Water use "facts" get skewed to represent the views of the writer. The #1 abuse is the amount of water "used" by farming. If you measure the amount fed into the field you get one number, if you measure the amount consumed by the crop, its a different number. Irrigation is far from perfect and the difference between inflow and consumed is either returned to the irrigation supply or percolates below the root zone and recharges the underground aquifer.
The second most misunderstood fact is most northern California farms are served by irrigation districts formed before statehood and had water supplies secured before the state and federal dams were built. These districts agreed to allow the government agencies to build the dams (which would impede the use of existing supplies) for the good of the state as long as their original rights were honored. Now many environmental groups love to claim farmers are using "subsidized water" to grow subsidized crops when the fact is the water owned by the water district is flowing through a public built dam but not owned or controlled by the public.
The water use below the Delta is a another can of worms I will leave to someone more knowledgeable but I will comment some is diverted but other supplies were secured/built long before the state/fed got involved.