Sprinkler 15

This is important, read it carefully!

Sprinkler Coverage:

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!

Jess Stryker's Landscape Sprinkler Design Tutorial

Step #3

Sprinkler Spacing

Page 1 of 11Sprinkler Spacing (Landscape Sprinkler Design Tutorial)

5/3/2009http://www.irrigationtutorials.com/sprinkler15.htm

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.



What if you need a smaller radius than the sprinklers available?

Almost all sprinklers have a radius adjustment device on them so that you can reduce the radius of the water throw. This is one way you can adjust for narrower areas. Keep in mind that for most sprinklers you can't reduce the radius by more than 50% without causing problems. The other solution for smaller areas is to use nozzles made to spray less far, or that spray a special pattern. An example of a special pattern would be the nozzles that spray a 4' x 30' rectangular pattern. These are commonly used in long, narrow areas.

Remember if you reduce the radius of the sprinkler you must reduce the distance between sprinklers by the same distance! Keep the coverage head-to-head!

Calculating the GPM for sprinklers when you reduce the radius is easy:

For spray heads you just use the manufacturer's chart. When you use the radius adjustment on a spray you are simply reducing the water pressure by closing a small valve in the nozzle. As the pressure drops, so does the radius. Just look at the manufacturer's chart for the radius you plan to reduce the sprinkler down to. Then read the GPM for that radius! For example, your designing for 30 PSI. The radius at 30 PSI of the sprinkler you selected is 15 feet with 1.85 GPM according to the manufacturer's chart. But you want the radius to be 14 feet. Looking at the manufacturer's chart you see that the radius of the same sprinkler is 14' at 25 PSI with 1.65 GPM. So the GPM of that sprinkler if you reduce the radius to 14' will be 1.65 GPM. That's because when turn the radius adjustment screw to reduce the radius to 14' what you REALLY did was reduce the pressure to 25 PSI!

For rotor heads the GPM stays the same no matter how



much you reduce the radius! That's because reducing the radius on a rotor doesn't change the amount of water coming out of the nozzle. To change the radius a small screw extends into the stream of water coming out of the nozzle. The tip of the screw deflects the water which "screws it up" (pun intended) so it doesn't go as far. This creates another problem, however, which is that it really messes up the uniformity of the water. So when you use the radius adjustment on rotors, you tend to get dry spots. This is one reason I strongly suggest that you use a smaller nozzle if possible rather than using the radius adjustment screw on the sprinkler. The other reason is that when you reduce the radius you really should also reduce the GPM of the sprinkler. Otherwise there will be a lot more water under the sprinkler with the reduced radius. Bottom line- use the radius adjustment screw on rotors only when nothing else will work.

Warning for rotors only: When designing systems with rotors do NOT rely on the manufacturer's stated radius for design. They get those distances by testing the rotors inside a building with no wind. The real world is harsher! If the gallonage of the rotor is less than 6 GPM the maximum spacing should never be more than 35' between rotor type sprinklers.

Stryker's Rule: the spacing in feet between rotors can never exceed the operating pressure in PSI at the sprinkler inlet (30 PSI operating pressure = 30 foot maximum spacing). Ignore this rule and you will be very sorry!

Sprinkler Precipitation Rate and GPM

The precipitation rate is the amount of water the sprinkler throws onto the area it waters, measured in inches per hour. (Inches per hour is how deep, in inches, the water would be after one hour if it didn't soak into the ground or run-off.) Precipitation rate must be considered when selecting your sprinkler heads to



eliminate water application uniformity problems (dry spots).

Spray Heads: Almost all sprinkler manufacturers make their spray heads so that you can mix and match nozzle patterns and the precipitation rates will still match for all the heads. This is referred to as "matched precipitation rates". Look for this feature when selecting your sprinklers. Important: do not mix different brands of spray heads and nozzles together on the same valve circuit without checking to see that they have the same performance specifications. Just because the nozzle will screw into the sprinkler body doesn't mean it's designed to work with that sprinkler!

