more pictures

more pix

Push a screw driver into the soil and see if there is any difference in resistance or depth between the brown areas and the green.
What watering schedule have you been using?

There is the same resistance and depth if I use a screwdriver. I have been watering every other day for about 25 minutes per zone.

If not a watering problem, move on to insect infestation. Have you seen any tiny moths fly when you walk through the lawn? Google webworms and see if you see any signs. I believe Billbugs are usually are a KBG problem, but check for them also. While you are down there look for any powder substance, leasions, odd color, or fiberous growths or "webs" on the blades. Any of those would indicate disease and I'm lousy at disease diagnosis but someone else here should be able to help you.

In your first picture there is a very clear ring shape. That is a fungal disease. Some people call it fairy ring, but there are fairy ring "diseases" that cause no damage to the grass.

I have had good luck with an organic approach over the years. I would apply ordinary corn meal at a rate of 20 pounds per 1,000 square feet now and repeat in 3 weeks. Search this forum for "fungus disease tiemco" and you should find what tiemco has to suggest for chemical fungal disease control. If you try the chemical approach first, you cannot then try the organic. You do need to do something ASAP, though.

This problem is usually caused by too much water applied too often. Watering every other day is only better than every day. People in Phoenix are watering 2x per week in temps over 100 degrees F every day for weeks. There are many considerations in watering frequency, but it would be in interesting argument to justify watering in your area every other day. Do you know how much water your sprinklers put out in 25 minutes? You can measure with tuna or cat food cans. My system takes 8 hours to fill a can but my neighbor fills a can in 20 minutes. You have to measure your own system. You should only need 1 inch per week including rainfall.

By the way, many people do not believe that a fungus population can be that helpful or harmful. Take a look at the size of your circular area. You have enough pathogenic fungus in that entire area to kill your grass. I have a ring in my lawn that is 60 feet across. Those fungi are powerful creatures.