Water usage with emitters

I know my PR (3.2) and my run time. The only info that I do not know is area. What is a simple way to estimate the area for my drips? I usually have 1 drip going to one plant. Should I just use a standard number per drip and then add up all my drips (i.e. each drip is 1 sq ft and I have 15 drips so the zone area would be 15?)

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@Modawg2k How did you get from GPH to PR ? I’m interested to learn about the area as well so thanks for the post.

Here’s some info on drip irrigation fro Washington State University. Drip Calculator

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I actually emailed Rachio before and had a back and forth. They provided me the following doc. I have 2 GPH drips, and they said to plug in ‘1’ for area.

With correct soil, it seems like the number I get from the doc is pretty good. My drips run for close to a 30 minutes and my plants seem to be doing pretty well. Precip Rate

So that’s why I’m wondering if I should plug in 1 for each drip to help get close to actual water used.

@Modawg2k For some reason that spreadsheet isn’t editable by me, but I think I know what it’s doing. @emil describes the equation in this thread.

So the equation summarized there is key:

When calculating your nozzle output (precipitation rate, or “PR”), you’ll need to factor ALL heads/emitters. Here’s the equation:

PR = (96.25(A*B))/©

96.25 = constant that converts gallons per minute (GPM) to inches per hour. It is derived from 60 minutes per hour divided by 7.48 gallons per cubic foot. times 12 inches per foot.
A = #heads -or- #emitters
B = flow rate (GPM) per head -or- emitter
C = zone square footage

I’m an engineer, so I went into way too much detail and calculated the units for the 96.25 constant, which are 96.25 (mininftft)/(galhr). I have 1GPH emitters for shrubs, but the PR equation above is for GPM, so I have to use the value of (1/60) gal/min. So

PR (in/hr) * area (ftft) = 1/60 (gal/min) * 96.25 (mininftft)/(galhr) = 1.6 ftft*in/hr

Assuming a 1ft x 1ft area, the PR = 1.6 (in/hr)

A while back I dialed in my settings according to the ‘Water Use it Wisely’ watering guide. I plugged into their site that I wanted the watering time for a 3’ shrub canopy, and it suggested 8 hours. From the Rachio app. I eventually got to 7.5 hrs by creating a custom nozzle and using a PR of 0.2 in/hr. Now, going back to the last equation,

PR (in/hr) = 0.2 in/hr = [1.6 ftftin/hr] / area
area = 1.6/0.2 (ft*ft) = 8 sq. ft

So when I dialed down my PR to get the 7.5 hrs I was targeting, I was inadvertently ended up making an assumption of an 8 sq.ft area in this equation. Does that make sense ? Now remember that I assumed a 3’ diameter canopy for the shrubs in the “Water Use it Wisely” site. A 3’ diameter circle translates to an area of 7.1 sq.ft. That’s eerily close to the 8 sq. ft number that I ended up at in the estimate above, especially given that I my duration is 7.5 hrs instead of 8 hrs.

I could very well have made some basic mistake somewhere here. If not, it seems to me that:

  1. The area to use for your PR estimate should be an average of the canopy sizes in your yard. Perhaps the zone area would then be the average canopy size * number of shrubs/emitters.
  2. I question the use of 1 sq.ft. for the area estimate, but as I mentioned I might have made a very basic mistake. I’d love to hear from @emil or @franz, or any of the other experts.

Sorry for all of the math, but I needed to do it to make sense of it all!

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My way is ghetto, but seems to work, I use 1 square foot per emitter.

Do you know where the assumption comes from ?

Because I’m using 1 emitter for each square foot I’m covering.

My trees have 7-8 and my shrubs usually 1-2 and my pots have misters hooked up to the drip,so trying to get within 10% accuracy is far more work than gain.

At the end of the day the area for a zone is only accounted for in estimating water usage. Next week I finally get my flow sensor online so I don’t care about this number.

Not to mention a drip system with pop on emitters is a heterogenous system and trying to jam it into a system that assumes symmetry is a lost cause in my opinion.


It’s a bit strange, maybe because of what they ask us to do for desert watering. In my case, they recommend on the order of 8 hours. You’ve gotta figure that in 8 hours the radius covered by the watered area will grow wider and wider the more that the area under the plant is saturated. If our expected duration were shorter I would guess that the 1ft approximation would be pretty good.

