02/09/12

The area watered by each sprinkler must overlap substantially the area watered by the adjacent sprinkler. This overlap may seem like a waste at first, but it is a very important necessity. Without this overlap it would be impossible to design sprinkler systems that provided uniform water coverage.

Have Doubts? See for yourself, it only takes a couple of minutes to prove! Grab a piece of paper and draw circles on it so that all areas of the paper are inside a circle, but no circles overlap. You can't do it, can you?

Important!
Sprinklers are intentionally designed to require 100% overlap of watered areas. That means each sprinkler throws water ALL the way to the next sprinkler in each direction. READ THAT AGAIN!

That's right, 100% overlap of watered areas is REQUIRED or you will get dry spots! This is known in the industry as "head-to-head coverage or head-to-head spacing". A lot of those free design brochures you find in stores get this wrong. They don't show enough overlap! The writers of those brochures think you are going to look at the overlap and buy the brand of sprinkler that shows the least heads. So they try to make it look like you can use less heads with their sprinklers. After you've bought the sprinklers if you have dry spots, well hey, it's YOUR problem now! You'll probably just buy a few more sprinklers to fill in the gaps. $$$ Ching, ching!

Hey, you're spraying water on my head!
Rule: Sprinkler Radius = distance between sprinklers

One more time: The water from any single sprinkler should actually get the sprinklers on each side of it wet!

Now that I've told you that you SHOULD use head to head spacing I'm going to backtrack a bit and tell you that you can space a few of the sprinklers slightly farther apart as needed to work around odd shaped areas. I still recommend that you keep at least 80% of the sprinklers at head-to-head spacing! Take the sprinkler head watering DIAMETER and multiply it by 0.6 to get the absolute maximum distance that should ever occur between any two adjacent sprinklers. (Remember most manufacturer's give you the radius of the sprinkler, you need to multiply by 2 to get the diameter.) For example, 15' radius spray heads should never be more than 18' apart (30' diameter x 0.6 = 18'). Note that we rounded to the nearest foot. If the sprinkler system is in a windy area I suggest the majority of the sprinklers be spaced at 45% of the diameter (that's closer than head to head!), as winds over 10 mph really mess up the sprinkler patterns.

Back when I designed my first sprinkler system in High School I wondered why they wanted so much overlap of the sprinklers. It seemed to me to be nothing more than a ploy to sell more sprinkler heads! I was smarter than that, so I stretched them out to save my folks some money! The result was big dry spots, and my parents wound up replacing the sprinkler system a few years later. (They never said anything about it to me, I just noticed the new sprinklers a few years later on a visit home from college.) Ouch! Not a good start for a future irrigation expert! Now that I'm a bit wiser and more knowledgeable I realize there is a good reason behind the head-to-head coverage. Unfortunately, it's rather hard to explain. The perfect sprinkler would put out a pattern of water that is heaviest right next to the sprinkler, then uniformly declines out to the radius. So the farther you move away from the sprinkler, the less water falls on any given patch of ground. When we test sprinklers for water coverage we set up a series of cups between the sprinklers to collect the water that falls. That way we can see how much water falls at various distances from the sprinkler. In the diagram below you can see what happens when there are various distances between the sprinklers.

In example "A" the sprinklers are just barely overlapping and much more water is falling in the cups next to the sprinkler heads. But the middle 3 cups are only getting ½ the water of the cups next to the sprinkler. If you watered long enough to keep the middle green, the areas around the sprinklers would turn to mud! In example "B" we see that moving the sprinklers closer together has evened up the amount of water a bit more. However the areas near the heads are still getting 25% more water than the other areas. Not enough to cause mud, but you would definitely see rings of greener grass around the sprinklers! Example "C" shows almost head-to-head spacing. The cups are almost all uniformly full! So don't stretch the distance between sprinklers.

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