"This drought is having a devastating impact on our people, our communities, our economy and our environment, making today's action absolutely necessary," the Republican governor said in his statement.
Mandatory rationing is an option if the declaration and other measures are insufficient.
The drought has forced farmers to fallow their fields, put thousands of agricultural workers out of work and led to conservation measures in cities throughout the state, which is the nation's top agricultural producer.
Agriculture losses could reach $2.8 billion this year and cost 95,000 jobs, said Lester Snow, the state water director.
