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Sprinkler Coverage
09/06/10
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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