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A few other comments if you decide to incorporate something like this. I don’t mean to belabor the obvious, just passing along some knowledge I gained.
– Perhaps obvious, but rotary or spray heads outside the US are measured in mm/hr and not inches.
– Drip systems are measured in either gallons per hour (gal/hr) or liters per hour (l/hr) and thus you might want to support those units for entering per-zone watering rates and amount of water required.
There are a few python libraries out there for calculating PM ETO and alternatives. A google search turned up several, many of which are based on FAO’s underlying library like the one here. Here is another: http://python.hydrology-amsterdam.nl/modules/evaplib.py
The key weather data inputs appear to be:
Minimum and Maximum temperature (C)
Mean temperature (C)
Dewpoint temperature (C) or relative humidity (%)
Mean windspeed (m/s)
Atmospheric pressure (hPa)
Total solar radiation (in Jules)