In areas where temperatures fall below freezing, irrigation systems need to be flushed of water for the winter. Any water remaining in the lines has the potential to freeze and burst. Using an air compressor, it's not that difficult to winterize drip irrigation. For many drip systems, a small portable unit such as those used to power pneumatic nail guns will work fine; you're going to blow out lines that normally operate under fairly low pressure, so a larger unit probably won't be necessary unless you have an extensive drip system. If you don't have or can't borrow a compressor, check tool rental companies.
Turn off the water to the irrigation system.
Determine a connection point to the air compressor. If your irrigation lines start at a sillcock (spigot), the hose end that screws onto the sillcock is an excellent point. If you have a more elaborate irrigation system, find a suitable blow-out point such as a yard hydrant or drain plug.
If you have a backflow preventer, you can leave it in place if you will be blowing air up to it. If it has valves, shut them now. If you need to use the downstream connection point of the backflow preventer as your blow-out point, remove the backflow preventer and store it in a place away from freeze danger. Backflow preventers may be damaged by compressed air blowing through them.
Make a hose adapter by assembling the fittings and adapters as necessary to transition from the air hose to the appropriate hose thread of your blow-out location. Use Teflon tape on each male thread, making sure to wrap the tape in the direction of the thread. Tighten all connections with pliers or wrenches.
Screw the completed hose adapter into your blow-out point.
Set the compressor output pressure to 30 to 45 psi to start. Adjust the pressure to a higher setting later if needed, but start low to avoid damage to the drip line.
Attach the air hose from the compressor to the hose adapter and monitor the water/air output from the drip line. If you have an irrigation controller, you need to manually cycle through the active zones to blow them all out. To prevent freeze-related breaks in drip irrigation lines and valves, blow out as much water from them as possible. Use several cycles from the compressor for each line if necessary.
If possible, temporarily uncrimp any hose ends in the drip line to help evacuate the water. Downhill crimps work best for this.
When air without significant water comes out of all of the drip emitters on a line, the line is sufficiently winterized.
If you have a backflow preventer and didn't remove it earlier, do so now.