Irrigation + on field capacity chart not consistent with watering times


#1

Good Afternoon,

As an example, in one of my zones the current watering time is set to 51m with a hunter MP rotator with a precipitation rate of .48"/hr.

Yesterday the zone ran for 51m as scheduled in the Flex Daily, but the Soil moisture chart showed only .24 was applied under “irrigation+ ->Flex”.

My calculations show that .408" of water should have been applied to that zone during that time period.

I’m trying to diagnose a generally wet lawn, and I don’t want the irrigation to be inadvertently applying nearly double the neccessary water.

Thanks in advance,
D2


#2

I reviewed your account and used Front Left Lawn as an example to attempt to explain how amounts and durations are calculated. The math behind the moisture level is a bit less straightforward as it may seem. Unfortunately we cannot use direct formula of precip_rate * duration to calculate moisture level. The way flex daily determines the amount it needs to water per zone is using this formula:

DoW = RZD * AD * AWC

where
DoW - depth of water
RZD - root zone depth
AD - allowed depletion
AWC - available water capacity

Then we determine scheduling multiplier which accounts for nozzle efficiency and uneven zone distribution using this:

SM = 1 / (.4 + .6 * efficiency)

where
SM - scheduling maultiplier

Then duration required to water the depth of water is calculated like the following:

duration_minutes = 60 * DoW * RA * SM / PR

where
RA - user runtime duration adjustment
PR - nozzle precip rate (in/hr)

Based on our logs there were a few updates to advanced settings to this zone in last 30 days
one on 08/28 and few on 09/17 (today). Here is the breakdown of previous and current settings

As of 08/28
efficiency = 0.8
RZD = 4 in
AD = .5
AWC = .12
PR = 0.48 in/hr
RA = 1.50635800456472 means that duration was increased by user in schedule settings by approx. 50% from system derived duration at some point.

Plugging those into formula gives:
DoW = 4 * .50 * .12 = .24 in
SM = 1 / (.4 + .6 * .8) = 1.13636
duration_minutes = 60 * .24 * 1.50635800456472 * 1.13636 / .48 = 51.3524

Which correlates with irrigation event yesterday

Current settings
efficiency = 0.8
RZD = 6 in
AD = .5
AWC = .12
PR = 0.45 in/hr
RA = 1.50635800456472 same as before

Gives
DoW = 6 * .50 * .12 = .36 in
SM = 1 / (.4 + .6 * .8) = 1.13636
duration_minutes = 60 * .36 * 1.50635800456472 * 1.13636 / .45 = 82.165

Notice that after the zone settings update the schedule will now water 82 minutes. I hope this helps understand the calculations behind flex daily durations/moisture levels. A detailed discussion on this subject can also be found in this thread Refresh my memory on the moisture chart. Please let me know if you have more questions.


Fixed Schedule - Skip notifications coming at last minute
#3

Thanks @theflexdude.

I appreciate the in depth response.

So in summary.

To avoid my “soggy” fungus ridden conditions, I need to do the following:
Core Plug my Lawn to determine root depth.
Verify my soil type with the mason Jar test(will repeat)
Verify PR: Completed

Assuming all of those are Nailed down, and the sun conditions are correct, what do I do when the grass is still “wet” in the late afternoon? Should I start with what is recommended after those changes, and then step back watering timing until the ground isn’t wet?

I really would prefer two “heavy” watering over 3-4 “lite” waterings.

Thanks in advance,
D2


#4

One more question:

If my Root depth is 3" currently (as an example) and I would like to get it to 6", do I set the target at the Root level I would like to achieve, or step it up in increments?

If my logic is correct, going longer roots will increase watering time, and decrease frequency. Is this correct?

Thanks,
D2


#5

Yes - in order to have longer durations and less frequent waterings we need to increase root depth, this will force the system to think that it can hold more water. I would step it up in increments if you are setting it higher than the actual root zone depth of your crop and wanting to exercise it to grow deeper. In this case the crop may struggle when depleted until the root has grown. Also crop type and crop coefficient is something to consider. Those control how quickly your crop loses moisture which affects frequency. If soil is still saturated by the time next watering comes this means that ground hasn’t lost sufficient amount of moisture, while the system thinks it should have. This means we either over-watered and/or depleted too quickly. Adjusting crop coefficient and RZD will help fine tune this. If you adjust the duration in the schedule you will apply a coefficient to durations, which means that the system will think that the same depth of water is achieved using a user specified time. This is a fine knob to tune and last resort. It is always best to get a perfect combination using zone settings. Currently you are adjusted 50% more than recommended, I would start with a brand new schedule with defaults and see what this gives. Let me know if you have any questions!


#6

Thank you.

I’ve deleted the schedule and remade a new flex daily.

The Durations have gone up, but I suspect that the frequency will go down… it’s hard to tell because of the weather here lately…

Is my assumption correct?

Thank you.
Pete


#7

Yes, please take a look at moisture graph and see what your frequency looks like based on current weather and tweak it that way.


#8

Thank you for this thread. It really gives insight to how the settings interact.