Image copyright Getty Images Image caption Officials estimate that 94% of California is now in severe drought or worse

California regulators have accepted an offer from farmers to reduce their water usage by 25% in an effort to combat the record four-year drought.

A group of several hundred farmers made the concession after state officials threatened to cut their water rights.

California has already demanded a 25% reduction in cities and towns.

But critics say state Governor Jerry Brown has not done enough to reduce usage by the farming industry's powerful water rights holders.

The deal between California's Water Resources Control Board and the farmers - who have until now resisted pressure to commit to cutting back - is a sign of the worsening impact of California's drought.

The US Drought Monitor said on Thursday that 94% of California was in severe drought or worse, and farmers used 80% of all water taken from the land in the state.

The agreement is the first of its kind in more than 30 years, and involves farmers in the delta of the Sacramento and San Joaquin rivers.

Others in this category - known as senior rights holders - are irrigation districts, utilities with hydropower stations, and cities, including San Francisco.

Thousands of the state's junior water rights holders - those whose claims to water usage date back only as far as 1914 - have already had their water use curtailed this year. Those are mostly farmers getting no federal irrigation deliveries.

Under California's system, junior water rights holders have to stop taking water from rivers and streams so there is enough flow left to satisfy the demand of those with older claims.

Image copyright AP Image caption California's lakes have lost trillions of gallons of water over the past four years

Image copyright AFP Image caption Previous mandatory water restrictions did not apply to farmers

About 350 farmers met on Thursday to discuss an attempt to avoid deeper mandatory cuts to their water allowance.

"That doesn't necessarily mean they'll all participate" in the voluntary 25% reduction, said Michael George, the state's water master for the delta. But he said he believed many would.

The farming industry has come under fire in recent weeks from residents who have been forced to turn off their sprinklers and time their showers under threat of heavy fines.

The agreement gives the farmers until 1 June to present plans for how they will make the proposed cuts.