Total California water storage levels are near their lowest in a decade, risking unsustainable groundwater depletion if the drought persists, researchers say. Using data from NASA's GRACE satellite, a study led by Jay Famiglietti at the University of California, Irvine, found that as of November, total water storage in the state's river basins -- the combination of all of the snow, surface water, soil moisture and groundwater -- had declined to its lowest point in nearly a decade. Water losses between November 2011 and November 2013 -- the early phase of the current drought -- were particularly steep, researchers said in a university release Monday. "That's the steepest decline in total water storage that we've seen in California since the GRACE mission was launched in 2002," Famiglietti said. California Gov. Jerry Brown recently declared a drought emergency. The research focused on the Sacramento and San Joaquin River basins -- the state's two largest -- and the source of most its water. The region also encompasses the Central Valley, the most productive agriculture region in the country, which depends entirely on the surface and groundwater resources within the river basins to meet its irrigation needs and to produce food for the nation. The ongoing drought represents a growing threat to groundwater supplies in the Central Valley, typically viewed as a strategic reserve that supplements sparse surface water supplies in times of scarcity. Steep declines in groundwater storage are typical during droughts, the researchers said. If the drought persists, "Central Valley groundwater levels will fall to all-time lows," Famiglietti said.