Their California mansions, lush with green lawns and vegetation, guzzle as much water as 90 homes -- but the astronomical bills are a drop in the bucket for them.

After years of searing drought in the state, authorities and activists are scrambling to find ways to get the ultra-rich to turn off their sprinklers and get on board with conservation efforts.

Some, like the news site Reveal, go for the sledgehammer: expose the identity of the residential "mega-users" and how much water they are using -- "drought-shaming" -- to push them, and people like them, to act.

A year ago, Reveal reported that 100 residential water customers in the luxury neighborhoods of Los Angeles, like Bel Air and Beverly Hills, were consuming millions of gallons of water a year.

One of the exclusive properties, according to the Center for Investigative Reporting site, used 11.8 million gallons (45 million liters) of water in one year, "enough for 90 households."

Reveal dubbed that household the "Wet Prince of Bel Air." Authorities have refused to identify the big water-wasters, so Reveal turned its sights on satellite images and other clues to unmask them.

Last month Reveal identified what it said were seven "likely culprits," presumably including the "Wet Prince of Bel Air."

Among them were a former television network chief, an investment banker, a film producer and an heir of a retail giant's fortune.

The authorities have their own "shaming" site -- www.savewater.ca.gov -- that encourages individuals to report when they see water being wasted, whether its watering on the wrong day or at the wrong time, such as under a hot sun or washing down sidewalks.

Social media's finger-pointing hashtag, #droughtshaming, has been aimed at celebrities like reality television stars Kylie Jenner and Kim Kardashian, singers Kanye West and Barbra Streisand, comedian Amy Poehler and actor Tom Selleck.

- Shame or diplomacy? -

Marty Adams, a top executive at the Los Angeles Department of Water and Power (DWP) said that water shaming draws attention "but there's no proof it works better than anything else."

"Shaming is not really the most direct way to reduce water use," said Stephanie Pincetl, professor of environmental policy at the University of California, Los Angeles. She prefers higher bills, fines and shutting off the water.

While there are no limits on how much water can be used as long as an individual pays for it, California Governor Jerry Brown, who declared a state of emergency in 2014 due to the drought, has ordered urban water consumption to be reduced by 25 percent.

The state has publicly rated municipalities on their compliance with the conservation program.

The sprawling city of Los Angeles has banned a number of actions, including running water into the street and washing a car with a garden hose.

For around two years, a "water police" has been patrolling Los Angeles but its handful of agents hand out relatively symbolic fines of around $100.

The DWP recently beefed up its arsenal against big water-guzzlers, which is expected to take effect this month, Adams told AFP.

- Bills 'so small' -

Despite the drought, water remains fairly cheap. But a fourth category of billing has just been created for the big consumers.

Those water customers will have to be audited and could face fines starting at $1,000. For repeated violations over a long period of time, the fines could reach $40,000.

Adams said the ultra-rich usually do not waste water knowingly.

"For a lot of them, water bills are so small compared to gas or power or phone bills... they have no sense they are using too much," he said.

The DWP has taken a diplomatic approach, and sent letters to the top guzzlers.

The mega-users, once contacted, generally have been receptive.

According to Reveal, the former head of Warner Bros, Robert Daly, says he has cut water on his estate by 35 percent since 2014 and replaced lawns with artificial turf and pebbles.

Adams said that rich, poor and everyone in between needs to make an effort. Angelenos overall have cut back water usage by about 20 percent since 2013, he said.

The stubborn pockets of drought remain a critical problem in the huge state of California. The southern region is growing increasingly dependent on buying water, especially from the Colorado River, whose lower levels are blamed in part on climate change.