# Seeking Precipitation Rate Calculation Guidance



## Movingshrub (Jun 12, 2017)

I need to some help understanding how to calculate the precipitation rate for smaller areas within a zone.

I understand how to calculate the PR for a giving area.

PR = (96.3 * GPM) / ( spacing between sprinklers * the spacing between sprinkler rows)

I see this as, PR = (96.3 * GPM) / Area covered by irrigation; If this assumption is wrong, please advise.

See attached photo. 


My concern is trying to determine whether I'm over/under/correctly watering the areas with different quantities of overlap. Some areas are covered by one sprinkler, others are covered by four. Assuming all of the sprinklers are matched precipitation, should I assume areas with a four are getting four times as much water? Initially I would have thought yes, however, the application by each sprinkler is not perfectly distributed across the throw zone. The rainbird irrigation design manual notes the peak water is closest to the irrigation head and then drops to less water as you get further away from the head, and points out that at 60% of the radius is where the drop off is likely to be insufficiently irrigated to support plants, hence suggesting head to head coverage.

So my concern is, if I have an area that's overlapped by multiple irrigation heads, but that's beyond that 60% threshold of the sprinkler radius, am I giving that areas sufficient, insufficient, or the appropriate amount of water? What happens when I push the sprinklers tighter than that 60% threshold, am I then overwater?

Moreover, what happens when I change the throw of the sprinkler head with something such as an R-VAN or MP rotator? is that 60% of the max radius where the application rate drops down to insufficient, or is it 60% of the radius of the adjusted throw for that situation? As an example, if I have an R-VAN 24, adjusted to 18 feet, is the 60% threshold 10.8' or 14.4'?

Lastly, how do you account for when you are knowingly overthrowing? I expect to get water on my driveway or my neighbors yard due to the weird shape, so that water doesn't really count for my lawn. Should I just artificially increase the area to include the overwatered areas when calculating my PR? I would think yes, until I'm throwing water against a neighbors fence.

@Rain Bird Corp Feel free to chime in. I live in math for my job and the design manual is confusing as F to me on the above points.


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## g-man (Jun 15, 2017)

I hoping someone answers this for you, but don't hold your breath. I think it is too complex to do without having detail information of the spray pattern of each head to get the precipitation rate individually.


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## Movingshrub (Jun 12, 2017)

g-man said:


> I hoping someone answers this for you, but don't hold your breath. I think it is too complex to do without having detail information of the spray pattern of each head to get the precipitation rate individually.


I'm not looking for someone to tell me the PR for each section I'm happy to do the math. I am just struggling with this feeling that this is more art than science.

To me this is really two steps.
1. What's the area?
2. How much water is being applied?

1. Easy to do
2. Not as easy to do. I struggle with this part because the sprinklers don't throw an even application of water over their span of throw. So I'm left trying to figure out what percentage of the output is being applied at that distance, and at what rate does the application drop off to zero, then figuring out what amount of the sprinkler radius is being applied to the section.


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## dpainter68 (Apr 26, 2017)

@movingshrub Would it not be easier to just do an irrigation audit?&#128556;


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## MMoore (Aug 8, 2018)

dpainter68 said:


> @movingshrub Would it not be easier to just do an irrigation audit?😬


only if the system is in place. this could be a design q for a new install.


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## dpainter68 (Apr 26, 2017)

MMoore said:


> dpainter68 said:
> 
> 
> > @Movingshrub Would it not be easier to just do an irrigation audit?😬
> ...


I was assuming (you know what happens when you do that) that it was for his current setup since this is how his yard is laid out. Although he could be updating/changing the current design.


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## Movingshrub (Jun 12, 2017)

MMoore said:


> dpainter68 said:
> 
> 
> > @movingshrub Would it not be easier to just do an irrigation audit?😬
> ...





dpainter68 said:


> I was assuming (you know what happens when you do that) that it was for his current setup since this is how his yard is laid out. Although he could be updating/changing the current design.


It's both. I want to know the math for a new install, because my existing plan for one section is so busted, that an audit is insufficient. I see a few choices. 
1. Come up with my best guess, install it, and hope for the best. 
2. Come up with my best guess, install and assemble entire layout above ground, connect it into the irrigation system below ground, then perform an audit.
3. Test individual sprinkler heads one at a time to determine application curve


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## dpainter68 (Apr 26, 2017)

@Movingshrub I was under the impression that the rotary nozzles precipitation rates were constant across the throw, even regardless of the arc or distance of throw (vs what you said the RB manual said about 60%). For example, the MP rotators have "approximately" a 0.4"/hr precip rate regardless if it's the MP1000, 2000, or 3000 at 90 deg or 360 deg (according to the Hunter material). I'm not sure if the RVAN's are the same or not. I came across this on Hunter's site about the MP rotators concerning uniformity:


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## dpainter68 (Apr 26, 2017)

Also just found this in the Hunter specification for MP Rotators:
"The standard MP Rotator family, designated by the black canister, has a matched precipitation rate of approximately 0.4 in/hr across any arc and radius to better match soil intake rates and prevent runoff. "

and

"3.5	Application Rate
A.	Models MP1000, MP2000, MP3000, MP3500, MP Corner, MP Strips shall produce and maintain a matched precipitation rate no greater than 0.6" per hour throughout the arc adjustment range and radius adjustment range, (up to 25% of radius reduction), when spaced at 50% of wetted diameter."

