SAN FRANCISCO—California farmers face being cut off from federal water imports for the second straight year, in an unprecedented move likely to worsen crop losses in the nation’s biggest agricultural state.

Citing declining reservoir levels amid a four-year drought, the U.S. Bureau of Reclamation announced Friday its initial allocation of supplies from the federal Central Valley Project would again be set at zero, as it was in 2014. The agency said the allocation amount could be revised upward if the state gets more precipitation.

...