Northern California is experiencing one of the wettest starts to a water year in the history of record keeping, dating back to the turn of the 20th century, weather officials said. The California Department of Water Resources maintains an eight-station index, where it measures rainfall at eight locations throughout the mountainous region of Northern California, ranging from Mount Shasta City down to the American River Basin. It compiles the rainfall at each station to arrive at a number that indicates the amount of precipitation Northern California is receiving. Thus far in the 2012-13 water year, which begins Oct. 1 and …