# Use of Efficiency in Watering Time Calculations

I’ve come across two of what I call discrepancies, or at least areas (2 of many) that I don’t understand about the Rachio results. In trying to understand operation of the Flex Daily system better, and to be able to fine-tune it to my needs, I’ve found:

Using 70% sprinkler head efficiency, 0.5 “/hr nozzle rate and the constant (in these examples) of my Test Zone, Rachio calculates watering 122 minutes resulting in an application of 0.84” of water to my zone. For this given Test Zone, Rachio will ALWAYS calculate 0.84" of water, applied about every 4 days.

Fine. Now, if I change the nozzle rate to 1"/hr, Rachio calculates 61 minutes of watering, giving the same 0.84" of water. Exactly half the time, for twice the flow, same water output. Excellent!

Finally, if I change the nozzle rate to 0.25"/hr, Rachio calculates 180 minutes of water, giving 0.84" of water. Whoops! With half the flow, to give the same amount of water, the time should be twice what it was for 0.5", or 224 minutes. But it’s not. That, I think, is a problem. Can someone explain it.

Similarly, with a constant 0.5" flow, and producing the same 0.84" of water, I get these times for 3 different efficiencies:
50% - 143 minutes
70% - 122 minutes
100% - 100 minutes

With half the efficiency, to get the same amount of water, the time should be doubled. But it’s not, it’s only increased by 43%. (About the square root of 2 = 1.414) Again, is the correct? It doesn’t sound right. Remember, the ONLY thing being changed here is the nozzle flow rate and efficiency. Rachio calculates the same 0.84" of water will be applied in all cases. How can that be?

I know that Rachio uses sqrt of efficiency to determine how much to water. So For example, if the nozzles are putting down 0.5"/hour @ 100% efficiency, then at 70% efficiency it would be putting down 0.5"/hour * sqrt(0.7) = 0.418"/hour.

sqrt is fine in this case. It smoothes out the numbers.

I am not sure why things are behaving the way they are when you change nozzle rate to 0.25"/hour. Maybe there is a limit on watering time of 3 hours (180 minutes)? In practice, are there sprinkler heads that put this little water per hour? It would take forever to water multiple zones. And it would be a costly mistake if the user puts wrong rate and ends up watering for hours and hours without this limit.

(How the heck do you embed quotes on this forum. Still can’t figure it out.)

(I just wiped out a half hour’s typing - you’re welcome - because I’m learning more about Efficiency.

moisture said:
“I know that Rachio uses sqrt of efficiency to determine how much to water.”

Reading more (and I’m going to do more yet, of course) Efficiency has to do with how evenly the water is applied to the zone. 100% means all the catch cups fill evenly. A lower percentage means it’s uneven (not sure exactly how yet, but will find out), and Rachio attempts to increase time/water to adequately water areas that receive less water. But using Efficiency directly, with a lower efficiency, can greatly overwater the rest of the zone. So Rachio uses the square root of the Efficiency, moderated the correction some. It may be a SWAG, just trying to get low efficiencies from producing ridiculous times, or may be base on science/geometry. I don’t know, but thinking about it this way does make sense. Now.

You can highlight the section with the mouse, and hit Reply. It will automatically add the text for quote.

Hey, that works. Thanks.