Drip math?

full disclosure, i am brand new to rachio and managing my own irrigation. new home, new system, early learning curve stage.

one of our perennial beds has drip installed. its rainbird hose. packaging says .9 gph with emitters every 12". unfortunately its all buried in mulch. best guess is there is ~350 linear feet of hose down over ~830ft2 of bed with ~30 emitters.

google says .9gph = .015gpm

using the formula rachio support sent me to determine PR for custom nozzle setup, this is what I’ve got.

PR = (96.25(30*.015))/830 = 0.05218373493

am i doing this correctly?

when i emailed rachio support back to confirm I was on the right track, Jared replied “Typically, 1 GPH equals approximately 1.60 in/hr. So if your drip hose is 0.9 GPH, then it should be close to 1.5 inches/hour.”

so now i’m thoroughly confused. what am i missing?

There is no way that one gallon per hour equals 1.6 inches per hour. A quick Google search brought up dozens of resources for a drip runtime calculator. See what you find and let us know.

Take a look at this and it should get you on the right track. (As an example, I have a continuous drip line with emitters spaced out 12-36" apart. The huge majority of my emitters are .5GPH, and I ended up defining a custom nozzle of .2 inches/hour)

@mckynzee, is there someway to get word to support that the Rachio provided calculator doesn’t work well for the drip emitters that snake through a bed for watering annuals/perennials? Several of this worked through this last summer, and the referenced link works great, and I think is pretty understandable. It would be good for support to have that link in hand.


what point do you think Jared @ Rachio was trying to make? i would have assumed the team at Rachio would be expert in water delivery, but I can’t make sense of his reply.

I think the calculator that he used probably works great for drip systems that work in a grid, and some people probably have that. Rachio support is some of the best I’ve seen, and their customer service and responsiveness is great. But having worked with help desks in the past, it would be impossible for each person who answers the phone to know everything.

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Just to clarify some of the confusion with the approach of using the equation above is the area parameter. If 0.015gpm minute is not being applied uniformly across the area, as it more or less would be in sprinkler-based lawn system, the full area of the zone shouldn’t be used. With drip systems the wetting pattern is far from uniform.

The method that @Linn linked to calculates the PR based on total amount of water to be delivered, and eliminates the use of area in the PR calculation. The downside is that you need to have an idea of how many gallons that you want to deliver, but if you do it’s much easier to understand in my opinion. The calculator spits out an area number, but it’s only used in the software to give you reasonably accurate estimates of how much water is being applied when your zone turns on. In that case the area only matters if you care to follow the Rachio usage estimates. In the method that you were given to calculate PR area matters a lot for your PR, but how do you figure out the area of the wetting patterns above ?

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wow, this looks like it could be a pretty great spreadsheet. if only i had a clue what any of it meant. i didn’t realize i’d need a PhD in plantology to program this darned controller out. i have no idea how many gallons each of my plants needs. nor how many gallons per watering cycle, assuming i knew what a watering cycle was. and don’t see any moisture levels button in the zone edit screen to view flex daily moisture graphs.

my lines are more or less in a grid and have equispaced emitters all of the same rate. in which case, is there an easier way to go about it?

hmm, so i guess my next step is to try and figure out how many gallons (per something?) each of the below need?

Heavy Metal Grass : qty 13
Walkers Low Nepeta : qty 31
Liriope Muscari Big Blue : qty 129
Fothergilla Gardenii Mount Airy : qty 9
Blue Angel Hosta : qty20
Kousa Dogwood National : qty1

@seuss, you can try something like this, but when I tried it for my garden it didn’t give me what I was looking for.


It has the simple inputs that you’re looking for, but I’d be concerned that the output will not end up watering your plants properly. That’s what happened in my case. Feel free to try it and see.

Do you know how long you water for now? If so, just use that time, multiply by the number of emitters you have, and the gpm, and you have the total number of gallons delivered. At that point you can use the calculator we linked to.

