the barrel has a lid at the top...sealed. I make a hole at the bottom to attach water hose to it. Then I turn on the water hose. The water volume in the barrel increases, no doubt about that. When It takes 7/8 of the barrel. Will the air in the top 1/8 of the barrel be compressed? I ask because the air has no way out. It has to be somewhere in the barrel. Needless to say that the barrel is empty but air to begin with.