Farmers in drought-stricken California with nearly guaranteed rights to water are bracing for historic orders Friday to stop diverting water from the San Joaquin watershed. Elsewhere, other farmers are trying to avoid a similar fate by offering voluntary conservation to stave off deeper cuts.

Some questions and answers about this pivotal development in the state's drought response:

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WHY IS THIS HAPPENING?

California is in its driest four-year stretch on record. Winter provided little rain and snow to replenish rivers and streams, meaning there is not enough water to meet the demands of farms, communities and wildlife. The State Water Resources Control Board is monitoring conditions in rivers and streams across the parched state and deciding who gets to divert water. Even those with long-standing legal rights to water are under scrutiny.

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WHAT ARE WATER RIGHTS?

The rights allow holders such as cities, irrigation districts serving farms, and corporations to take water directly from rivers and streams. The first to claim the water are the last to have supplies curtailed. Users who obtained rights to divert water after 1914 are the first to be cut off to ensure there is water for senior water rights holders with claims dating to the Gold Rush. Landowners with property that touches waterways have riparian rights — the strongest of the senior water rights.

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WHAT'S ALREADY HAPPENED?

Thousands of farmers and others with more recent, junior water rights in the Sacramento and San Joaquin River watersheds have been ordered to stop diverting water for the second consecutive year. Less than 30 percent have told the board they are complying.

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WHAT'S NEXT?

The board on Friday plans to order those with claims to water in the San Joaquin River watershed dating before 1914 to stop pumping from rivers and streams. Riparian rights holders are scheduled to be curtailed by mid-June. Friday's order would be the first restriction on senior water rights holders since severe drought the late 1970s, and the first in memory for the San Joaquin, which runs from the Sierra Nevada to San Francisco Bay.

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HOW IS THIS ENFORCED?

That's the challenge. Regulators lack enough sensors, meters and other technology to make sure water isn't illegally diverted. Water rights curtailments are instead enforced by an honor system, complaints and field investigations. Some curtailment orders are easily followed because there's no water to take from streams.

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WHAT ARE RIGHTS HOLDERS DOING ABOUT THIS?

Senior water rights holders see their claims to water as ironclad after they paid top price for land with nearly guaranteed water in dry California. Some of their attorneys have threatened litigation, saying the water board has no authority over them. Other farmers with water rights in the Sacramento-San Joaquin River Delta are offering to voluntarily conserve 25 percent of their water in exchange for assurances that they won't face additional cuts in the middle of their growing season.

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HOW IS THE STATE RESPONDING?

Thomas Howard, executive director of the State Water Board, says he'll announce by Friday whether to let riparian water rights holders take voluntary cuts to avoid curtailments. He says his decision hinges on whether the voluntary conservation would save enough water to reduce the strain on rivers and streams that are drying up. His decision would extend to waterfront property owners in the entire basin of the Sacramento River.