn't very conclusive but it does appear that at least for some, keeping the chlorine level up prevents the CYA from at least completely going away. For those that let the chlorine go to zero, having the CYA go away is more likely. There are exceptions in both cases, but that seems to be the trend.

Richard