This is true for the zones. I’m just trying to make sense of the GPH to PR recommendation. In the end I just tried to make sure that I got to the recommended duration and roughly the frequency, which I did. It’s just nice to know that the data I put in is close on most parameters.

Any defined correlation between the canopy size and the recommended root depth, and therefore duration, might make it easier to set up some sort of start-up calculator for new users. (ie. “What is the average diameter for the shrubs in your zone ?” … the answer plugs in a number for root depth. “What is the flow rate of each emitter per shrub and how many are there” … the answer, combined with the previous root depth calculates the PR.) The existing Flex algorithms take it from there for frequency and interval. A nice side benefit would be a rough estimate of zone area for those that don’t have a flow sensor.

Just my crazy thoughts, I have many. :confused:

If I ran my drip for 8 hours, my pots would be swimming pools. I’ll assume this is a desert requirement, I run my drips for 45 minutes 1 time a week. I also have vegitation that doesn’t do well in wet soil so I have to account for those as the smallest emitter I can get my hands on is .5gph. So I have to add extra emitters to account for the smallest possible watering.

Again, you are assuming symmetry. You are also assuming liked water requirements for various vegitation… Drip systems have to be physically tuned to their environment.

Take my yard, I have dahlias, hydrangeas, Japanese maples, atlas cedars and some cypress tress all on the same drip. My average will factor out the dahlias completely, so sticking to 1 foot per 2gph emitter is the only way I can keep it easy enough for how frequently I add/remove cultivars from my cutting garden and some new areas I have been building out over the past 2 years.

Also keep in mind, I have to account for the most frequently watered vegitation as well. I did get off my but last year and put my veggies on their own zone, but other vegitation will have to wait for some time, big daddy wants champion Bermuda first.

True, but what else do we have if they’re all on the same valve, other than adding extra emitters? I know it is a rough approximation, but the biggest obstacle I see right now for Rachio is smart setup. I’d guess that 95% of their customer base is, or what it could be, wouldn’t spend nearly as much time as you and I have on setup. If some non-ideal assumptions get them 70% of the way there with something they can easily understand, such as average canopy size, maybe it would be of benefit. Anyway, I’m ultimately just interested in the baked in intelligence in all of these calculations. Thanks.

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You have the right idea, I personally think rachio would be better served with using irrigation companies to install and setup.
My friend that runs one is currently learning the iro because he is getting more and more requests for efficiency but nobody wants to do it themselves, I guess that makes sense, because ppl don’t build out their own irrigation system. He and I have spent time on and off learning so he can setup customer systems.

It’s such a tricky problem, it could be modeled, but there are an undefined number of variables in this model. I wonder if this approach would be more palletable, ask the user for each plant type, the recommend the emitters for each plant and the run time. I’m sure the client app would ship with a terabyte sized plant lookup table, and the code would be madness.

I couldn’t agree more. What happens down here is that there are just a bunch of blanket recommendations made. “Run for 75 mins, 3 times a day, twice a week”, or “Water your lawn once in the morning, once in the afternoon, and once in the evening for 12 minutes, every other day”. These are the recommendations made even when the person recommending them has never seen your yard, don’t know your flow rates, or even your vegetation type. I’ve been dealing with it for years and it’s been a constant, confused frustration. Discovering Rachio and exchanges with the folks on these forums is finally giving me the ability to put some sanity on it all!


ive been following your posts awhile but i’m still having issues with my xeriscape. i have 2gph emitters on 40-80 plants. they are a mix of lantana, silverleaf, blue grass, mexican honeysuckle, and palo verde. by math i found around 2 in/hr? does that seem correct? i’m i calculage .12 to .14 soil absorption, just north of phx.

That PR sounds way too high. Have you run through the drip calculator here? Drip Emitter Calculator for Precipitation Rate & Area

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nope, thank you for the link! i have a large yard and like 40 plants in the front and 40 in the back on 2 drip valves. trying to save some money, epcor is killing me. my bill is around 180 in the winter and 350 in the summer.

No problem. The calculator should definitely help. Just be sure to walk through it all and don’t skip any steps. I’ve been using flex in the desert for a couple of years now with no issue once I had the settings dialed in. Be sure to keep an eye on your vegetation as we transition to summer. You’ll may need to adjust things a bit so you don’t lose any plants. Once I adjusted my settings for the first summer I haven’t touched it, and I’ve saved a whole bunch of water!


oh man, does the new version of the app not have the bar charts anymore? i cannot find it like the old app.

found it! its in zone - soil moisture.

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