Smart Cycle Question


Would love any resources discussing ASA. That said, my concern is that there are two equations for calculating run time before cycling occurs and one gives me a max run time of 60 minutes and one gives a run time of 1800 minutes from the same data. If the Iro is limited to a max run time of 3 hours as Emil pointed out above, then the tank isn’t even close to being filled in one case or we are over watering by 2/3. Is this possible? Is this right?


Great discussion. We’ll revisit this internally to make sure everything is correct.

The calculation comes from WaterSense testing documentation. The numbers calculated from the equation seem reasonable. Are there any that you disagree with? We list them on the Smart Cycle support article.

@plainsane & @derek_USSI – I will reach out to our IA contact and see if they can direct us to the missing documentation (AI-CLIA Training Manual Pg. 73 (September, 2004))

@derek_USSI, this is a valid equation for determining the run time to apply water. It’s used to convert inches of water to be applied divided by the precipitation rate of the nozzle into minutes of run time. However, we shouldn’t confuse it with the ASA calculation, which determines when water will begin to pool on the surface of soil.


Are you sure? Wouldn’t that be:
RT = ET(landscape) / PR(net) = Hr/day x60 = min/day?
ET(landscape) = ETo x Landscape Coefficient (K)
PR(net) = PR of nozzle x DU


When I say the documentation is wrong, I mean

If I was clear enough in my post and you knew that read on.

I’m dense and more often wrong then correct but the documentation states to subtract ir from pr which provides a ratio of quantity over time. Now taking an awkward but still physically possible scenario that my homie Derek pointed out which forced me to think about this, ask your self this question, if uptake rate of the soil is .1 inch an hour and the precipitation rate is .1 inch an hour we end up with an application rate of zero, seems to be impossible and causes a divide by zero when in fact we know that we are applying at the rate of .1 inch an hour. So that has a bad odor of infinity to me.

Now, obviously I’m running around this thread acting like I know a lot and that is wrong and dangerous, so this is the thought exercise I went through.

Using this formula in your docs:
ASA = Allowable Surface Accumulation
PR = Precipitation Rate. The amount of irrigation water applied per unit of time.
IR = Soil intake rate
Rt (max) = 60 (ASA)/(PR – IR), minutes
I assume the reason that ir is removed from pr is the fact that the ir will be removed from Asa as we water so we account for that by adding in more watering time to reach Asa.

So for me, and I might spend the time to validate this formula, i will adjust Asa to become a ratio for a moment.

Asar = (Asa / 60) / pr
Runtime = (asar + ir) * 60

What I have done here is converted asa to a ratio like pr, and computed a new ratio that defines how many times one goes into the other (we could have used algebra and made pr concrete by multiplying by 60, but who cares). Then we add back in the “loss” ratio of water from our desired asa that is absorbed by the soil from our watering interval and convert that to a real/absolute/concrete number/value.
Like I say, I could be way off base here because math and time is hard as hell and even more so when combined. I’m just trying to further clarify my reasoning as to why that formula is ever so slightly incorrect to the realities of the universe/world.

@derek_USSI I have not forgotten about ya, I’ll post some links tonight and also pm you some of my favorite jump offs that do not directly relate to this thread.

Peace out y’all and sorry for the confusions I have caused and presented

Point 1, there is some confusion for my self because we are focusing on a microcosm in a macrocosm. I can’t fix that.
Point 2, while in the shower, I think my brain farted, maybe a shart, you all be the judge. The removal of ir from pr is not done for accuracy, it is done for chronological efficiency. It in no way should have an impact on the amount of water applied overall.
I apologize so much for devolving this thread into an intellectual shit show. I still stand behind my assertion that the formula in the docs is wrong, but the reasons of infinity I posted in this reply and as I asserted early pr should not in fact be divided by ir, that is very wrong and I was not fully grawking my own thoughts.


Can I ask as I know little (nothing!) about ASA, if the chart says that for loam with no slope there is an allowable surface accumulation of 0.30 inches before runoff occurs, how does a soil accumulate surface water?

And if the PR is less than the IR can a soil accumulate surface water?


First if pr is less than or equal to ir then you can not accumulate water on the surface until the soil reaches its saturation point, but then you have other problems about to creep up.

Second, have you see water on a hydrophobic surface, like on the surface of a freshly waxed car? Water has a tensile strength all be it, very weak. If you hit the youtubes you will see some serious black magic videos. My favorite is the molecular water pump. That thing is intergalactic as hell.

As I stated in a previous reply, many things can effect asa like soil ph, water ph, water chemistry itself (how much calcium, etc). Certain fungus like many types of fairy ring spin out mycelium which are hydrophobic and will increase asa while decreasing ir.

