The Obama administration underscored the threat of climate change to Western water supplies Tuesday, releasing a report that projects less snowpack in the mountains and reduced flows in major rivers — all of which spells a much drier future for places such as California.

The report estimates that diminished runoff from the Sierra Nevada will prompt an average 9 percent drop in state reservoir levels by the end of the century and a 3 percent dip in deliveries to cities and farms from the major water projects.