Rotors: Rotor-type heads aren't quite as easy. You must select the appropriate nozzle size for each rotor in order to match the precipitation rates. A simple illustration will help explain. Rotor heads move back and forth across the area to be watered. The rotation speed is the same regardless of whether the rotor is adjusted to water a 1/4 circle or a full circle. So the stream from a 1/4 circle head will pass over the same area 4 times in the same amount of time that it takes for a full circle head to make one pass over the area it waters. With the same size nozzle in both, a 1/4 circle rotor will put down 4 times as much water on the area under the pattern as a full circle rotor will. (Remember that after every quarter turn the 1/4 circle rotor reverses direction and covers the same area again!) To match the precipitation rates between these sprinklers, the quarter circle rotor must have a nozzle that puts out 1/4 the amount of water that the full circle nozzle puts out! A half circle rotor must have a nozzle that puts out 1/2 the water of a full circle. This is why when you buy a rotor-type sprinkler head they often include a handful of different size nozzles with it. Wait, there's more (don't panic yet, there is a simple solution forthcoming)!

If you have rotors that are adjusted for different radii you will need to adjust the nozzle size to compensate for the radius change also! For example if most of the rotors are set for a 30 foot radius, but one is adjusted down to 20 ft., the 20 ft. one will need a nozzle 1/2 the size. (Remember: when you reduce the RADIUS by 1/3 you reduce the AREA by a little more than half.)



Avoid using rotors with nozzle flows that are less than 2.5 GPM, except in corners (quarter circle patterns). Flows under 2.5 GPM give very poor coverage due to the tiny water stream. Even a slight breeze will distort the watering pattern and give you dry spots. I strongly suggest that you stick to using nozzles as close as possible to the GPM of those in the cheat chart below.

O.K. Now that you understand the principles, let's simplify this a bit by using a cheat chart...

Unless you really know what you're doing (in which case you wouldn't be reading this tutorial), you should stick with the nozzles on this chart:

Jess Stryker's

Quick & Dirty Guide for Rotor Nozzle Selection

1. Find the section of the chart with your desired spacing. 2. Find the pattern (1/2, full circle,etc.) of the sprinkler. 3. The chart tells you the GPM the nozzle must have.

4. Use a nozzle size that comes close to matching both the PSI - GPM combination. 5. Ignore the radius given by the manufacturer. 6. Be sure to read the notes below the chart!

For 20-29' spacing between sprinklers-

1/4 circle . . . 30 PSI - 0.8 GPM 1/2 circle . . . 30 PSI - 1.6 GPM 3/4 circle . . . 30 PSI - 2.4 GPM full circle . . 30 PSI at 3.2 GPM Important: see notes below!

For 30-39' spacing between sprinklers- 1/4 circle . . . 40 PSI - 1.5 GPM 1/2 circle . . . 40 PSI - 3.0 GPM 3/4 circle . . . 40 PSI - 4.5 GPM



full circle . . 40 PSI - 6.0 GPM

For 40-55' spacing between sprinklers- 1/4 circle . . .55 PSI - 3.0 GPM 1/2 circle . . . 55 PSI - 5.5 GPM 3/4 circle . . . 55 PSI - 8.0 GPM full circle . . 55 PSI - 11.0 GPM

Important Notes:

It is critical that the water pressure (PSI) at the sprinkler be as high, or higher, than the distance between the sprinklers in feet (per Stryker's Rule). For example, if you space the sprinklers 45' apart, you must have at least 45 PSI of pressure at the sprinkler inlet. That's the pressure at the sprinkler inlet, not the total pressure available. Remember, you will lose pressure in the pipes and valves, so the pressure at the sprinkler inlet will be lower than your available pressure! Go back to the tutorial pressure loss pages to figure out how much pressure will be lost in your sprinkler system.

