Brown ordered state agencies to take actions to prepare for emergency conditions and called on Californians to reduce water consumption voluntarily by 20 percent.
"We have to recognize this is not a partisan adversary," Brown said at a news conference in his office at the State Building in San Francisco.
"This is Mother Nature. We have to live within the resources we have," the governor said.
Brown's proclamation says state water supplies have dipped to "alarming levels," with mountain snowpacks at 20 percent of normal for this time of year, reduced surface flow in rivers and significant drops in reservoir and groundwater levels.
The proclamation includes a series of executive orders requiring state agencies to aid affected farmers and communities by expediting water transfers and releasing stored water from reservoirs.
State agencies were also ordered to develop water conservation plans. The Department of Forestry will hire additional seasonal firefighters, Brown said.
Brown said he hopes to get federal aid to deal with the drought but said he did not know specifically what that aid might be.
Brown said he hopes the environmental analysis of his Delta Plan, which proposes two 35-mile tunnels to divert water to Central and Southern California, will speed up. Some conservationists and local officials have opposed the plan.
Brown said allocating water in California entails conflicts between northern and southern and urban and rural parts of the state, but said, "We all depend on one another."
California Farm Bureau Federation President Paul Wenger said he welcomed the proclamation.
Wenger urged Brown to lead a campaign for increased water storage facilities to provide more flexibility in the face of volatile weather patterns.
"Conservation alone won't solve our chronic water supply problems. California must commit to improve its water system," Wenger said in a statement.
Outside the State Building, a group of about 25 members of several environmental groups chanted and carried signs urging Brown to end fracking in California. Fracking, the hydraulic fracturing of underground rock to release oil, uses millions of gallons of water.
David Turnbull, campaigns director for Oil Change International, said, "To allow water-intensive fracking for oil to continue in a drought is to deny the reality of what California's farmers and communities are facing every day."
Counties encourage residents to reduce water usage
Following Brown's call for Californians to reduce water consumption by 20 percent, some Bay Area counties are offering incentives to residents willing to cut back.
Contra Costa County is offering cash to people who install more efficient plants. For those willing to lose the lawn, they will pay 50 cents per square foot to help residents replace sprinkler-fed lawns with drip-irrigated gardens full of drought tolerant plants.