Snow levels in the Sierra Nevada — a source of about 30 percent of the drinking water for Southern California — have more than quadrupled in one month, according to a manual survey conducted Thursday by state hydrologists. Water content contained within the 90.3 inches of snow measured at Phillips Station in the central Sierra Nevada contain a water equivalent of 28.1 inches, up from only 6 inches on Jan. 3, the state Department of Water Resources reported.