BTW, a watering cycle is just the time that it waters. For example if you water every other day now for 20 minutes, your interval is every other day and the cycle duration is 20 minutes. Whatever method you use should end up with numbers similar to what you have now.

Finally, you don’t need the PhD to use the controller. You can just used a ‘Fixed’ schedule and tell it the interval and duration that you want;. The downside is that it won’t change as the climate changes, so you may overwater in certain weather or underwater at other times. You need the PhD only to optimize. :wink:


If you don’t see ‘Moisture Levels’ in your zone, then you’re already using ‘fixed’.



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I would create a custom “sprinkler” precipitation rate of .2 or .3 inch per hour for the drip. If it is too wet or dry, trim back or ramp up. I keep hoping Rachio will create a drip setting. Make sure you put in the right soil, plant type and root depth. If you are growing vegetables, go with a setting for annuals. Drip seems to confuse everyone. A good resource is the Ewing Irrigation mobile app. One you load the app open and click on tools. You will find resources to allow you to convert drip gph to inches per hour. Hope this helps. Are you using 17mm drip tubing with inline emitters. This is the best drip method for non desert areas.

You are doing the correct math. To put it into perspective, a pop up spray nozzle has an average precipitation rate of 1.5 inches per hour and run time is typically 10 to 15 minutes. Most people will run a drip zone for at least 1 hour. I just saw your plant selection. Since these are new plants you must run more frequently until they are established. Where are you located? Normal rainfall and mulch will help, too.

I agree with @robertokc. Start here and watch how it does.

I just had to respond to the “i didn’t realize i’d need a PhD in plantology to program this darned controller out” and the answer is you don’t, but the more time you spend on programming your controller and and fine tuning the more water savings you will get out of it and the better off your plants will be! You COULD just setup the controller to mirror the exact same minutes per zone that you had on your old dumb controller but you wouldn’t really be getting much out of it :slight_smile:


Let us know how things go. I wonder if the guy from Rachio was just off some decimal points on drip. Maybe he meant .15 inch per hour. No PhD needed in plantology. Try to keep it simple. I have had my Rachio for a year. It is not 100 percent perfect all the time, but I really like the customized program for every zone.

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Take a close look at this picture, and you can understand some of the problems with translating a single dripper rate in gallons per hour to a precipitation rate.

  1. A single dripper will drip down into the soil creating a “cone shaped” area of wet dirt below the dripper. The further down the moisture flows, the greater diameter of the cone, and hence, the greater volume of soil that needs to receive moisture. So, it is not a linear relationship. This will expand outwards through capillary action to produce a “moisture circle” as shown in the picture above. Twice the watering time would not necessarily produce a watering depth twice as deep.

  2. The more clay that is in the soil, the more tendency to spread the moisture laterally. Sandy soil will have a cone with a steeper angle and will not spread laterally as far.

  3. AFTER the watering is done, these moist circles will continue to loose moisture to the dry areas of soil between these wet areas. So, this leaching of water away from the moist soil will affect the ability of the soil to retain the moisture it has received and continue to have this available to the plant roots.

  4. So, when you are watering “spots” surrounded by dry soil, you have to adjust the frequency of watering adjust for the increase rate water is being leached out of the wet “cones” by the surrounding dry soil. What this means, compared to an automated formula such as Rachio uses, is that the “rate” of moisture decay in the soil being calculated as a function of soil type, plant type, temperature and wind speed", is going to be UNDERESTIMATED by the Rachio calculator, which is geared to calculate for a uniform rainfall situation or uniform precipatation rate where is there NO leaching of water away from the watered spots.

  5. The DEGREE to which the Rachio formula will underestimate the rate of moisture decline in the wet areas is a function of how close the “wet spots” are to each other. The further apart they are, the more the water will be leached away from each wet spot because there is more dry soil all around it that continues to leach water away by capillary action.

  6. Since the spacing between plants varies considerably for each user, it is not possible to have a single formula that will accomodate both the spacing between the plants, soil type and plant type, to determine a more accurate estimate of how much or how often watering has to occur to keep adequate moisture around the roots.

