Rich Pedroncelli / AP Frank Gehrke, chief of snow survey for the Department of Water Resources, right, reads the weight of a sample held by climatologist Michael Hall near Echo Summit.

SACRAMENTO — The Sierra Nevada snowpack, a key source of California's water supply, has fallen well below normal levels, state officials said Thursday, increasing the likelihood of water shortages this summer.

Department of Water Resources scientists found snowpack water content averaging only 67 percent of normal throughout the 400-mile-long mountain range after the state experienced its driest two-month period on record.

Levels were 88 percent of normal in the northern Sierra and about 60 percent of normal in the central and southern regions.

Frank Gehrke, the snow survey chief at California's Department of Water Resources, said dry, sunny conditions in March and April melted what was an average snowpack earlier this year. In addition, soils parched from last year's drought are soaking much of the early snowmelt.

"It's a knock-out punch to have that combination," Gehrke said in a telephone interview from Echo Summit.

At the summit just south of Lake Tahoe, scientists measured 3.3 inches of snow in a meadow on Thursday. That's only 11 percent of what is expected there at this time of year.

The amount of water running into streams and reservoirs is only 55 to 65 percent of normal, according to the figures collected by the Department of Water Resources.

That's one of the reasons federal and state water managers have reduced water exports this year from the Sacramento-San Joaquin Delta to Southern California and the Central Valley.

Water deliveries also have been cut to comply with a federal judge's order that limits pumping from the delta by as much as 30 percent to protect the delta smelt, a threatened fish species. About 600,000 acre feet of water — enough water to supply 4.8 million people for a year — has not been pumped as a result of the restrictions, said Resources Agency Secretary Mike Chrisman.

The pumping restrictions, last year's drought and this year's dry conditions have left the state's reservoirs lower than normal. Lake Oroville, the state's principal storage reservoir, is less than half full.

"It's going to be a rough decade," said Tim Quinn, executive director of the Association of California Water Agencies. "You will see mandatory rationing, I believe."

Officials in Roseville, about 20 miles north of the state capital, issued a drought alert Wednesday because the city is not getting its full allotment of water from Folsom Lake.

Officials at the East Bay Municipal Utility District have said water levels are so low, its board of directors might have to vote for mandatory water rationing when it meets later this month.

Chrisman said it was too early to say whether the state would ask cities and farmers to issue mandatory rationing, but he suggested Californians voluntarily water their lawns less frequently, buy energy-efficient washing machines and low-flush toilets.

Although this year's water picture is bleak, hydrologic conditions don't yet merit a drought declaration, said Elissa Lynn, chief meteorologist at the Department of Water Resources.

Although the state's rivers are still low, projections show the average flow from this dry spell will be 15 percent to 20 percent higher than it was between 1987-92, California's last drought.

Gov. Arnold Schwarzenegger said the most recent snow survey underscores his argument that California should conserve more water and build more dams.

"These actions are vital to protect our environment, economy and quality of life," Schwarzenegger said in a statement.

"I know that legislative leaders share my goal of comprehensive water reform," he said. "But time is running out. The longer we wait, the worse our situation becomes."