In addition to what Jack said, get a good programmable timer that can be programmed for multiple start times. Instead of watering each zone for 21 minutes (or whatever), water each zone for 7 minutes, 3 times. The soil can only absorb a certain amount of water before it starts to run off. Dividing the total watering into multiple short sessions allows you to water deeper, and wastes less water. Water only at night, of course. There is a web site that can help you know how much water your lawn needs given the current weather. Let me see if I can find it...