LOS ANGELES — Farmers with rights to California water dating back more than a century will face sharp cutbacks, the first reduction in their water use since 1977, state officials announced Friday. The officials said that rights dating to 1903 would be restricted, and that such restrictions would grow as the summer months go on, with the state facing a prolonged drought that shows few signs of easing.

“Demand in our key rivers systems are outstripping supply,” said Caren Trgovcich, the State Water Resources Control Board’s chief deputy director. “Other cuts may be imminent.”

It is too early to know the practical impact of the cuts, which prohibit farmers from taking surface water. State officials have warned of such curtailments for months, and many farmers and agricultural water districts prepared for them by increasing their reserves or digging new wells for groundwater.

Still, the dramatic move is a sign of how dire the drought has become, as the snowpack in the Sierra Nevada mountain range — which normally supplies water to the state through the summer months, as it melts — is at a historic low. Only once before in the state’s history have the most senior water rights been curtailed. But now, with the drought persisting into a fourth year, state officials say that more reductions for so-called senior water rights holders are nearly certain, and the need for additional cuts will be evaluated weekly.