There are also a few things you can do prior to having your irrigation system blown out by a professional. In a nutshell: 1. Go to your crawlspace or basement and turn off the water supply. 2. Outside, go to the backflow prevention device. Turn the ball valves to a 45 degree angle. Use a small screwdriver to open the two drain valves. 3. Cover the backflow with an old blanket or towel. For extra protection, cover that with a trash bag and secure with duct tape.