Looking for alternative weather APIs the Open-Meteo weather service might be the perfect solution for OpenSprinkler watering calculations. Open-Meteo offers hourly and daily weather parameters for 7-day forecasts and last 2-days histories. For example, the parameters precipitation_sum and et0_fao_evapotranspiration can be easily used for calculating the watering percentage in accordance with the ETo Method. Please have a look at https://open-meteo.com/en/docs for details.
Open-Meteo APIs are free for open-source developer and non-commercial use. I’m not sure if this suits the need for the OpenSprinkler service, but just want to let you know that such a service exists.
https://pirateweather.net is a drop-in replacement for the Dark Sky API and uses NOAA data provided by AWS’s open data program (https://registry.opendata.aws/collab/noaa/). It includes NOAA High Resolution Rapid Refresh (HRRR) data, which provides 15-minute forecasts out to 48 hours for the continental US, southern Canada, and northern Mexico in 3 km increments. (GFS, with 18 km resolution, is used for the rest of the world.) Precipitation probabilities are computed using the Global Ensemble Forecast System (GEFS). By running different variations parameters and inputs, 30 different versions of this model are run at the same time, providing 3-hour forecasts out to 240 hours. Since this is a 30-member ensemble, if 1 member predicts precipitation, the precipitation probability will be 1/30. (You can use https://weather.pirateweather.net to see the data on the web.)