Et > awc

Hello. I use a flex daily schedule. The soil is mostly clay (I’ve read all the articles on how to set the parameters and have done a mason jar soil test). The root depth is set to 3 inches. When I realized the default is 6, I took some samples. I do think 3 is more realistic. The soil probe wouldn’t go much further than 3. The AWC is set to .15 - the default for clay. I moved the coefficient to .90 and the watering cadence has increased to daily. However, it’s been so hot and dry here in the mid-Atlantic that there is more ET than AWC every day with no change in sight, eg. .20 or .22 of ET. Does the fact that the system calculates ET>AWC mean I have other settings set incorrectly? If it can only hold .15, how is .22 evaporating? Should I be trying to increase the amount of irrigation to matche the ET? My understanding is that a flex schedule will only add enough to fill the AWC. Thanks!

@paulamccullough89, any reason Allowed Depletion is set to 30%? That is very small. The default is 50%. Since you already adjusted the root depth to 3", you should have Allowed Depletion back to 50%, and see how it works out. By doing this, you would increase watering from 0.14" each time to 0.23".

ET is calculated from reference ET and the crop coefficient. Is the crop coefficient correct for the type of grass you have? If it is cool season, the default is 0.8 I think, and warm season is lower. Higher crop coefficient increases ET.

Hello and thanks for the note. I moved the allowed depletion so that it would water more frequently. I was playing with that and the coefficient. I read that the coefficient is the best way to adjust frequency but that I should move it about 10% at a time. So I moved it from 80% to 90%. If I set the MAD back to 50% I was afraid it wouldn’t water frequently enough.

So, it sounds like I should try MAD = 50% and coefficient = 90%. When they were at 50 and 80 it definitely wasn’t getting enough water.

You have already set the Root Depth to 3", and that should take care of watering more frequently. Did you change Root Depth after adjusting MAD and Crop Coefficient?

What signs do you look for to determine that it is not getting enough water? Is the grass turning brown? For cool season, you should water the grass less frequently during spring, so that it can have better resistance when summer drought comes. And in the fall, you can help the grass roots go deeper by core aerating. And during the season, you can cut your grass higher to promote longer roots.

Another note is that AWC is 0.15 in/in. So if I’m following that correctly, for the 3 inches of root depth, the soil will hold (0.15 x 3) 0.45 inches of water. Thus losing 0.22 inches of water is feasible (at least in terms of the raw numbers). Others in this thread are asking good questions about your settings, I would continue to ensure those are correct.

Also the USDA has a soil study online. Set your AOI and soil depth and the system will give you an estimate for AWC in your area.

FIrst, I think 3" is too shallow for root depth. I know of no lawn/crop that the /desired/ root depth is that shallow. Assuming a lawn of some kind here, even if the roots ARE only 3" deep now, I think you WANT them more than that. I would think at least 6". The fact that you cannot penetrate deeper than 3" with the soil probe does not mean that your roots are not at that depth; it means that soil below that point has no water, which I think is a bad thing. I have clay too, and when dry, it’s hard to penetrate the soil at all. But after a thorough rain/soaking I can easily push it in 8" to 10". So, IMHO set your Root Depth to at least 6" (you might confirm the crop or grass type so we don’t go off on tangents here).

I also agree with others that the allowed depletion is too low, especially with your shallow roots. I’d definitely move that up to 50%. Then, assuming 6" for root depth, your soil will hold 0.15 x 6 = 0.9", and at 50% MAD, Rachio will apply 0.45" at a time. Meaning it will water about every other day. IMHO you do NOT want to water every day; it will result in shallow roots and more chance of stressing the grass.

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Hi all,

Thanks for all the help. I actually had already looked up the survey but it didn’t make much sense to me. Here is a screen shot in case you have any input:

The only reason I had root depth at 3 inches is because the company that installed the system had it set that way. They had 3 inch root depth, clay soil type, MAD 50%, cool season grass, crop coefficient 80%, efficiency 70%. The grass was really lush and green in the spring this year - we had the 2nd wettest April on record. I didn’t even get the system turned on until almost mid-June. But then it started to have brown patches that looked like not enough water. That’s when I tried to adjust things to get more water into the system. I guess I could call the company and inquire why they thought 3" root depth was appropriate. Sounds like that could be part of my problem. I knew about trying to train the roots to grow deeper by not overwatering but I can’t seem to get the parameters right.

Happy to have any input!

Thank you .


I see all loam down to 60”. Specify the AOI, by drawing a box around your house using the AOI tool. Then look at The Soil Data Explorer > Soil Properties and Qualities > Soil Physical Properties > AWC. Then specify the depth you want to evaluate (example 0 inches to 6 inches).

Thank you. I looked at the AWC rating 0-6 inches. Lucky me that my land literally has no rating. However, if I expand the AOI a little and capture my street, the common woods behind, then the street behind us, I can see some information. The houses right behind me and the common space between have about .15 AWC so I think that’s a good setting. The clay\sand\silt of the woods right behind is 22\55\23 but the neighbor on other side of woods is 17\48\35. I think that means I should mark my land as loam, not clay. So to recap:

cool season grass (unchanged)
soil loam (change from clay)
AWC .15 (unchanged)
root depth 6 (changed from 3)
MAD 50% (changed from 30)
efficiency 70% (default)
crop coefficient 80% (changed from 90)
nozzle in/hr - rotor head is .45 in/hr, rotary nozzle is .7 in/hr

I’ll give it a try and see how it goes!