So far, I have 4 feet of water in mine and the water level hasn't drope at all. We seem to get a rain about once a week, sometimes two weeks go by without anything.
I have a screen over my drain pipe that has 2 inch squares on it that's very visable and easy to see. It has not droped a single inch so far.
If it does, then I think I have a bad spot in my dam and I'll have to find it and fix it.
Eddie