Automatic watering systems save time and water and will, in the long run, give you a healthier lawn. By setting the timer for the exact length of time that your grass needs watering, you will never have to worry about running off and forgetting that you left the sprinkler running.
To gauge how long to leave your sprinklers running, place a shallow pan, like a cake pan, off- center of the sprinkler's spray pattern. Measure the amount of water in the pan every 10 minutes. This will tell you how long to leave the sprinkler running for the exact amount that you want the lawn watered.