I have a barrel that collects rainwater from the gutters on my house. At the bottom of the barrel is a manual 'faucet' with a round handle for turning the flow on and off. On that spigot, I have installed a "Y" fitting. One side of the "Y" goes to the irrigation hose which I turn on when the garden gets dry and there is water in the barrel. This irrigation hose goes down a slope and is always below the bottom of the rainbarrel. The other side of the "Y" goes to the bottom of a clear 1/2" plastic tube that is mounted alongside the rainbarrel so I can see how much water is in the rainbarrel without opening it. The top of the tube is open. Now comes the curious thing. With the rainbarrel full and the plastic tube showing full, I turn on the 'Irrigation' side of the "Y". The water in the plastic tube immediately drains all water and stays empty while the irrigation is on. When I turn off the irrigation, the plastic tube fills back up. So I have 2 questions. 1. How do I get around this? I really want to know the water level as I am irrigating so I can use a certain amount and then turn off irrigation. I might want to use 10% every day until empty. 2. How can this be explained?