>> I have 21 irrigation zones plus an additional 10 faucets with timers and drip systems.
Impressive! Do you walk around flipping valves several times per watering cycle, to hit each zone, or do the zone valves have timers also?
I'm just starting out, learning, and don't really trust the 9-Volt-battery-powered timer I bought in some big box store. I'm somewhat fearful that some coupler will let go while I'm at work, and blow city water at 30 or 45 psi through a 3/4" hose. The jet from one little 1/4" tube (0.17" ID) was impressive enough.
I read about a water-powered Y valve ("Galcon Alternator Valve", $20) that flip-flops from Zone A to Zone B each time the water flow re-starts. Or the 6-outlet automatic distribution valve (a "stepping valve" version) for $70. It steps to the "next outlet" each time the water flow stops for more than 60 seconds, and then re-starts.
