Please find below the results of my comparison. The results are based on German weather conditions and may differ for other climate regions.
Zimmerman Local PWS:
I’m hosting an own weather server consisting of a Raspberry PI connected to my local WiFi network. A WeeWX installation running on this Raspberry PI retrieves the Netatmo PWS weather data every 5 minutes from the Netatmo cloud service. The OpenSprinkler weather service runs on the same Raspberry PI and uses this weather data as input for the Zimmerman watering scale calculation. The weather data needed for the OpenSprinkler watering scale calculation reflect local weather conditions. However, there exists an issue with short rainfall periods. The precipitation amount isn’t correctly summed up by WeeWX. The total precipitation listed by WeeWX is less than actually summarized by Netatmo. Therefore a negligible chance of watering the garden a little bit more than needed exists.
Additionally, I`m using the Meteoware Service to retrieve the Netatmo PWS weather data every 10 minutes from the Netatmo cloud service and forward the data to Weather Underground. Weather Underground can be interfaced by the OpenSprinkler weather service if a PWS station name and a valid API key is acquired by Weather Underground. The official weather service provided by OpenSprinkler, as well as a local copy of this weather service running on my Raspberry Pi can be used for weather adjustments. The weather data needed for the OpenSprinkler watering scale calculation perfectly reflect local weather conditions, including correct precipitation measurements.
The official OpenSprinkler weather service by default uses the DarkSky API. At my hometown DarkSky provides only weather reports for the whole Nuremberg area. Especially the amount of rain differs a lot from the actual situation at my home. This means weather data delivered by DarkSky does not reliable reflect local weather conditions. It makes little sense for me to use DarkSky for my OpenSprinkler installation. It will lead for most of the time to insufficient watering of my garden.
The Evapotranspiration (ET) Method:
The ET Method sticks to DarkSky as weather provider for reason that Local PWS and Weather Underground data lacks the ‘solar radiation’ parameter. As already mentioned, DarkSky provides untrustworthy local weather data for my home location, which results in insufficient garden watering. Without having access to a local weather station the ET Method will still be nice to have but isn’t usable for my OpenSprinkler installation.
In summary the Zimmerman method in conjunction with my Private Weather Station (PWS) is the best suitable weather adjustment method for me. Both weather sources, Weather Underground and WeeWX provide acceptable weather data for the OpenSprinkler Weather Service. Hosting an own weather server isn’t always needed. At least for Netatmo Personal Weather Stations cloud services like Meteoware can be used to fill the missing link between Netatmo and Weather Underground.
The official OpenSprinkler weather server uses caching so the watering percentage during a day does not vary. Hosting an own weather server or using Weather Underground relies on the last 24 hour’s data for the Zimmerman calculation. The calculated watering scale will be updated during the day in accordance with the ongoing weather changes. My experience shows that this isn’t an issue in regards to the garden watering needs. It is more important to have access to a trustworthy nearby weather data source or a Private Weather Station.