Drip irrigation is the most efficient system of watering plants. Even if you are using harvested water from a rain barrel, you don't want to let that precious resource go to waste when watering your garden.
Using a rain barrel can save you a significant amount of money in a season. For each inch of rain that falls on 500 square feet of roof, you can collect 300 gallons of water to use in your garden, or your lawn. If we ever got rain.