I think you’re just missing my point that not all irrigation is done via sprinkler and “depth” isn’t really relevant. I’m not concerned about error really… there is simply ALWAYS going to be variables that can’t be accurately accounted for (save for perhaps hydroponics labs). Sprinklers are both simpler (if you don’t care about error) and more difficult (if you do) but at least the relationship, whatever it is, is mono-dimensional and there’s a direct relationship between rain and irrigation since they both are applied the same way.
Drip isn’t.
Subsurface drip really isn’t.
And I maintain that for a relevant (not necessarily accurate) correlation, that dimensional divide must be addressed. Now, that can be done by the user and the math is just on them. But, I think it can (and should) be in the software to make changes in the math easier to deal with.
Let me try to construct an example (that is, unfortunately, going to mix metric and imperial because I’m Canadian and that’s what we have to deal with):
My backyard is roughly 1000 S.F. I have ROUGHLY 1000 emitters in a more-or-less grid at root depth below the lawn. Each emitter… assuming it is functioning correctly… puts out 0.4GPH. So… with no help from OS, I *could* calculate how many gallons per minute gets put onto (into) my yard (or read my meter that tells me it is 25L/min actual), divide that by the square footage and multiply that by the time the zone runs and come up with a depth. Now I’ve manually calculated a figure to directly compare to rainfall and we’re all roses, right? Sure… but now I decide that my lawn is over watered and I want to change that to 8 minutes. Hmm… now I have to recalculate the weight rainfall has because same rainfall is now a larger percentage of my irrigation. And I have to do that manually. Manually = yuck. Computers are particularly good at math, so if instead I could input the 1000 s.f. area and the 25 L/Min flow then the software – which already knows how much time the zone runs for – can dynamically calculate depth and compare that to rainfall. Less work for me = yum!
Even with sprinklers I think my method is better because it doesn’t require you to go out with a tuna tin and a ruler to figure anything out. Provided you have a meter to check (and if you don’t, you still have your tuna can and ruler) you *KNOW* the volume applied. Enter size of coverage for that zone (not head) and you’re back at my yummy world above.
If you don’t have a meter to know the flow, then it isn’t as convenient and – I concede – it would be better to tuna-can-measure depth and compare that directly to rainfall data.