A staggering amount of California's water is used for alfalfa, which gets shipped to China to feed cows. In essence, we are allowing certain farmers to sell CA's water to China.
The water is owned by the farmers, not by California. Probably the state should eminent domain it, but I have no idea what it would cost to do so.
Outlawing alfalfa growth in California from allocated water would not conflict with anybody's "rights". They can grow something else there that uses less water. Or (and this is something being done, in the Sacramento River delta) flood the land and collect carbon credits for sinking swamp vegetation.
Much of the farmland in the Sacramento River delta is well below river and, indeed, sea level, some over 20 feet below, behind dikes (called there "levees") which threaten to collapse in the next big earthquake, pulling in a toxic load of salt water from the bay. (This has happened more than once; the dikes are not in better shape today.) Flooding fields gradually is hoped to lead to a sustainable future for the river. Erstwhile farmers could also float solar panels and collect more revenue. Raise freshwater fish and shrimp, besides.
California agriculture north of Bakersfield does not use up quite so much water as allocation seems to indicate; the water evaporates and is blown to the Sierra Nevada mountains where much of it precipitates, and ends up in rivers again.