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The zimmerman calculation is
The formula used to set the scale is as follows:
• Humidity: Average humidity for the previous day above 30% is subtracted from the weather adjustment, below 30% is added to it. Example: 77% Max Humidity + 26% Min Humidity / 2 = 51% Avg Humidity, 30% – 51% Avg Humidity = -21% Humidity Adjustment
• Temperature: +4% for each degree Fahrenheit above 70, and -4% for each degree Fahrenheit below 70.
• Precipitation: -2% for each hundredth of an inch precipitation from today and yesterday. Example: 0.12″ rain today + 0.05″ rain yesterday * 100 = 17 hundredths of an inch of rain * -2 = -34% Precipitation Adjustment
So I would suggest to have the 3 parameters configurable:
* For Humidity: be able to modify the 30%
* For temp: be able to modify the +-4%
* For Precep: be able to modify the +-2%
With this extra addition, I think everybody can build its own custom Zimmerman method that fits his needs.
The ultimate would be to have also more impact of historical rain (in my region). Because sometimes it rains during a thunderstorm 30l, so this means the next couple of days no irrigation is needed. With the Zimmerman method only the previous day is taken into account.
But I know it is difficult to create a rule for this because going more days in the past has a lot more items that will influence the decision how much to water.