If the state project is forced to cut deliveries, it would take some time for effects to be felt by customers, as many districts have other sources of water, such as local storage and groundwater supplies, the Los Angeles Times reported.
Still, Friday's announcement from the State Water Project is quite significant, officials said.
"This is the first time in the 54-year history of the State Water Project that projected water supplies for both urban and agricultural uses have been reduced to zero," said state Department of Water Resources Director Mark Cowin. "This is not a coming crisis ... This is a current crisis."
Gov. Jerry Brown urged Californians to reduce water consumption by about 20 percent, the New York Times reported.
"Today's action is a stark reminder that California's drought is real," Brown said in a statement Friday. "We're taking every possible step to prepare the state for the continuing dry conditions we face."
Brown declared a drought emergency in the state in January.