Flex schedule miss-calculation?

Had this happen the other day with my flex daily schedule…heavy rain storm before 2 am did not direct system to not water. And then it watered again the next day also!!!

IF you have any insight into why this happen I would appreciate a response, thanks.

My guess on Jul 21 is that the forecast and the actual amount of rain that you got were different, and that’s why it watered.

The system is set up that once you get more precipitation than the field capacity, it will take your moisture balance up to a max of 110% (the logic is that once you are at field capacity, the soil just can’t hold any more and so it runs off). There is some rounding, so in your case the math for this zone takes you to 107%.

You watered on Jul 22 because you started out with a moisture balance of .16 in and with the Crop Evapotranspiration you were going to use .16 in, which brings you down to 0%, so the system had to water that zone since you were going to reach your MAD.

Sure wish you’d send a little of your rain my way!!! :wink: I’m having the opposite problem where I have a thunderstorm predicted every afternoon, so the system thinks its going to get rain, and then it doesn’t materialize or it skips my neighborhood!

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Ok…I guess that makes sense but why did it water again this AM?

Same reason as Jul 22. Today you started with a moisture balance of .15 in, but you were going to use .17in today, so you would have been below your MAD, so it had to water.

You might want to have the community take a look at the setup of this zone. Only watering .15 in at a time might be a little low. Can you post a pic of your zone setting?

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are you saying that the 1.65" of rain basically was run off and did not contribute to my MAD?

Here is my setup… Thanks for your interest!

Your available water looks low to me. I have clay with available water of 0.15. I haven’t looked at other soils, but I would expect them to be higher.


Actually, soils like sand are lower and is 0.05, meaning that water, for lack of better explanation, flows thru it quicker and isn’t retained as well. I guess the better question for @edbianc is asking if you are sure your soil is just sand?


The run-off actually has nothing to do with MAD. Your field capacity the way you have things set up is .3 inches, so some of that water didn’t run off. Depending on what time it rained, it added somewhere between .15 in (to finish filling to capacity) and maybe even .16 in more (from what your plants used that day). The other Remainder probably added a little bit to get soil to the saturation point and then the rest ran off.

Thanks everyone for your suggestions. My soil in Aiken, SC is sandy and very porous. Here is the latest details on moisture. It looks to me like the system is trying to keep MAD at 100% by watering almost enough to balance crop evap. Still does not answer the original question of why it watered on July 21 with 1.65" of rain…

Thanks for sending in a ticket to support on this one, @edbianc. It allowed me to take a closer look at your account settings in order to advise. I’ll reply to that ticket with greater detail, but you asked that someone comment here. You have a somewhat unique use case here. So, for the benefit of the community at large…

Seems like there are two main issues at play…

First concern | why is Flex watering [almost] daily?

Modawk hit on what I think is the crux of this, when he spotted the soil type - Sand.

After examining your settings, it looks like you have a lot of them “dialed in” (i.e. - less default values than we often see)
So, I’m pretty sure you know your soil. Just in case you haven’t, or for the benefit of other readers, some recommended reading – What type of soil do I have?
From that article:

Sandy soils are the opposite of clay and generally drain too fast, and so are unable to hold onto any nutrients long enough for a plant to use them. Organic matter helps to hold onto water and nutrients, and as with clay, it’s almost impossible to add too much

This is a valid conclusion here, and it sounds like it matches what we’re seeing. The zone in question (zone 5) is paired with a seemingly-identical zone 4, in a Flex Daily schedule. I’ll leave a few additional notes on this in your support ticket, but…

This brings me to your primary question.

Second concern | Why did Flex water on a day with so much rainfall?

Linn’s initial guess (below) is pretty dead-on here.

It appears Flex did its check that day at 3:16 AM, for a schedule set to start at 4AM. I’m not sure what it saw exactly at that point, but… it decided your warm season grass needed water.

In your first post, you mentioned that on that day there was a …

However, I’m not seeing that reflected by your weather station, which shows 0.00 precip until 6am, then a static 1.27in daily total for 10hrs. At about 4pm, a final shower seems to have pushed it to the 1.65in total you see for the day. See: http://www.pwsweather.com/obs/archive/2016/07/21/KSCAIKEN23.html

Of course, the Rachio can only go off the data it’s given. It could be that your selected PWS, at almost 3 miles away, didn’t record the same rain you (or your grass) saw.

I hope this helps clear things up a bit. Flex has a lot of moving parts, but if you’d like to “game” it a bit there’s good info in this support article on what goes into each “lever” : Flex Schedule Tips


Just FYI, it’s not trying to keep your MAD at 100%, it’s keeping it from going BELOW the 50% you’ve set. On each day, your starting moisture balance is less than what the plants are going to use that day, so it has to water.

Also. Have you checked your soil type in the Web Soil Survey? There might be a possibility that your soil has a lot of sand in it, but is not pure sand which would affect your AWC.

IMHO an available water capacity of .05 in. (based on soil type) is really low, and looking at your irrigation amount of .15 inches (shown in moisture graph details) and seeing your crop evapotranspiration of roughly .16 inches daily, you will almost always be running a deficit and watering every day.

I like @Linn’s advice. Having a soil type truly of sand seems questionable. Thanks @Lucas for providing a lot more context and detail.

Can you even grow warm season grass in sand? i would assume the roots would be super shallow.


Wow! Thanks Lucas for the great info.

Concerning the timing of the rain…my memory probably failed me in this case and the PWS is probably correct.

Still sticking with very sandy soil…let’s call it loamy sand. I will change the settings to this to see what happens. I believe it will reduce (slightly) the frequency of watering.

I used to water 3x/week fixed at 50 minutes each with my old controller. Now its more frequent (5-6x/week) but only 15 min/time. Grass (centipede mixed with Bermuda) seems to be thriving with the more frequent but less time watering. Probably due to the sandy soil not being able to hold much moisture. Temps have been around 95 during the day.

And yes Franz, the roots are very swallow, in many areas <4 in.