  7. THE SOLUTION. Set up the calculation for soil type and plant type. After a watering occurs, probe the soil to determine if close to the planter if the moist area of soil is extending down deep enough to cover the presumed depth of the plant’s root. IF NOT, then you must increase the watering time. This can be most easily done by specifying a precipitation rate equivalent for the dripper that is actually SMALLER than calculated.

If you have a 1 gal per hour dripper, and find it makes a “wet spot” that is about 2 sq ft in surface area, then the approximate equivalent precipitation rate would be 1 gal per hour / 2 sq ft = 1 gal per hour / 288 sq in = 231 cu inch / 288 sq in = 0.8 in / hr precipitation rate. If this precipation rate entered into the Rachio calculation does not penetrate the soil to the entire depth of the root, then you might reduce this rate by 30%, and check the next watering cycle to see if it is penetrating deep enough.

  1. The second adjust needed to to compensate for the increased leaching of water away from wet spots to the dry surrounding soil. The adjustment needed to compensate for the increase rate of moisture decline at the wet spot, is to increase the FREQUENCY of watering. The easiest way to do this is adjust the moisture slider to re-water when moisture has fallen a higher moisture level than the default setting (80%). You might set it to 50% or 40%. Now you have the irrigator watering not only longer (to increase th depth of the moisture “cone” with each watering, but also more frequently, which will compensate for the increased leaching of water from the wet circles to the dry surrounding soil.

  2. After 2-3 watering cycles, and some soil probing, you should have made enough adjustments for having the moisture depth to match the depth of the plant roots, AND, the soil should be the appropriate dryness of moisture when re-watering is triggered again.

Probe the soil immediately after a watering cycle to determine if the depth is adequate. Probe the soil immediately before watering cycle to determine the soil has reached a reasonable state of dryness for re-watering to occur.

  1. Given the variety of plants, soil conditions, and relative spacing of the drip immitters, there will never be a perfect formula for everyone to use. However, with 2-3 watering cycle “adjustments” early on, you should arrive at a very acceptable watering depth and frequency of watering to matching the rate at which soil moisture is declining.

After these adjustments, the Rachio should be able to maintain near ideal watering with seasonal changes that affect watering needs.


Thanks all for the feedback and additional direction. Part of the reason I went with Rachio against the recommendations of several professionals was the reputation of the community. The commentary in this thread being confirmation of that decision.

I was of course half joking about needing a PhD in plantology. And am fully aware I can make this controller dumb like the one it replaced. But that would defeat the purpose, so trying to make good use of the tools capabilities. I am simply on the early end of the learning curve and was confused in part by Rachio’s reply to my original question.

@MarkSanDiego when you reference probing the soil after watering, are you referring to simply digging down and visually inspecting. or am i supposed to be doing this with an instrument / moisture meter of some kind?



You would use a soil probe designed to remove a small core of soil.


If you want a simple gadget that will work and can be purchased inexpensively, you could use a 2-3 ft piece of 1/2" angle iron. This is simply two 1/2" bars of steel welded together at right angles. You insert this into the soil, twist it, Sometimes you can use a simple piece of rebar 3/8" or 1/2" and simply push it into the ground, and then pull it out. There are enough irregularities on the surface of the rebar that drag some soil out with the rod and you can see how moist it is.

The problem with using an ordinary shovel is that the shovel would probably cut some of the roots of the plant. However, if you placed a dripper on the ground with no plant nearby and simply used it as a “test” dripper, then you could dig down with an ordinary shovel and see how deep and how wide the wet area of ground is below the dripper. You need to wet the area as large and as deep as you anticipate the plant roots will be for the type of plant you are watering.

Water moisture instruments are highly unreliable unless you are willing to spend a significant amount of money.

My comments in general are specifically regarding the setting of the Rachio for drip irrigation, which is unique its variability with respect to moisture calculations. Traditional spray heads do NOT have this problem, and usually you can use the calculations of the Rachio with no adjustment needed.