Well no, rainfall is not reported (nor can it be) as a measure of volume. It is a depth – which is 2 dimensional. IF you’re presuming we’re talking about sprinklers, then yes you can measure how much simulated rainfall depth you receive and compare apples to apples… but that presumption is not valid. A more universal approach is measuring volume for the irrigation side and then comparing that to rainfall. It is also a lot easier and more accurate than putting a cup on the lawn (which isn’t possible if you’re sub-surface irrigating said lawn). That volume is given by your water meter – assuming one exists. THAT is volume… ie. 3 dimensions. Actually 4 (time) And it isn’t directly relate-able to rainfall in any meaningful manner without knowing the irrigated area, which is simple enough to estimate/measure, and time (controller knows this).
Simple weighting is linear, and I don’t think it is sufficient without a relationship between measure A (rainfall) and measure B (irrigation). Or tell me I’m wrong. Here’s an example: I run my lawn irrigation for 12 minutes. It rains for 3mm. How in the world do we make an adjustment based on that information?
We make the same point about indoor vs. outdoor… it is necessary to have a per-zone adjustment, which isn’t how it is currently (unless I’m missing something).