Martjane43,

This is an interesting situation. I have a couple of different thoughts but none of them make much sense. The first is that when it gets cold outside the water becomes ice and expands. But even if the pond froze solid it would not expand enough to go from half full to totally full. The other is that when it gets cold the ground water under the pond rises. The only way that this would make sense is that you have a drainage line that freezes up and doesn't allow the ground water to drain out and it rises in the ground pushing the water up in the pond. If this is not what is happening then to have a better idea what is happening I need more information. First is how big is the pond and how deep? The more important information is what else happens when the weather gets cold. Does it rain when it gets cold and dry out when it gets warm?
Mike

Thanks for your interest Mike! The pond is about 3' wide and
2 1/2 feet deep. We dug it deep so the fish could survive the winter at the bottom, which most of them did. As the weather gets colder it has not gotten cold enough to freeze over. Usually as it gets cold it rains as well. The drainage in our yard is crummy. Now that it is warmer out the pond is more consistently low, even when it rains!

Maryjane43,
The answer is clear now. The ground freezes during the winter. The pond will thaw out sooner then the ground. So when it rains when it gets cold the water in the ground can't drain as it sits on top of the ice under it. This water pushes the liner up along with the water in the pond. If the pond was full of water this would not happen unless the water in the pond is lower then the water in the ground. As the water in the pond maintains the same level as the water in the ground the water is allowed to escape the pond as the water in the pond rises to maintain ground water level.
Mike