Found here: https://www.hunterindustries.com/documents/?f[0]=im_field_product_line:162&f[1]=im_field_product_type:234


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## g-man (Jun 15, 2017)

The precipitation rate of 0.4in/hr is only if you do the square pattern. That means four nozzles in each corner then the entire area inside the square will get around 0.4in/hr. But a single nozzle will not deliver 0.4in/hr to an area.


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## dpainter68 (Apr 26, 2017)

g-man said:


> The precipitation rate of 0.4in/hr is only if you do the square pattern. That means four nozzles in each corner then the entire area inside the square will get around 0.4in/hr. But a single nozzle will not deliver 0.4in/hr to an area.


Understood. I was coming at it from the question of the 60% in shrubs post and how changing the distance of the throw would change the precipitation rate. I believe that would be applicable on rotors but that's not my understand of rotary nozzles.


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## Movingshrub (Jun 12, 2017)

To follow up, what I'm really aiming for here is the math to support a densogram. However, without knowing the distribution rate curve, it's challenging to create a densogram.


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## hsvtoolfool (Jul 23, 2018)

Fascinating topic, and I expect Hunter or Rainbird can give you some insight. I've contacted Hunter via email several times and found them very responsive. And while I'd love to learn the answer to this very advanced question, my instinct is that you're over-thinking the problem. Maybe I'm wrong, but I think you're looking for an impractical level of perfection. All you want is the grass to be the same color and to use the least amount of water, right? This is like an architectural house plan with tolerances specs at 0.001".

You've placed the heads for the best possible coverage. Now group the heads into more zones than normal. This allows you to adjust the run-time for each zone to get the desired precipitation in an audit. More heads just means a shorter run-time for that zone. I'd put the narrowest wedge-section on a zone, the middle wedge-section on a zone, and the full-sized areas on two or more zones as needed.


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## Movingshrub (Jun 12, 2017)

hsvtoolfool said:


> Fascinating topic, and I expect Hunter or Rainbird can give you some insight. I've contacted Hunter via email several times and found them very responsive. And while I'd love to learn the answer to this very advanced question, my instinct is that you're over-thinking the problem. Maybe I'm wrong, but I think you're looking for an impractical level of perfection. All you want is the grass to be the same color and to use the least amount of water, right? This is like an architectural house plan with tolerances specs at 0.001".
> 
> You've placed the heads for the best possible coverage. Now group the heads into more zones than normal. This allows you to adjust the run-time for each zone to get the desired precipitation in an audit. More heads just means a shorter run-time for that zone. I'd put the narrowest wedge-section on a zone, the middle wedge-section on a zone, and the full-sized areas on two or more zones as needed.


Over thinking? Maybe. My more intense evaluation is due to the irrigation layout working well for my bermuda lawn, but not working well for my PRG overseed last year. I ended up with washout in some areas and under-watering in others. That opened my eyes to the possibility of the same problems in the other areas of my lawn that I didn't overseed at the time. Furthermore, when I sprigged my front yard this year, that showed me some different short comings. My objective is to fix the shortcomings that I've identified after living with the existing irrigation layout through three sprigging projects and one PRG overseed. As an example, I ripped up the entire sidewalk strip layout and changed it in 2018 after seeing how bad the coverage was when using a straight line layout of pop-ups, and using the 15 CST style nozzles. I've since staggered the popups and switched to R-VAN Side Strip nozzles, with much better coverage as a result of both changes.

I plan to rent a trencher to fix some known problems in my front yard. I know I have a problem with my side yard and would prefer the solution to be correct rather than just stabbing in the dark. It's both a triangle and a rectangle. It's the north side of my house, so some shade, with a small slope, and also has an oak tree competing for water. As a result, I'm looking for a solution that provides a sufficiently slow precip rate, that won't wash out an overseed or have run-off, won't drown the areas next to my house at the same time, won't overspray onto my vehicles, but also has good coverage so as to avoid dry spots during the summer or during an overseed. This area is about 1500 sqft. I've got 80+ PSI, plus the ability to put down 22GPM in the area.


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## hsvtoolfool (Jul 23, 2018)

Movingshrub said:


> Maybe. My more intense evaluation is due to the irrigation layout working well for my bermuda lawn, but not working well for my PRG overseed last year. I ended up with washout in some areas and under-watering in others.


Wow, that's a challenge! What do golf courses do for their winter greens? Hand water with a really soft spray nozzle until the seed germinates? Would a plug aerator and sand top-dressing help keep the seed in place? Maybe you need a teeny-tiny precipitation rate running for short periods many times a day. Something like ten 2 minute soaks each day. Good luck!


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## Movingshrub (Jun 12, 2017)

hsvtoolfool said:


> Movingshrub said:
> 
> 
> > Maybe. My more intense evaluation is due to the irrigation layout working well for my bermuda lawn, but not working well for my PRG overseed last year. I ended up with washout in some areas and under-watering in others.
> ...


My challenge was that it was bare ground, rather than a traditional overseed, due to the leveling effort. I also had shift irrigation head alignment during installation to throw around some trees, which have since been removed. My goal was to reduce PR so as to minimize the chance for run-off. I ended up having to basically do two seedings. Once with the right amount of water for the majority, and then another for the overwatered areas, while also using my sidewalk strip sprinklers, aimed back across the sidewalk, to water areas that didn't get enough water the first time around. It was definitely a piecemeal solution that I would prefer to avoid in the future.


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