Two parched years - punctuated by the driest spring in at least 150 years - could force districts across California to ration water this summer as policymakers and scientists grow increasingly concerned that the state is on the verge of a long-term drought.

State water officials reported Thursday that the Sierra Nevada snowpack, the source of a huge portion of California's water supply, was only 67 percent of normal, due in part to historically low rainfall in March and April.

With many reservoirs at well-below-average levels from the previous winter and a federal ruling limiting water pumped from the Sacramento-San Joaquin River Delta, the new data added a dimension to a crisis already complicated by crumbling infrastructure, surging population and environmental concerns.

"We're in a dry spell if not a drought," said California Secretary for Resources Mike Chrisman. "We're in the second year, and if we're looking at a third year, we're talking about a serious problem."

Chrisman stopped short of saying the state would issue mandatory water rationing, which appears possible only if the governor declares a state of emergency. Rather, the burden will fall on local water agencies. Many, such as San Francisco and Marin County, have asked residents and businesses over the past year to cut water usage voluntarily by 10 to 20 percent.

Others have taken more drastic steps.

In Southern California, the water district serving about 330,000 people in Orange County enacted water rationing last year, due in part to a ruling by U.S. Judge Oliver Wanger reducing water pumped from the delta by about a third to protect an endangered fish.

The East Bay Municipal Utility District announced in April that it was considering water rationing, price increases and other measures in response to critically low reservoirs. The district, which serves 1.3 million customers in Contra Costa and Alameda counties, will vote on the measures this month.

"If you catch a third (dry) year, then you're looking at a supply that's so low you can't manage it well anymore," said Charles Hardy, spokesman for the district. "That's when its starts to hurt businesses and people across the board."

No industry faces bigger changes than agriculture, which uses about 80 percent of California's available water; the remainder goes to urban areas. Some experts say they believe the balance could shift toward urban areas.

Already, some farmers are switching to crops requiring less water and letting fields go fallow. One water agency official recently talked to a Southern California avocado grower who cut his trees back to stumps and won't begin growing again until water supplies recover.

"We have a lot of water, but we also use a lot of water," said Jeffrey Mount, director for watershed sciences at UC Davis. "From an economic perspective, it makes sense moving water from agriculture to urban uses."

In fact, some farmers are already selling their water to urban districts. But there is no easy system for transporting that water, and the infrastructure required would be extremely costly.

Californians have suffered through droughts before.

A deep, two-year drought in the late 1970s drew discussions about dragging glaciers down from Alaska or filling huge plastic bladders at river sources and dragging them by tugboat to users, Hardy said. Consumers endured rationing during a longer drought in the late 1980s and early 1990s.

After those dry periods, water conservation initiatives kicked into high gear. Low-flow toilets and showerheads became the norm, and homeowners started filling their yards with drought-resistant plants. Today, that might not be enough in a state with a population expected to reach nearly 50 million by 2030.

In addition to possible restrictions on watering lawns and washing cars, water prices could spike - at least for those who use too much.

The district serving 330,000 customers in Orange County has developed a type of water profile based on household size, yard size and average temperature in the area. Using that data, water managers have come up with base water allocations; above that level, water bills jump.

"If you really want to use more water there, you're going to pay for it - and (the district) uses the extra funds to finance conservation investments," said Ellen Hanak, a senior fellow at the Public Policy Institute of California in San Francisco. "There's a lot of room for innovating in that area - some places are doing it, but there's hardly any penalty for the extra water."

It is unclear whether this dry period is a full-blown drought. Much like economic recessions, droughts can be diagnosed only in retrospect.

However, it is certain that if the dry conditions that began with the low 2006-2007 snowpack levels continue, they could have a cascading effect. The dryness of 2006-2007 contributed to this year's poor water supply totals, said Elissa Lynn, chief meteorologist with the California Department of Water Resources.

"We're losing a lot of what we did have as snow melted into the ground," Lynn said. "It's either in subsurface, waiting to come down, or it's going into soil moisture because we had a dry fall."

There is also a small chance that dry windy conditions blew snow straight from the mountains into vapor, she said.

Not all Bay Area agencies face the same challenges, because they get water from various sources: San Francisco and the Peninsula from Hetch Hetchy, East Bay Municipal Water District from the Mokelumne River watershed and the Santa Clara Valley Water District from a combination of reservoirs and the delta. Some local water managers say their supplies look good. Marin County, for instance, said its reservoirs are at more than 100 percent of capacity.

Nevertheless, stricter water controls could be a continuing part of California's future. So might large-scale projects that aim to use water in new and better ways.

"We're facing some pretty grim circumstances that call for some bold action - recycling water, desalinating water," said Tim Quinn executive director of the Association of California Water Agencies. "Above and beyond that, we have to invest in the sustainability of this system that our grandfathers constructed in the middle of the last century. It was developed with the convenience of human beings in mind, not aquatic beings."

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Tips for conserving water

Even if water rationing is not mandated, there are a number of things you can do to help. Here are some:

Lawns: Water between 9 p.m. and 6 a.m.

Cars: Use a bucket and a hose with shutoff nozzle to wash cars or go to a car wash that recycles and reuses its water.

Yards: Don't water more than three days a week or on consecutive days.

Laundry: Put full loads into front-loading machines.

Leaks: Find and repair, particularly in toilets.

Driveways: Use a broom, not a hose, to clean them.

Pools: Cover pools and hot tubs when not in use.