About 5 months ago I released a working version of the OS that would calculate ET and adjust watering times accordingly. The ET is calculated using data from WU weather data (the primary script is hosted on my server), this way no one has to work out every reporting method for ET. The method also uses an hourly calculation so it will adjust for real world conditions that vary from predicted. It also adjusts for predicted values so that it will withhold water if rain is predicted in the near future. Having used it all of the 2016 watering season I had great results with only 2 days of over watering amounting to a total ‘waste’ of about 25 gallons of water, compared to the near 500 gallons saved versus the previous year (adjusted for vastly differing weather patterns–I averaged and subtracted deviations of outliers). The entire program is available on my fork of opensprinklers main github if you wish to use it as a starting point. I don’t support it anymore as it seems interest in the product is high, but interest in integration to the primary firmware is low, essentially I do not have the time to support the project as it takes away too much time from my job.
Most of the firmware matches the 2.1.6 version of the firmware and as it sits is compatible with all devices. Please be aware of the use of code from other authors as I did not start from scratch. You can see the information HERE.