If you havn't already done so, it's time to winterize your irrigation system before more freezes occur. Any freeze can damage an irrigation system and come spring you will have repair issues on your hands that may, at the very least, be tedious or, at most, expensive.
I don't know much about the technicalities of in-ground irrigation systems, I simply expect them to work when I turn them on. I dig the holes and trenches when pipes have to be repaired, while my husband is the brains behind making the system work. I do know that the first thing to do when winterizing is to shut off the water supply to the irrigation system and to drain it. Then, run the timer/controller through its normal cycle to open each of the valves to relieve water pressure and help drain lines. After that, turn the controller off or use the "rain mode" to shut off the signals to the valves. This maintains your programs and the clock stays on. On the other hand, you might just shut off the power to the controller, but then you will need to reprogram the timer next spring.
I found a thorough article on winterizing irrigation systems online at www.irrigationtutorials.com/winter.htm. It says to remove the backflow preventer, then drain and store it in a warm place for winter. Or, simply drain it and then reinstall it. The first method sounds more reliable against deep freezes. If you do leave the backflow preventer installed, wrap it and all exposed pipes (not sprinkler heads) with insulation to reduce damage from early and late freezes. After that, the article says remove the water from the risers and cap the risers. This site had a lot of good information and I suggest you check it out for a detailed winterizing process.
Drip systems also need winterizing. This requires that the lines be drained of water. After turning off the main water supply to the drip system, disconnect the hose from the filter. Wrap the filter connection with a plastic bag to prevent dirt from getting into the tubing. Open the ends of all the drip tubing to allow the water to drain. Lift the tubing where you can to help the water drain out or blow the water out of the line using an air compressor. Then close the ends of the tubes. Happy winter.
JoAnne Skelly is the Carson City/Storey County Extension educator for University of Nevada Cooperative Extension and may be reached at firstname.lastname@example.org or 887-2252.