Rainwater is excellent irrigation water — it is pure water, with no dissolved or added chemicals; it is at air temperature when you apply it, and it is free for the collecting. If you live in an area where water is of poor quality, or where it is expensive, rationed, or in short supply, consider saving the rain water that falls on your roof.
Saving rainwater can be as simple as placing a barrel under a downspout or as complicated as digging an underground cistern and pumping system. Barrels can be old-fashioned rain barrels, modern plastic barrels, used wine barrels, or used oil drums (thoroughly cleaned out). Water is easiest to collect if you purchase or make a diverter that lets you direct water either into the barrel or onto the ground when the barrel is full.
Unless the barrel is lower that the garden, you can irrigate directly from it with a low-pressure drip system. Fix a tap in the bottom of the barrel and run drip lines to the garden. The few pounds of pressure available in the barrel because of its height are enough to feed water to the drip system.