(Adds quote from environmental group)

By Dan Whitcomb

LOS ANGELES, May 22 (Reuters) - California water regulators on Friday approved a plan by some of the state's most senior water rights holders to voluntarily cut water use by 25 percent in exchange for assurances that they would not face further curtailments during the growing season.

Under the first-of-its-kind agreement, so-called riparian growers in the Sacramento-San Joaquin River Delta who participate in the program have agreed to either reduce water diversions by 25 percent or fallow one-quarter of their land, said Felicia Marcus, chair of the State Water Resources Control Board.

Riparian land borders natural waterways such as rivers or streams and the roughly 4,000 growers with such farmland in the delta hold some of California's priority or most senior, and typically inalienable, water rights.

"We're in a drought unprecedented in our time. That's calling upon us take unprecedented action. We're in uncharted territory here, facing hydrology we've never seen," Marcus said.

"This proposal helps Delta growers manage the risk of potentially deeper curtailment, while ensuring significant water conservation efforts in this fourth year of drought," she said.

The deal comes as California considers curtailing water diversions to senior water rights holders in the state for the first time since the late 1970s.

Marcus applauded the voluntary cutbacks a creative solution by the riparian growers but conceded it might be unpopular with other senior water rights holders

Water officials declined to estimate how much water the plan could save in the drought-parched state, saying it depended on how many farmers took part in the voluntary cutbacks, but called it an important symbolic move as the state grapples with severe water shortages.

Riparian growers represent only a small fraction of the state's farms.

Peter Gleick, president of the environmental advocacy group Pacific Institute, said the deal was an indication of the growing acceptance of the severity of the drought.

"We're in such a deep hole now that even senior water rights holders have to rethink what they're doing," Gleick said.

California is in its fourth year of a devastating drought that has prompted Governor Jerry Brown to impose the state's first-ever mandatory cutbacks in urban water use, up to 36 percent in some communities.

Brown had been criticized for largely exempting agriculture from those severe restrictions.

California grows nearly half of all U.S. fruits and vegetables, mostly in the Central Valley, and ranks as the top farm state by annual value of agricultural products.

(Reporting by Dan Whitcomb; Additional reporting by Lisa Baertlein; Editing by Sandra Maler)