If someone uses watering agents this can also significantly increase ir while as the same time decreasing asa.


Interested! You’ll have to send me links. And can’t find any info on Pace Turf website. Maybe I need a membership?


Yea they are a membership but they leak stuff from time to time
Non of these videos are directly relevant but, you will pick up on a few things that you can continue with on other sites.

Here is pace turf, what they publish, be selective from you jump offs

The gypsum verse calcium was relevant for another thread in here, but I was too lazy to follow, nor did I care about be right.

I love following this guy, his Ph.D. Is in turf pathology but he interacts with the industry’s best. He will publish he g+ video conferences from time to time, but they age off which is why I cant post certain links.
He is awesome and I’m not advertising his services, I think I got lucky, but when I started using trynexapacethyl, I sent him pictures which he review and educated me in agronomics and the virtues of stolon pruning.

I’m going to pm you some articles that will not directly apply to you, but they are ppl that you want to google, and lurk.
A great iPhone app is turfpath. Dr. Kaminski authored it and I get constant turf boners every week from it as turf managers report on it weekly. You will not learn from it most of the time, but it always provides great Google couch wear.

Here is a video,that runs away on glysopahate but you can pick up tide bits about asa and provides good jump off points for more hyperbolic videos that you can glean good info on Asa.

The great thing rite now is that turf management is on the cusp of becoming a well respect science discipline so a lot of these forward thinkers are trying as hard as they can to leverage any technology available to crowd source any respectable information obtained from unknown but fully capable managers on the course. These managers setup sentinel patches to test any idea or conjecture they read before rolling it out on mass scale. These YouTube videos are so informative and are often found from the recommendations of YouTube.

Now my most favorite videos (so much fun to wAtch but short) (science warning) great jump off spot

This one took me 10 bowls of weed to,get through but there is some amazing information in here (there references at the bottom)

Edit: doob, can’t believe I didn’t mention mica woods, that guy is a bad ass as well…he deserves more credit than he gets as well. Here is a jump off point Here you will see pace turf and mica in a typical video conference


@plainsane You are so above this question!!! I love that video of water moving up hill !!!
I’m simply asking how to reply to my multi million dollar homed clients and I getting physics that would send them into a tizzy! That is so awesome!!!


Maybe not.
ASA is the allowable amount of accumulative water on the surface before runoff.
(PR - IR):
If PR is the water going out and IR is the water able to be absorbed then PR - IR is whats left of the water going out that isn’t absorbed.
So if you divide the the Allowable amount on the surface by the remaining water not being absorbed you get the amount of time it will take using the water not absorbed to create that allowable accumulation.
So maybe the formula is fine.
The question might be how do you deal with run times that are large because of landscapes with deep roots (big gas tanks eg trees with 25" roots)? If you put limits then you’re not filling the tank and/or it stops being a flex schedule and becomes a fixed schedule. On the other hand you can get watering every 19 days for 340 minutes.


That is the point I was making with this comment,

Which is why I changed the formula to
Asar = (Asa / 60) / pr
Runtime = (asar + ir) * 60
This way it never introduces a divide by zero. But more importantly I feel it is more descriptive as to the intent. Aka, not reduce the accumulation amount befor computing a ratio, as this infers a removal when In fact we need to add back in , it is an addition. But I’m an engineer, not a mathalete so I strive for simplicity and clarity (completely subjective too).

So hopefully they break up watering across days for deeper root zones, not sure, the cap at 6 hours I think, Hence my other statement about us focusing on a microcosm. One thing I know, which is why I didn’t give this thread much thought, the rachio guys are on top of their game. Minus one oversight that I feel is not strait forward to correct (scheduling across iro’s to accomodate water pressure) everything been meticulously thought out. I’m willing to bet that they implemented something close to my ghetto formula.


From Robert D. von Bernuth (Technical Adviser - Irrigation Association)

You are correct in thinking that the run time until runoff increases without bound as the PR approaches the IR. In effect, what it is saying is that if runoff is the criteria, when PR is equal to or less than IR, you can run indefinitely. That is the definition of IR and runoff. It is the rate at which soil will accept water, so if you are applying water at or less than the rate the soil will take it, it will not result in runoff. Of course, there are many other criteria to consider. For example, the run time shouldn’t exceed the time to fill the root zone. If the time to runoff is less than the time to fill the root zone, it would be necessary to “cycle and soak”, and time to runoff would take precedence. If time to runoff is more than the time to fill the root zone, the time to fill takes precedence.

I have extensively researched this equation, and cannot find the origin. However, I have checked it against several other methods of estimating time to runoff initiation, and it gives reasonable results. I believe there is a better way to estimate time to runoff, and it is covered in the Irrigated Soils student manual.

