We are on a well so we are thankful for a good rain to fill our pool. We drain/backwash so that the skimmer works and let evaporation take over from there. If we have a big rain expected, such as tropical storm, we drain some water prior so we won't have have the problem of water behing liner. I am interested to know if we could retrofit an overflow valve to our existing pool...that would certainlly be helpful if we were out of town and couldn't manage the water.