See the additional discussion in this thread: viewtopic.php?f=28&t=564&start=10#p4134
I have a prototype that uses the water balance method and looks ahead one day for rain. Every midnight it generates a watering schedule for the day by looking at the running water balance (where ET is subtracted and rain and irrigation are added). When the balance is negative, irrigation is needed. However, if the forecast for the day calls for enough rain to make up the water deficit, the irrigation is not scheduled that day. The next midnight, the calculation repeats. If the forecast rain fell, then all is well and the watering delay was justified. If the rain didn’t come, the water balance will be even lower and watering will be scheduled unless an even larger rainfall is forecast for that day. The case not currently handled is if there is a large amount of rain forecast more than a day in the future. It would be easy to check for this; the concern is the uncertainty of the forecast the farther it is in the future, and the possible stress on plants while waiting for the rain. Also, looking more than one day out the check should include water depletion via ET, but I don’t know of a source that forecasts ET. I’m doing some experiments to predict ET, but it’s too soon to tell how accurate this might be.