Welcome to the Forum! We've never had a whole state join before!
Although sometimes it seems like all of Texas is here. It sounds like this is the perfect time for improvements. You need to know what kinds of soil(s) your pond is in before attempting repair. It sounds like you might be losing water to seepage which, combined with Summer evaporation, accounts for your seasonal level drop.
You should be getting a lot of good advice from dirt experts. I highly recommend you order a copy of
Perfect Pond: Want One? and pay special attention to the sections on fixing leaks and renovations. It'll be the best money you ever spend towards your pond.