Q. I don’t understand why I can’t apply the same guidelines from your tutorial and choose 2 or 3 heads with 70 foot spacing? That would mean a lot less sprinkler heads on my large acreage lawn. Other than not being able to aim them as selectively, I’m missing the reasons I shouldn’t go this route. But you caution against it, so I’m sure I’m missing something.
A. Someplace around 55 foot spacing things start to get all screwy. They do make sprinklers that will shoot that large radius. They are pricey, the cost works out about the same per square foot irrigated regardless of the spacing (funny how that happens!) The problem is there’s just too much wind drift, evaporation, etc. at those wide spacings. Plus to get water to fly those long distances you need big, heavy water drops with lots of momentum. Those big drops just beat the crud out of the lawn, and cause compaction of the soil. Think of what it would be like if a really hard rain storm occurred each time you watered. Where the huge droplets don’t compact the soil they may erode it. Golf courses and parks have fought this problem for years. Most city parks have now settled for 55′ spacing rather than deal with the grief of citizen complaints about dead grass.
The bigger radius heads work better with pasture grasses, where long unmowed grass blades soften the droplet impact and a few dry spots and general “ugliness” aren’t as important.
It also takes lots of water pressure and volume to get that water out there. 70 feet radius means you need 70 PSI and 30 GPM at each sprinkler head. That means probably 85 PSI or more coming out of the pump. Most systems with big sprinklers like that run at over 100 PSI of pressure, which means lots more wear and tear on the system, and a shorter life-span. With those high pressures, design becomes critical, mis-design a single thing and it is unforgiving; water hammer can rip the whole system apart in a big hurry.
Then there is the safety issue. You ever been hit by a 30 GPM stream of water flying from a nozzle at 70 PSI? I have, it knocks you on your butt and hurts like hell! Keep in mind that the really big impact guns used on farms reverse with enough force to kill you if you are struck in the head by the sprinkler arm. Liability is the biggest reason that parks and golf courses are ditching the big water guns for smaller sprinklers.
Bottom line is that using big radius sprinklers just gets really tricky and the results are ho-hum at best. It’s not a good solution in the vast majority of situations. If you do want to mess with it, get professional help with the design. Most of the sprinklers over 70′ radius are only sold by agricultural irrigation dealers.