In a historic declaration atop a Sierra summit barren of snow, Gov. Jerry Brown on Wednesday ordered California’s first-ever mandatory statewide water restrictions, threatening hefty fines for communities — and potential rate hikes for residents — that fail to hit stepped-up conservation targets as the worst drought in state history enters its fourth year.

The governor’s emergency order comes after a year of requests for voluntary conservation — and a record-breaking warm and dry spell culminating in the worst April snowpack in recorded history — have failed to alarm many Californians enough to cut back on water.

Now, facing criticism that the state has done too little to respond to the drought, Brown has unleashed a wide range of water-saving directives, calling for a 25 percent statewide reduction in urban water use from 2013 levels; boosting enforcement of water waste; requiring drip irrigation at new construction; cracking down on farmers who illegally divert water from irrigation canals; and investing in new water-saving technologies.

Most of the new rules would have to be implemented at the local level, and water district officials around California on Wednesday were scrambling to figure out how to meet Brown’s targets.

“This historic drought demands unprecedented action,” Brown said standing amid grass, dirt and rocks during April’s annual snowpack measurement at Echo Summit off Highway 50, southwest of Lake Tahoe.

He urged Californians to “pull together and save water in every way possible.”

Snow surveyors found no snow at the Phillips Station site — the first time that’s happened in 75 years of early-April measurements. In an average year, the site would have 5.5 feet of snow. Across the Sierra, electronic readings indicate the water content of the snowpack is only 5 percent of average.

That shattered the previous low record of 25 percent of the average April snowpack, set last year and in 1977. The implications are huge for the cities, farms and wildlife that depend on melting snowpack to yield water during the spring, summer and fall.

Snowpack traditionally is at its peak by early April, before it begins to melt. With the state’s historically wettest winter months now gone, the drought is now firmly rooted in its fourth consecutive year.

Until now, efforts to achieve the state’s voluntary 20 percent water conservation goal have been disappointing, with Californians cutting water use at homes and businesses only 8.8 percent statewide in January 2014 compared with January 2013, the baseline year used by state water officials.

“We’re not getting the level of effort the situation warrants,” Felicia Marcus, chair of the State Water Resources Control Board, said in a Wednesday news conference. “We need to take it to the next level.”

Not every community will have to meet the new 25 percent statewide target; some will be higher, while others lower, she added. For instance, places that have already conserved — such as San Francisco, Santa Cruz and Pleasanton — are likely to have lower targets than places where consumption remains high, such as Bakersfield. Specific regulations will be issued in May.

Local water agencies will be tasked with enforcing the new mandates, and those that fail to hit their targets could be fined $10,000 per day.

So far, the impact of the drought — now entering its fourth year — has been felt most acutely by farmers, wildlife, and residents in lower-income communities that subsist on shallow private wells.

The new order means much more aggressive conservation by urban residents, especially “homes on very large lots with a lot of landscaping,” said Marcus.

Local agencies are likely to use different tactics to force compliance, such as tiered prices and enforcement measures, she said.

“It wasn’t a surprise,” said Abby Figueroa, spokeswoman for East Bay Municipal Utilities District, where customers last year cut back 12 to 13 percent. “We’re looking at what we need to ask to get 25 percent across the board.”

The move from voluntary to mandatory cutbacks will mean adding staff, she said. It has not yet issued fines, but calls, emails and visits excessive water users.

Santa Clara Valley Water District last week called for 30 percent reductions, increasing its voluntary conservation target from 20 percent, said spokesman Marty Grimes. The district is already limiting lawn watering to two days a week.

“It’s not important to us whether every single individual meets it or exceeds it. We’re just looking at bottom line numbers,” he said.

In addition to the 25 percent statewide cutback, Brown seeks to prohibit new homes from irrigating with potable water unless water-efficient drip irrigation systems are used; ban watering of ornamental grass on public street medians; require campuses, golf courses, cemeteries and other large landscapes to make significant cuts in water use; expand a rebate program to replace old wasteful appliances; replace 50 million square feet of lawns with drought-tolerant landscaping; and help local governments in lower-income communities to defray the cost.

Agriculture has already been cut back, with the federal Central Valley Project allocating no water to farmers and the State Water Project providing 20 percent of allocations, said Mark Cowin, director of the California Department of Water Resources.

But there will be new rules that allow inspectors to ensure farmers have stopped using water, after official curtailments.

“Ag is already in a deep, deep hole, and now certain urban and suburban folks are going to feel the same scarcity,” said Chris Scheuring, an attorney who oversees water policy for the California Farm Bureau Foundation. “That tells you how deep we are into this thing.”

The new rules will not restrict farmers from pumping groundwater, Cowin said. Although there will be a new requirement that some groundwater basins be measured more closely, “this is not a great time to ask folks who won’t receive surface water to also restrict groundwater,” he said.

The state also hopes to help people, expanding its emergency aid to low-income communities where wells are running dry, such as Tulare, Fresno and Madera counties. Water tanks, supplies and bottled water will be delivered to places where all available water has been exhausted, said Mark Ghilarducci, director of the California Office of Emergency Services.

“Desperate times call for desperate measures,” said Marcus.

Staff writers Nicholas Weiler and Kerry Klein contributed to this report. Contact Lisa M. Krieger at 650-492-4098. Follow her at Twitter.com/Lisa M. Krieger.