Unfortuneately I dont have a copy of the Irrigated Soils Student Manual. :cry:


Uh shaw, isn’t that everybody’s favorite toilet reading?
But seriously, reading that email reply did give a little chub. He makes far more sense than i do.


@derek_USSI, couldn’t have said this any better myself.

It’s important to remember that drip nozzles (emitters, bubblers, & misters) do not use Smart Cycle since their PR is usually below IR and they need to run longer to fill the large soil reservoir due to the deep root zone depth.

@plainsane, correct! When running a watering schedule with Smart Cycle turned on, the Iro divides the total watering time for each zone, into a number of cycles. The number of cycles per watering time is determined by the maximum number of cycles needed by any one zone within the watering time.

This is exactly what Smart Cycle does, automagically given the zone attributes provided :wink:

Neither do I, but I’ll see if any of our sources know of said student manual.

Thanks for all of your research into this topic @derek_USSI. This has been a great discussion and helps us to double check our work. I hope my comments/notes helped. Let us know if you have any other open questions on the topic.

Best, Emil


But Hunter’s MP Rotators have a PR(gross) of just 0.39 in/hr. With a DU of 80% puts it down around 0.3. This would be less that the IR for most soils excluding those with clay.


The Landscape Irrigation Scheduling Coarse (Online) by the Irrigation Association teaches
Runtime (max) = iR/PR * 60
(with RT(max) being the allowable max time before runoff not max time for water application).
With Slope Multipliers as follows:
5:1 - 0.90
4:1 - 0.75
3:1 - 0.50
2:1 - 0.25


How is that value derived? Is that a single rotor for its max coverage area (is my assumption)? I would assume if you follow proper guidelines we end up with a much higher effective percip rate due to the overlap in head coverage. I have 4 head that all overlap with each other so I have an effictive pr of 1.9 (I have higher flow rate nozzles installed).


@derek_USSI, as @plainsane asked, how did you calculate this value? If your efficiency (DU) is 80%, you’ll need to run the sprinklers longer to achieve a net PR of .39 in/hour.

The Landscape Irrigation Auditor Handbook (3rd Edition) provides the following equation for determining your watering times:

RT = D/PR * 60

  • RT: run time (minutes), lower boundary
  • D: depth of water to apply (inches)
  • PR: Precip Rate (in/hr)

Taking the lower boundary RT, DU is used to calculate the upper boundary RT, which is simply multiplied by the scheduling multiplier – which is a fixed valve based on the DU %.

Assuming DU is 80%, the scheduling multiplier would be 1.14.


  • RT: unknown
  • D: .50 inches
  • PR: .39 in/hr

RT (lower) = 76.9 = 77 minutes

RT (upper) = 77 * 1.14 = 87.78 = 88 minutes

The Irrigation manager should select a run time between the upper and lower boundaries; the Iro takes the average of the two.

From here, the RT would be cross referenced to the Smart Cycle table as outlined in this support article. Please note, these are maximum allowable watering minutes as outlined by the EPA for WaterSense testing.

I hope this helps. In short, Smart Cycle works by taking the RT (either Rachio calculated, or user set) and is cross referenced to the Smart Cycle table. The number of cycles is determined by the maximum number of cycles needed amongst the zones in the watering schedule; i.e. if one zone needs 2 cycles and another needs 4 cycles, all zones will be cycled 4 times.

@plainsane, good point. The only way to calculate the true precip rate is to conduct a catch cup test.


I don’t have to derive it. It’s straight from the MP Rotator nozzle chart.

And this is most likely what one would use when customizing a nozzle.
That said you can derive it by the following formula:

PR (gross) = (96.25 x gpm) / (H x R)

gpm = flow of full circle sprinkler
H = head spacing
R = row spacing

And yes, not from one nozzle, but from one type of nozzle that all have a matched precipitation, weather by nozzle design( Rainbird MPR’s) or by choosing the appropriate nozzle output (rotors). But this only gives you the water leaving the nozzle, PR(gross). And yes this would be a higher amount of water as this would assume all water leaving the nozzle gets to where it is suppose to. The water hitting the ground would be this number multiplied by your distribution uniformity (DU) giving you your PR(net). And yes you can find your DU and your PR(net) by attempting a catch can test.
But how do you do a catch can test for a shrub zone?
And how accurate would they be if like in the video the homeowner walks out with the 9 catch cans, to his 1/4 acre back yard grass?


I may not have been clear. You said that “It’s important to remember that drip nozzles (emitters, bubblers, & misters) do not use Smart Cycle since their PR is usually below IR” but if one creates a custom nozzle of an MP Rotator, which has a PR of 0.39 in/hr (below the IR of most soils) the software will not allow it to Smart Cycle the same as drip nozzles aren’t allowed?