Here is an interesting page about water supply and use in California:

Water is life. It's also a battle.

California imports zero water for Central Valley agriculture. That all comes from the Sierras. The Colorado River Aqueduct swings South, and comes into San Bernardino South of the mountains. It ends at the Metropolitan Water District, which supplies water for the greater LA and San Diego areas. LA also gets a lot of it's water from the Eastern Sierra. That's the system Mullholland built. MWD also gets some water pumped South from the Sacramento River Delta. So getting less water from the Colorado River will not directly affect Central Valley agriculture, unless MWD gets more of "their" water pumped South. (That's actually more expensive than desalination!)

The big problem for Central Valley ag is low rain and snowpack in the Sierras. We do "import" a little bit of water in far Northern California, but that's just the natural flow of the Klamath River. Actually, you could even say we don't "import" Colorado River water, since the Western shore is in California for about 195 miles. Every drop of water Southern California desalinates, cuts the amount we need to take from the Colorado River. That page I quoted lists the cost of desalination versus transportation (and everything else). They say desalination costs 40% more than water transportation, but desalination probably includes the profit the desalination company makes, and water transportation is heavily subsidized by cheap electricity rates. When I compared the number of MWhrs of electricity used for transporting water from the Colorado River per acre foot, to the amount the desal plant uses per acre foot, they were about the same.

We're flying electric helicopters on Mars yet you can't turn on your clothes dryer in Texas. That's because scientists are in charge of Mars, and Republicans are in charge of Texas.