SAN FRANCISCO — Gov. Jerry Brown on Friday declared a drought emergency in California as the state struggles with the least amount of rainfall in its 163-year history, reservoir levels fall and firefighters remain on high alert.

“We are in an unprecedented, very serious situation,” said Brown, who asked California residents and businesses to voluntarily reduce their water consumption by 20 percent. “Hopefully, it will rain eventually. But in the meantime, we have to do our part.”

The drought declaration also streamlines the rules for water agencies to transfer extra water from one part of the state to another, easing shortages. It also directs the state to hire more seasonal firefighters, limits the landscaping of highways and raises public awareness.

Brown was governor in 1976 and 1977, one of California’s most severe dry periods in the 20th century. The most recent extended drought was from 1987 to 1992.

The last governor to declare a drought emergency was Arnold Schwarzenegger, who did so during a period of low rainfall in 2008 and 2009. Brown lifted that declaration in 2011 after a wet winter.

Asked how his prior experience as governor during a drought might help now, he replied, “I don’t know that I kept my notebook from 1977.”

However, the state won’t hesitate to redirect whatever resources are necessary, Brown told reporters. “When the house is burning down,” he said, “you have to pour water on the fire, and if that costs money, we’ll spend money.”

Although California has a Mediterranean climate and periodically experiences drought, current conditions are particularly dry.

The Sierra Nevada snowpack on Thursday was 17 percent of normal. And last year, most cities in the state received the lowest amount of rain in any living Californian’s lifetime. The rainfall records go back to 1850.

For the past 13 months, a huge high-pressure ridge in the atmosphere has sat off the West Coast, diverting storms that normally would bring winter rain northward to Canada.

As a result, reservoir levels are low, farmers and ranchers are suffering, and fire danger is at an extreme level.

Brown’s declaration generally won praise from Republicans and Democrats, environmental groups and farmers.

“It’s entirely appropriate for the governor to declare a drought emergency, and we appreciate his timely action,” said Paul Wenger, president of the California Farm Bureau Federation.

He called for more state spending on dams and reservoirs, which he hopes will be included in a water bond proposed for the November ballot.

“Farmers across California face wrenching decisions today, as well as in coming months,” Wenger said. “Will they have enough water to plant crops, to water their livestock, and keep trees and vines alive?

“An additional concern is how many people they may have to lay off as a result of water shortages. Any way the state and federal governments can provide assistance in adding water to the system will help.”

So far, farmers have been affected more by the dry conditions than most California residents.

Although many residents think that population growth is the main driver of water demand statewide, it actually is agriculture. In an average year, farmers use 80 percent of the water used by people and businesses — 34 million acre-feet from a total of 43 million acre-feet that is diverted from rivers, lakes and groundwater, according to the state Department of Water Resources.

Most of the water goes to irrigate crops. Without rain, many farmers have been heavily pumping groundwater in the Central Valley, and some areas expect that thousands of acres of fields will be fallowed this summer.

In the Bay Area, large water districts that serve the majority of residents say they will wait until March or April to make a decision about whether to put mandatory restrictions in place. Those include the Santa Clara Valley Water District, the East Bay Municipal Utility District, the San Francisco Public Utilities Commission and the Contra Costa Water District.

The reason for the delay, officials said Friday, is that many have water in storage and have been running conservation and rebate programs for years, so they are not in a crisis.

“If it remained bone dry, we would have to look at restrictions,” said Contra Costa Water District spokeswoman Jennifer Allen. “But it’s still too early to say what supply we will have.”

The Santa Clara Valley Water District has a year’s supply stored in underground aquifers, and another year’s supply is stored underground at the Semitropic Water District near Bakersfield.

“We are hoping we can wait until the end of the rainy season to propose a target,” said Marty Grimes, a spokesman for the Santa Clara Valley Water District. “What happens in February and March could change the situation.”

Brown’s declaration also:

Directs state agencies, led by the Department of Water Resources, to execute a statewide campaign to encourage and promote water conservation, with a goal of reducing water usage by 20 percent.

Requires the Department of Forestry and Fire Protection to hire additional seasonal firefighters.

Urges cities and water districts to update their water management and drought plans.

Orders all state agencies to conserve water, including placing a moratorium on new, nonessential landscaping at public buildings and along highways.

Requires state officials to speed approval for voluntary water sales and transfers between willing districts.