# ET and root depth

So I’ve been thinking about this for some time now. How fast soil loses water is a function of daily ET (and soil type, but I’m ignoring it in the context of this question). Now, the transpiration part of ET depends on the plant type. The evaporation part is the function of the temperature. However, the longer the root depth, the deeper we go, the amount of water evaporating from that depth should go down. So, as the value in the root depth field is changed, shouldn’t ET be adjusted (I imagine the amount of evaporation between 4" and 12" is quite substantial). How does Rachio’s algorithm handle this?

ET and water usage is not a function of root depth. You might say it is, as it regards the ET value for trees versus shrubs versus grass, but each plant type has its own Crop Coefficient, which takes that part of it into account. Soil type doesn’t really affect it, although mulch or ground covering does.

Basically, ET is calculated from the FRET ( Forecast Reference crop EvapoTranspiration), which is the estimated depth of water that would evaporate and transpire from a reference crop under the forecast weather conditions on a daily basis. While it may be an actual Crop, I’ve heard it defined as the water that would evaporate from a pan of water sitting on the ground. Temperature, humidity, wind, sun, etc. affect that value.

Then the ET is calculated from the FRET value by multiplying it by the Crop Coefficient. A sun/shade coefficient is also considered, maybe something more.

Anyhow, for a given type of crop, the root depth does not play into how quickly water is used (the ET factor).

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I see. What you are saying, is that the root depth should already be reflected in the crop coefficient for a plant. So as a young tree matures, for example, and its roots get deeper, the crop coefficient may change. Although, as a tree matures, it also gets more leaves, so while evaporation may decrease as roots get longer, the transpiration will increase. So what we really IDEALLY/TECHNICALLY need to do is adjust the crop coefficient based on standard industry tables.

Another important tidbit I picked up from your comment is that Rachio’s Crop Coefficient on the Advanced page is a combination of a plant’s crop coefficient and its location’s sun exposure. Which means that we have to be careful adjusting that number to a specific plant type. For example, I have a zone that is entirely roses. I COULD adjust the crop coefficient to a number that is more aligned with roses than general shrubs, but then I’d also somehow have to account for the fact that my roses are in a semi-shared located. And as far as I know, Rachio doesn’t give any guidance on how to do this.

No, the ET includes the sun or shade exposure factor, the Crop Coefficient does not. However, the FRET value does include sun, so for a cloudy day, the FRET would be lower, also lowering the ET. And Rachio changes the ET based on your local shade/no-shade area.

But Crop Coefficient is not affected by shade; it’s just a percent of the standard value, which does consider cloudy days, but not local shade.

Hmm. I’ve always assumed that every knob from the Zone Edit screen had an equivalent on the Advanced screen. In other words, one could either exclusively use the regular screen (i.e. novice users) OR the advanced screen. But it sounds like the Sun Exposure field is not reflected on the advanced screen, so it’s impossible to configure the entire zone just from the advanced screen.

Well, you can’t get to the Advanced screen, without going through the Standard screen first. So yeah, if you intentionally set the Sun/Shade setting incorrectly, then set the rest on the Advanced screen, you will not have accounted for Sun & Shade. Same with Slope; it’s set on the Standard screen.

So you CAN do everything from the Standard screen; the Advanced screen is simply to fine-tune the variables.

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