It's fun to do,what rachio's canned vegitation is set to is good enough.
Yea, that is why you can earn Ph.D. In these areas now. Look at pwp, it is surprising how squishy that concept is.
Micah woods is a good source to follow, pace turf, dr. John Kaminski at psu. They are heavily turf focused though.
Personally I computed my et and it was not worth the effort. It's a heavily involved process that you can google. I had computed 5% higher than rachio and could not tell a difference with their default (could not adjust it way back).
But in short, the et is the rate at which a plant moves water from the ground into the roots then through the leaves. Most of the water is separated via photosythesis to get the hydrogen for constructing chloroplast and cellulose while releasing the oxygen. Some water is just released for cooling purposes.
Infinite monkey cage has a fantastic podcast on this process, to show just how little we really know about the whole process and to highlight why there is growing need to understand quantum mechanics. It appears that plants utilize quantum adherence for photosythesis, which compared to solar cells, are crazy efficient. It's a very bizarre podcast but I like Brian cox a lot as he is the closest thing we will ever have to a Carl Sagan imo.