The indoor outdoor issue would require a per zone indication, which is not done now as far as I can tell, so on that we agree.
What I was getting at is the ability to flatten the math out to 1 common dimension, depth. Rainfall is most commonly measured by filling a cup of a known size(volume) and dumping it out when it fills, then counting the tips. This volume is then extrapolated as a depth covering an undefined area, though for their calculation they will know the area the rain is hitting. If we assume the area for the sprinkler and the area for the rain are the same, depth is our only concern for both and now we can compare them. I make this assumption knowing that rain will not hit every plant with exactly the same amount of water, and that a sprinkler will vary if the pressure goes up or down, but at the same time knowing I’m not building the next lunar lander and some error is acceptable.
Sprinkler manufacturers will typically provide you with how many gallons per minute or liters per minute a single nozzle or head will distribute. They will also give a coverage area in square feet or meters. Using those 2 things and assuming the head is provided sufficient water volume to operate at design specification, you can calculate coverage depth. This would only have to be done once. Now that you have calculated coverage depth, you can use the rain depth over the same coverage area as an apple to apple comparison. This is simply Volume divided by Area, leaving you with depth. Converting known volume and known area to inches per hour can be done using: Precipitation Rate = (96.26 x Total GPM)/Total Area. 96.26 = (60/7.48)*12. 60 = minutes per hour, 7.48 = gallons per cubic foot, and 12 is inches per foot. So my sprinkler head and rain measurement are now the same, a precipitation rate. We will know the time for our sprinkler and can get depth. We will know depth for our rain and can then subtract it directly as time from the irrigation time when they both hit the same area.
Next up is time to some unknown depth, we’ll pick 1mm over the coverage area. I can run and measure the water, or calculate it if I know or can measure the volume of water provided to that single head in a certain amount of time. So let’s pick 5minutes, I run the sprinkler for 5min or calculate the output for 5 min, and now I have mm/5min. Divide the mm by 5 and I have mm per min. Let’s say I got 0.5mm per min. Watering for 12 mins means I applied 6mm of water to my coverage area. It rains for 3mm, these 2 measurements are now simply depth over my coverage area (yes there will be error if the measurement for the rain comes from a different area, but typically these errors are low enough to negate or work with), I can simply subtract or add them to get differences or totals. So to answer your question using my sprinkler it would be 12min*(6mmPR/3mmPR) = 6min Needed with 3mm rain accounted for…
If you don’t want to make a few assumptions, measuring actual rain and actual irrigation rates to each plant becomes a cumbersome process that typically won’t vary data enough to make a difference in the long run, but for some math is fun so hey, to each their own I suppose.
So to wrap up that mess I think we are actually on the same page most of the way down:
1) We need comparable measurements for rain and irrigation, which is easy enough to do without adding special equipment or measuring every single time we water. Once we have a good and acceptable measurement we can use that, this will allow for some error that we will have to accept or calculate out.
2) Using Depth over time, or Volume over time for both measurements is a must. The time will be assumed the same for both measurements when they need to be directly compared. This will have to be accounted for regarding saturation…min/max boundaries for rainfall should handle this well enough for sprinklers.
3) Linear adjustments will have error…When using a single dimension linear adjustment and some form of averaging will reduce error to an acceptable level for sprinklers. While yes I am a physicist, no I do not have the desire for, nor do I need a perfect calculation, just a really close approximation of perfect.
I might be skipping over some of the important parts here, so if you are still having trouble seeing how irrigation time and rain depth relate linearly or if none of this makes sense let me know.
EDIT: That last statement isn’t meant to be rude, just throwing out the idea that discussion brings better results…and re-reading your previous post I think we are saying the same thing, just I’m calculating volume (more time, less equipment), and you are measuring (with greater overall accuracy, faster startup, added equipment).