Farmers and other landowners who staked the earliest claims to California's water are the latest consumers to face mandatory cutbacks ordered by the state during the relentless drought.
Here's a look at those senior water rights holders:
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WHO ARE THEY?
More than half are corporations such as power companies that use water for hydroelectric dams. Claims are also held by rural irrigation districts that serve thousands of farmers throughout Central California and the Sierra Nevada foothills. Water departments in San Francisco and Los Angeles are among the biggest users, although those agencies have multiple sources of water and have yet to be curtailed.
HOW DID THEY GET WATER RIGHTS?
Establishing an early right to California water was as simple as going ahead and diverting it. The paperwork came later. San Francisco got the Sierra Nevada water that fed its lush gardens by tacking a handwritten notice to a tree in 1902. The state started requiring applications and monitoring consumption after 1914 but exempted previous claims from the process.
WHO WATCHES THEM?
The State Water Resources Control Board oversees the water rights system and decides when conditions are too dry to meet demand. Every three years, it requires senior water rights holders to report how much water they have diverted, used and conserved. Since regulators lack widespread remote sensors or meters, they depend largely on the honor system to guard against illegal uses and conduct field inspections in response to complaints.
HOW MUCH WATER DO THEY USE?
Trillions of gallons a year, but no one knows exactly how much. An Associated Press investigation last year found the data riddled with obvious errors. State water officials concede they need better real-time monitoring of flows and diversions. As it is, they generally enforce cutback orders when someone complains.
WHY DO THEY HAVE SPECIAL STATUS?
Much of the parched West was developed around the principle that water goes to those who claimed the limited resource first. Unused water may be sold or transferred. But California is unusual among Western states, in that it doesn't monitor senior rights holders as closely as it does junior rights holders.