STOCKTON — A 143-year-old piece of paper proves that Rudy Mussi has a legal right to water from the gently meandering Middle River that nourishes his family farm.

But the same piece of paper — a “certificate of purchase,” signed in florid 19th-century handwriting and faded to near illegibility — also is proof to a growing number of critics that California has outgrown its water rights system.

The venerable “senior rights” enjoyed by Mussi and about 4,000 other farmers, companies and public agencies — some dating back to the Gold Rush — could soon become the latest casualties of the historic drought.

More than a century ago, the state essentially guaranteed unlimited water from California’s rivers and streams to pioneers who struggled to turn wilderness into fertile fields that supported a young and hungry California. The rights then were passed down to the pioneers’ heirs or to the land’s new owners — who now use more water annually than Los Angeles, San Jose, San Francisco and Sacramento combined.

Once thought inviolable, these water rights holders now face their first real challenge in California history — and they are the focus of the latest installment in this newspaper’s series “A State of Drought.”

“If we were designing the California water system today, it would look very different from what we had,” said Peter Gleick, co-founder of the Pacific Institute, an Oakland-based think tank that focus on water issues.

“The system of senior water rights might have made sense 100 years ago,” he said. “But given our new realities, it is not going to work in the long run.”

The current approach “neither protects the environment nor ensures efficient use of our limited water,” he added. “It just clarifies who was there first.”

Gov. Jerry Brown has said that if the dry conditions continue, the state’s entire water rights system could be up for examination. And this month, the state for the first time ordered property owners to provide proof of these rights, triggering anger and a flood of historic and hastily retrieved documents from hundreds of farms, cities and irrigation districts.

Under the threat of a complete cutoff by the State Water Resources Control Board, Mussi and other “riparian” Delta growers — those who live adjacent to a river — agreed last week to use 25 percent less water than they did in 2013. And other senior rights holders in other parts of the state soon may be forced to completely turn off their pumps.

Farmers are firing back, hiring attorneys to assert that the state is defying statutes that honor their seniority. The water board’s order exceeds the scope of the state’s authority, the lawyers contend.

“Water always existed here — before statehood, before the state water board,” said Mussi, 62, driving his pickup along the miles of high earthen levees that protect his tomatoes, alfalfa, grapes and other crops from being drowned by the Middle River.

Unlike most others in the Central Valley, Mussi said, the Delta farmers can’t just drill wells to make up for fewer water allocations. That’s because their groundwater is so salty that it’s lethal to crops.

As the state stretches into its fourth year of drought, the pain of cutbacks is being felt across California. Cities and towns are being required to cut water use from 8 percent to 36 percent beginning in June, or face steep fines. And about 9,000 holders of “junior” rights — the newer farms — already have been curtailed for the second consecutive year.

State officials contend that it’s only fair to require senior rights holders to cut back. “It allows growers to share in the sacrifice that people throughout the state are facing because of the severe drought,” said Felicia Marcus, chairwoman of the water board.

But Mussi called the state’s “take it or leave” approach “extortion,” noting that he’s already tilled the soil, signed contracts with canneries and planted crops — an investment worth hundreds of thousands of dollars — trusting the time-honored system of water rights.

“It’s like me pointing a gun at your head and saying, ‘You don’t have to give me your wallet,'” he said.

Here in the Sacramento-San Joaquin River Delta — home to generations-old family farms amid a network of man-made islands and channels in the nation’s largest freshwater estuary — water is considered a private property right. Rivers drain onto the farmers’ fields, then back out again.

Water is almost a birthright in the Delta, where settlers dammed, diked and drained wetlands described as “nothing better than rotting turf and waving rushes … worthless in their natural condition” by a 19th century New York Times correspondent.

While most Eastern states recognize riparian rights, California and Oklahoma are the only states west of the Mississippi River that continue to recognize them — and they are governed by few laws and frequently litigated.

A second type of senior right — called a “pre-1914” right because that’s the year California established an official permit process for its chaotic and litigious water rights landscape — is equally historic. And, until now, it also has been subject to minimal state oversight.

Plumas County alfalfa farmer Robert Forbes contends that the state lacks the authority to take away his water. His family’s right to a ditch on a small Quincy reservoir dates back to 1870. It also supplies water to 11 neighbors.

While he’s voluntarily made big cutbacks, Forbes said, “My water rights are written into the deed, then passed on.”

The junior rights holders, who planted in the arid grasslands and deserts in the southern and western parts of the San Joaquin Valley after 1914, are even far down the pecking order and have already had their water cut.

A UC Davis analysis shows that California’s water is heavily oversubscribed, with five times more water committed to these rights holders than flows through all the state’s rivers and streams combined.

Because the state promised more water than it can deliver, farmers such as Mussi — who shares the farm with his brother, son, nephews and their families — are angry that their generations-old rights are being eroded.

“To entice people to come here, the state issued a patent, and the water rights came with it,” he said. “Now, it’s like me coming to you and saying ‘Hey, you have a house. One of those bedrooms, I’m going to use it.'”

Who, where and what rights will be curtailed in coming weeks remains to be determined, water officials say. Cutoffs will be based on flows in the watershed — and how long rights have been held.

To defend their place in line, senior rights holders have rushed their ancient documents to analysts in the Division of Water Rights in Sacramento.

Oroville’s Richvale Irrigation District asserted rights dating back to the 1870s for construction of flumes and pipes for long-gone Cherokee Mines. Yolo County’s water district rights dates back to the diversion of Cache Creek in 1856.

In the Sierra Nevada foothills, the Kelsey Cattle Ranch’s rights were secured by an 1859 ditch dug by Erastus Kelsey. Granite Bay’s San Juan Water District traces its rights to an 1853 gold mine on the North Fork of the American River.

The vast irrigation districts in and around Turlock and Modesto also hold senior rights. So does the city of San Francisco, whose mayor hiked into the Sierra in 1902 to nail a claim to an oak tree along the Tuolumne River.

The struggle for California is how to monitor, balance and enforce 19th and 20th century rights that are more abundant than 21st century water.

“The rights system is manifestly archaic and absurd in 21st century California, when the lowest-value uses have at the same time the highest legal priorities,” said Wade Graham, an adjunct professor at the Pepperdine University School of Public Policy.

When Australia was faced with a 12-year drought beginning about the turn of the 21st century, Graham said, its governments agreed to manage their water in the national interest rather than on local rights. Graham said he thinks California could create new legal and economic incentives to improve its existing allocation system, rather than a “seizure” of rights, “which is politically and perhaps legally untenable.”

The state has a constitutional obligation to “the reasonable use of water and the public trust — this is above water rights seniority,” said Jay Lund, director of UC Davis’ Center for Watershed Science.

Lund thinks the fundamental system still works. What needs fixing, he said, is its administration. There’s no timely system of reporting usage, and there’s too little funding to enforce penalties for overuse, he said.

All the political struggles and financial uncertainties are a far cry from Mussi’s childhood, when water was abundant and assured.

“We jumped in ditches to catch catfish. We helped with irrigation, starting the small siphon pipes. We worried about flooding and kept an eye on the levees,” he said. “We got inner tubes and jumped in the canal, floating from one end to the other.

“Here in the Delta, we always knew we would have water,” he said. “It’s always been here.”

Always, that is, until now.

Contact Lisa M. Krieger at 650-492-4098. Follow her at Twitter.com/Lisa M. Krieger.