# Run time calculation in Flex Daily schedule?

I’m having a hard time understanding the calculation of run time vs inches of water in the Flex Daily schedule. I would expect the zone run time X the Efficiency X the Nozzle Inches Per Hour = inches of Irrigation shown in the Soil Moisture graph.

For example I have a zone with the following advanced settings:
Efficiency = 70%
Nozzle Inches Per Hour = 1.5 inches/hr

According to the Soil Moisture graph, the Flex Schedule will add 0.48 inches each day it runs. And on those days, it runs that zone for 23 minutes. By my calculations that would only deliver 0.40 inches, which is about a 20% difference. It may not seem like a huge difference, but for zones with lower efficiency ratings, the difference is much larger. What am I doing wrong in my calculations?

If you adjusted the suggested duration we have to apply a multiplier to keep the amount applied consistent (even if it is augmented in a certain direction), or things start to unravel pretty fast .

Hi Franz,

I appreciate your input, I’m just not understanding how that answers my question. Except in the general sense that all factors are related.

Maybe I can clarify my question. I do not want to modify the duration. I just want to understand how duration is calculated. I.e. what is the formula for Total Run Time for a zone on a given day? I’m not asking for algorithms for calculating watering frequency or the desired quantity of water (expressed in inches of water in the Soil Moisture graph).

In the example I made the following assumptions:

A) Inches of Irrigation from the Soil Moisture graph/chart = 0.48
B) Nozzle Inches Per Hour = 1.5
C) Efficiency = 70%

I assumed that Run Time = A / (B x C), but apparently I am mistaken.

Thanks again. I’m a big fan of Rachio, have 3 controllers.

I’ve plugged it into my spreadsheet so I can play with things and see what happens – Here’s the calculation the best I can tell.

You will need this:
Scheduling Multiplier = 1/(0.4+(0.6*efficiency))
How much to water in inches = Available water inches * Root depth inches * Allowed Depletion

Duration = (60*how much to water in inches)/(in/hr)*Scheduling multiplier

Yeah … I’m sometimes that nerdy to want to know this. My calculations seem to come up with the same amount as Rachio comes up with.

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Thanks Linn. That gives me the same results as Rachio and plays into my nerdy side for sure. Can I ask how you came up with the formula for the Scheduling Multiplier? That’s definitely the variable that I oversimplified.

One of the Rachio techs/engineers posted the formula on a thread in the community here a while back. But it seems to be a standard equation to use. This link shows using it: https://ccuh.ucdavis.edu/measuring-DU-run-time

If you really want to geek out here is a good write up on the equation itself. I have to admit that most of it’s over my head!!! If you go down to Appendix B you’ll find the same equation used by Rachio – https://www.irrigation.org/IA/FileUploads/IA/Resources/TechnicalPapers/2009/RevisitingTheSchedulingCoefficient.pdf

This has most of the other information for full geekdom.

Thank you to Linn & Franz for such in-depth references. I’m not sure I am able to follow all of the info in those technical papers. But I did try! I think I was taking a very simple view of Distribution Uniformity aka Efficiency. And The Schedule Coefficient paper on the www.irrigation.org website seems to highlight the flaws in my assumptions. And while I’m still a bit fuzzy on how they arrived at this ‘preferred’ Scheduling Multiplier formula. I’m now able to accept that it probably does a better job of accounting for different ways of calculating DU and comparing that to the ‘adequate’ concept. I feel thoroughly geeked out now. So thank you again!

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