SACRAMENTO — Waste California’s water, risk a $10,000 fine.

Residents and businesses could soon face that threat after Gov. Jerry Brown on Tuesday unveiled legislation that would increase potential penalties on the most flagrant water scofflaws and allow cities, counties and water districts to issue fines without having to go to court.

“As this drought stretches on, we’ll continue to do whatever is necessary to help communities save more water,” Brown said after meeting with California mayors, including San Jose’s Sam Liccardo and Oakland’s Libby Schaaf.

But whether the proposal — which would boost maximum fines 20-fold from the current $500 — will ever take effect was unclear Tuesday.

It must first be approved by the state Legislature, and some top lawmakers, especially Republicans, said Tuesday they oppose the tough fines.

And although local governments have had the power under emergency state rules issued last summer to impose fines for wasting water, almost none have done so.

“The issue is not the size of the fine, but how localities are going to enforce it,” said water attorney Leon Szeptycki, director of Stanford University’s Water in the West program. “It is a PR move. But it will get people’s attention. This is part of a broad effort to get people to reduce their water use. They are trying everything.”

Meanwhile, the Brown administration late Tuesday refused to back down from strict conservation mandates it plans to approve May 5 that will force cities and water districts to cut water use from 8 percent to 36 percent this summer, depending on their per capita use, or face fines of $10,000 a day.

Representatives in some cities, particularly in the Sacramento area and Inland Empire of Southern California, where hot summers and large lawns have led to high per-capita use, have said in recent weeks the targets are unfair and should be relaxed to account for differences in weather.

“We understand this is a big deal and a challenge for everyone, but the drought we are in is serious and it calls upon us to rise to the occasion,” said Felicia Marcus, chairwoman of the State Water Resources Control Board.

The drought may last for many years, she said, and communities need to dramatically reduce the amount of drinking water that people are putting on lawns to head off severe shortages.

“It’s better to be safer than sorrier in the face of uncertainty,” she said.

Tuesday’s developments represent the state’s latest efforts to reduce water use as California heads into the long, hot summer in its fourth year of drought — the worst in the state’s 164-year history.

Under existing state rules, residents and businesses can be fined if they water lawns so much that the water runs into the street, sidewalk or neighboring property. It is also illegal to wash cars without nozzles on the hose, for restaurants to serve water unless upon request and for anyone to irrigate lawns within 48 hours of measurable rain. But enforcement is up to local government.

Speaking Tuesday at a Sacramento news conference, Brown also said he will direct state agencies to streamline environmental review of local water supply projects — like expanding San Jose’s water recycling plant — that Liccardo has been fighting for.

The $10,000 fines won’t be doled out liberally, Brown said, noting that “only the worst offenders” will be penalized this way.

“Fines are a tool, and they’re the last tool you use,” Marcus said.

Brown’s goal with the city-by-city water targets is to cut statewide water use 25 percent this year, compared with 2013.

“We think they’re reasonable,” he said. “We think they’re enforceable.”

The threat of steep fines should help cities enforce Brown’s order to cut urban water use after a state appeals court last week rejected San Juan Capistrano’s policy of charging punitively higher rates to consumers who use the most water.

Under Brown’s rules, cities that use little water per capita, such as San Francisco, Hayward and Santa Cruz, must cut water use by only 8 percent. Cities that use much more, like Beverly Hills, Bakersfield, Hillsborough and Atherton, must reduce by up to 36 percent.

Fresno must slash water use by 28 percent, but Mayor Ashley Swearengin said she’s confident the city will be able to meet the tough target because it cut 17 percent last year.

“Most everyone has already gotten into compliance,” she said. “We have just a few people who are well over the limit. As we work with them to get their water use down, we’ll be able to close that gap.”

San Jose’s three water suppliers will be required to cut water use 20 percent.

When it comes to fines, many water districts so far have not been willing to pull the trigger on offenders.

“We’ve focused more on education and outreach rather than hitting people on the head with a hefty fine,” said Abby Figueroa, spokeswoman for the East Bay Municipal Utility District, which has 1.3 million customers in Alameda and Contra Costa counties, and must cut water use by 16 percent.

“But the drought is getting worse so we may need to do more sticks than carrots for some of the more egregious water wasters.”

Contact Jessica Calefati at 916-441-2101. Follow her at Twitter.com/Calefati.