Triggering Irrigation based on Rainfall

I am looking for a way to calculate rainfall based on precipitation as reported by a weather service like Weather Underground. As precipitation happens it will add to a "reservoir" and as rainfall does not happen, the "reservoir" will be depleted. Once we deplete the reservoir to 0, it will trigger irrigation, which will then add to the reservoir and start the process again.

I have looked around and have seen some items that propose to do this but don't seem to work for me. This seems like a fairly straightforward effort. is there a plugin that would do this? I imagine it is an item that would be of interest to many.

I don't know of a source for localized rainfall data that's available. Weather Underground no longer provides precipitation data without a subscription. You might be able to web scrape the rainfall amount, but I would think that writing a plugin to do it would go against their terms of service. Dark Sky provides a predictive value (not a measured value) so that would be risky.

Once you have the data, it would be a very simple Python script (or plugin) to do what you suggest.