Every year, like a giant frozen reservoir, snow that falls across the Sierra Nevada mountain range slowly melts in spring and summer months, providing roughly one-third of the water supply for California’s cities and farms, from the Bay Area to Los Angeles. But at the current rate at which the climate is warming, the amount of runoff from Sierra snow into California’s largest reservoirs is heading for a dramatic decline — a 54 percent drop in the next 20 to 40 years and 79 percent in the next 60 to 80 years, according to a new study from scientists at Lawrence Berkeley National Laboratory.