I have finally uploaded to github.com/pbecchi/sensorserver OpenSensor that run on ESP8266 .
The code compute hourly ET0 based on data from nearby WU meteo station and use solar radiation from another station 40Km away. The ET0 are also computed from my house solar panels. Both value are very similar and correlate quite well.
The code read also the rainfall and can directely control OpenSprinkler water delay.
I use it now since 4 month with good results.
If you want more info on the code visit github.com/pbecchi/sensorserver or the topic https://opensprinkler.com/forums/topic/opensensor-logger-controller-for-opensprinkler-networks/