I’m not sure if the Raspberry or Beagle have reliable interrupt driven inputs like the AVR’s. This $4.99 rain gauge has a dry contact reed relay output and a phone jack style connector. I haven’t measured the rain amount per pulse yet. In any case, measured rain total readings would take some of the guess work out of watering time adjustments. http://www.ambientweather.com/amws1180rg.html
Some photos of the internal mechanism: http://www.philpot.me/weatherinsider.html