Answer: Flood irrigation is most effective but you'll need a very level lawn and you'll need to build a berm all around to hold the water. Then, you'll need enough water pressure to fill the area with several inches of water. This isn't very practical for most people. So, you'll need to work with what you have.
You may need to adjust your sprinkler heads to direct the water to the right places. Your grass will be healthiest if the roots of the grass receive water every time you water. Watering to a depth of 10 inches is best. To accomplish that, apply about 3/4 inch of water during each watering session. You can use a soil probe, or a long screwdriver to test the soil. About one hour after you have watered, push in the soil probe as far as it will go in easily. Did it go in 10 inches? If not, you'll need to water longer until it does. Most pop-up sprinklers apply about 4/10th inches of water for every 15 minutes. Impact sprinklers only apply about half that amount.
You can perform a test to see how much water your system is applying. Take 6 or more flat bottomed cans, like tuna or cat food cans. Place them around your lawn. Turn on the sprinklers for 15 minutes. Measure the depth of the water in each can. Use decimals for easier addition (1/4 inch = .25, for example). Add them all together and then divide by the number of cans. That will give you the average watering depth. If you see variations in the depth that are larger than a 1/4 inch, you probably need to adjust some of the sprinklers or repair clogged sprinkler heads.
Divide .75 inches (that's the 3/4 inches necessary for a health lawn) by the number of inches you measured as your average. Then, multiply your result times 15 minutes to find out how long you should be watering your grass. Most lawns in the Phoenix area should be watered every 3-4 days, depending upon how deeply you are watering.
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