A couple of nuke plants powering desalination and California could be exporting water to the east. Of course, that assumes a willingness to take their water issues seriously - it's not like they haven't seen it coming for several decades and could have all this in operation already.
For the record, California has an estimated 5.8 GW of wind turbine generation, and in the year 2019 alone, California ADDED 3.1 GW of solar generation capacity. This doesn't require a Nuclear plant or crazy economics or giant nationwide plan. The actual building, machinery, and pipework for desalinating would be a larger infrastructure project than the extra power generation to power it.
The only thing this requires is for Californian government to get their head out of their asses, and stop expecting everyone else to just bend over backwards for them.
I think the problem is that desalinating water (ignoring the environmental impact of the resulting brine) will produce water that costs at a minimum many hundreds of dollars per acre-foot. This is fine for urban use, but makes many of the current agricultural uses uneconomical. Nobody will pay $5,000/acre for water so they can sell their crop for $1,000/acre. All of these discussions dance around the fact that irrigation farmers are largely turning X dollars of water into 0.25X worth of crops, and the taxpayers are making up the difference. This is true of farmers in pretty much every state that uses colorado river water - if we erased 'water rights' and allowed the water to be competitively bid on, many farmers would either leave their land fallow or switch to less water intensive crops. We're basically giving away extremely valuable water at way below market rates, we're practically guaranteed to have a 'shortage.' It doesn't make sense to add enormously expensive infrastructure just so we can turn around and keep giving the water away.
