Still confused about initial run time calculations


I see that 2.5 will introduce even more intelligence into the ways to calculate how often to water and how long. But all of these calculations are based on an initial run time for a zone, and then increasing/decreasing it based on some variables.

I’m still a little confused about how the initial standard run time is computed for a zone. For instance, I create a zone, and the Iro determines a certain length of watering time based on the type of sprinkler head, and I assume the amount of sun and type of vegetation. That isn’t a lot of information. I haven’t told you how many heads I have in the zone or the size of the zone.

For example, if I had a zone with one head making a 180 degree arc covering a zone, that zone would get less water per minute than if I had a zone with one head making a 90 degree arc, tossing the same amount of water over half of the area. Also, my Rainbird 5000 rotating heads I guess have a standard max flow rate, but isn’t the flow rate also determined by my water pressure or other factors?

I’m just wondering how I know that the initial recommendation of the zone is accurate? If my zone should be watered 30 minutes, and the Iro recommends 45 minutes, then no matter how many other variables are used as multiplies, it’ll still never be right.

I’m not accusing it of being wrong…I just don’t understand. Could someone please elaborate? Thanks! And I love the product! I just want to learn more about it.


Well, it doesn’t matter about the psi or the head or the coverage BECAUSE rachio assumes that the irrigation zone was installed properly which means that each zone is receiving a uniform distribution.

Now don’t light into me yet, uniform cant always be achieved with with most geometries you find in yards. So obviously some areas in a zone get more or less, this is referred to as “efficiency”. 100% efficient means a uniform distribution.

Now to really answer your question, when your system is designed and installed, the installer should be able to calculate your precipitation rate. “Pr” in the head setup of an app.

So, yes, psi, coverage and other things go into it, but all off that collapses under the pr. And if your efficiency is under 65%, I would recommend tossing your installer a 9mm salad (unless you didn’t want to pay the premium for odd geometries).

So if you have a pr of 1.5 inches an hour, your soil type defines the volume of water the soil column can hold, so, at this point we know how much it can hold and how fast we apply it, the only missing variable is time, and that is determined my volume / pr = time. There is more that goes into it if you dig on the forums you can find a thread I was in that got too detailed.

now, the discussion of how you raise your zone efficiency when you have a zone mixed with 90/180/360 heads is another discussion, but psst, you have to change the nozzle on the heads to increase the 360 head flow rate and decrease th 180, and decrease the 90 even more.

That is really what you are paying for from the installer, the cheaper the installer, the less they think about the efficiency.


@plainsane is correct on all fronts.

IMHO the biggest factors are precipitation rate and correct vegetation (and soil type if you know it).

To accurately measure precip (and efficiency) one of the best ways is to do a catch cup test. Search the forums (catch cup) and you will find a lot of information on the topic. You can even create a custom nozzle after retrieving all that information. If you don’t have the time or desire, picking the closest nozzle type is definitely recommended.

Hope this helps.



I’ll add, I never realized what @plainsane mentioned this until I had to replace one and read the documentation that came with it. There are different nozzle inserts that can be put in these heads to match all the watering. One that does a 360 circle gets a larger flow than one that is set to 90 degrees.

Of course, like many other things at our house… it was done wrong… I did switch to MP rotators that automatically match PR.


Mine was installed by my home builders “landscape guy.” Not a lot of faith that he did any sort of calculation beyond making sure every part of the yard got a sprinkle.

Thanks for the in depth explanation though.