"Today I'm declaring a drought emergency in the state of California," Brown told reporters here, "because we're facing perhaps the worst drought that California has ever seen since records began being kept about 100 years ago."
The declaration come as the state suffers through dry conditions for a third straight year. It follows weeks of consideration by the Democratic governor amid pressure from lawmakers and water officials to act.
Brown had suggested for days that he was close to declaring the emergency, a formal act considered useful for raising public awareness and focusing the attention of federal officials who can accelerate some relief efforts.
In declaring an emergency, Brown directed state agencies to hire more seasonal firefighters, use less water and prepare a water conservation public awareness campaign.
Brown's appeal for conservation is voluntary, but he suggested the state could impose mandatory reduction measures if the drought lasts.
"As we go down the road - you know, January, February, March - we will keep our eye on the ball and intensify, even to the point of mandatory conservation," he said. "But we're not going to do that quite yet."
California is entering one of the driest winters on record after two dry years have already parched the state, depleted reservoirs and left streams and rivers running low. American River flows are at their lowest level in two decades, while Folsom Lake has receded so dramatically a Gold Rush-era mining town, long submerged, has been exposed.
The U.S. Department of Agriculture on Wednesday designated parts of 11 states, including California, as primary natural disaster areas due to drought. The designation, which includes Sacramento, Fresno and San Luis Obispo counties, makes certain farmers eligible for low-interest emergency loans to cover losses.
Meanwhile, water districts and local governments throughout the state have enacted conservation measures, including a water rationing order by city officials this week in Sacramento.
The last drought emergency in California was declared by then-Gov. Arnold Schwarzenegger in 2009 and lifted by Brown in 2011.
Last year, with the state becoming drier once again, Brown issued an executive order directing the State Water Resources Control Board and Department of Water Resources to expedite their processing of voluntary water transfers.
Editor's note: This post was updated at 10:44 a.m. to include additional remarks by Brown.
PHOTO: Gov. Jerry Brown addresses the media at Fresno City Hall on Monday, Jan. 13, 2014. The Fresno Bee/Eric Paul Zamora