Craig Miller at KQED has a useful roundup of what sort of shortfalls California water users might see this summer as a result of drought:

• Ag: “More than 400,000 acres of farmland were fallowed last year because of scarce water. Credible sources have estimated that figure could double this year.” That’s in the neighborhood of 10 percent of California’s irrigated acreage.
• Big munis will have to cut back, but will not run out of water. Some rural systems will be stressed: “Cowin hastens to add that ‘the vast majority of our citizens will not run out of water,’ some already have, mostly in rural areas where wells have gone dry.”
• Pressure on aquifers to make up for surface shortfalls will grow: “Groundwater resources will be stressed even more, as water-constrained farmers turn up the pumps to offset cuts in allocations from state and federal water projects.”