I started this thread and am now able to use data from my local PWS for the Zimmerman calculation, thanks to some excellent help. If this is a high priority for you, then I suggest you visit https://github.com/OpenSprinkler/OpenSprinkler-Weather and consider implementing a local weather-server. In my case I managed to feed data from my Davis Vantage Pro2 to a weather-server instance, where it is used to compute the desired water level.