According to our readings of the WaterSense documents, it seems implementing ET is necessary. We do have an initial ET implementation in the opensprinkler-weather github repository. We are doing some final testing right now and hoping to release it as soon as possible.
On a related note, I still don’t quite understand how some of the ‘dumb’ (i.e. none Internet connected) controllers are WaterSense certified, maybe they use just historic ET data without real-time weather data. Any ideas?