Only alterantive is to cut back some of the corn/soybean production in the midwest and grow some of the other crops grown in California. If that doesn't work, like you said, we can import water but other states need to cooperate and the federal government should chip in some funding. You can't just tell Californians to suck it up and figure this out by themselves when the state has >10% of the population plus many essential crops and industries.
I don't think there is a need to cut back on corn/soybean production. But we definitely need to be growing more fruits and veggies in the midwest.
Last year Illinois passed two laws: one prevents towns and cities from restricting vegetable gardens. If you want to fill your front yard with a hoop house, your city can't stop you. The other forces each county to establish guidelines for p2p food sales and prevents town and cities from stopping it. This needs to be adopted in other states.
Realistically, California could and should start a major water works project to import water from the east or pacific northwest.