Select the nozzle size closest to these GPMs without regard to the radius the manufacturer gives. For example, if you are looking at a 25' radius, the chart above says to use a 1.6 GPM nozzle for a half-circle rotor. But you happen to notice that the rotor manufacturer's literature says that at 25 PSI, a 1.6 GPM nozzle has a radius of 32 feet. So why am I telling you to space it at 25'? When the manufacturer tested the rotor on their test range (inside a large building with no wind) they measured a few drops of water 32' from the rotor. When you install it out in your yard it will not perform as well. You may still get a few drops of water 30' or even 32' from the head, but not enough to grow anything. You need to trust me on this one! Remember, if the sprinkler sprays too far, most rotors have a radius reduction screw that will allow you to very easily fix the problem. But, if the rotor does not spray far enough there is nothing you can do about it without a major expense! Best to play it safe.

You may want to make additional adjustments to nozzle sizes after installation to compensate for your specific conditions. Most rotors



now come with a "nozzle tree" that contains most of the different nozzles for the rotor, so you can change the nozzle sizes if you need to. Some manufacturer's don't offer nozzles sizes larger than 3.0 GPM for their economy-priced heads (providing those extra nozzles would probably cost them at least another penny in costs!). You may need to upgrade to the next better model line if you have a large yard! The larger size nozzles for 40' spacing are not available with most of the "mini-rotor" models sold for residential use. You will need to upgrade to the next model. Also, sometimes other nozzle sizes are available separately from the manufacturer, for example low angle nozzles. You will probably need to get these from a store that specializes in irrigation sales, rather than a hardware or home store. Look in the yellow pages under "Irrigation" or "Sprinklers", or try one of the online stores listed in the tutorial links pages.

There is a conflict between the nozzles recommended for the 20-29' spacing range of the chart and my previous advice to "avoid using rotors with nozzle flows that are less than 2.5 GPM". This is because the Nozzle Selection Guide assumes you will be mixing 20-29' radius rotors together on the same valve with 30' plus radius rotors. To keep from having enormous nozzles on the larger radius rotors I am recommending that you use smaller nozzles than I would otherwise consider for the smaller radius rotors. This is essentially a compromise. Sometimes it is not practical to obtain perfection! If all or a majority of your rotors will be spaced at 20-29' apart, then you should probably use larger nozzles than I recommend in the chart. In other words, use those listed in the chart for 30-39' spacing for the 20-29' spacing. This will help avoid problems caused by the wind blowing the spray out of the irrigated area. However, if your sprinkler system will be located in an area with little or no wind you can go ahead and use the smaller nozzles in the chart. What is little or no wind? Go outside in the evening or early morning when you will likely be irrigating. If you can feel the wind blowing even gently against your face, I would consider that enough wind to need the larger nozzles.

If you calculate the precipitation rates you will notice that the



shorter spacings result in a higher precipitation rate than the larger spacings. This is because the smaller heads with lower GPM rates are more susceptible to wind and evaporation, and thus it is assumed less of the water is actually reaching the ground. The higher precipitation rate compensates for this.

Windy Locations

If you are designing a sprinkler system for an area where the wind blows a lot you should look at the Irrigation and Wind FAQ.

Select Your Sprinklers

If you haven't started shopping for sprinklers yet, now's the time to start checking out what's available. Check out which sprinklers are available and look them over. Write down a list of the heads you think will work well for your irrigation system on your Design Data Form. Be sure to list the PSI and GPM for each head as given in the manufacturer's literature, along with the maximum spacing between heads.

One last warning!!!

Do not blow-off my advice on sprinkler spacing in order to save a few bucks on sprinkler heads! Right now you may be feeling pretty smug about how much money you saved by stretching the sprinkler spacing. But next summer you're going to look pretty stupid to the neighbors, standing out there with a hose watering the yellow spots your new sprinklers don't cover! I have a collection of "wish I'd listened to you" letters from people who didn't take this advice. I get a few more of these every year, and these are just the brave folks willing to confess they messed up. They all say you should listen to me on this!



Later on you will need to know the flow rate for each sprinkler you use, so it might be helpful to make some notes on the back of your Design Data Form showing the nozzle size and GPM you will need for each different sprinkler you plan to use. Otherwise you'll wind up having to look the information up over, and over, and over...

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