No new sources
The graphic above was pulled from the Little Hoover Commission (LHC) executive summary and shows that California has less than about 1 MAF water growth COMBINED available from new sources (surface storage, forest management, desalination, cloud seeding, rain dances and prayers) assuming we meet the low estimate in each category. Meeting the high estimate of increased surface storage has the greatest impact of that bunch, but probably requires significant new infrastructure (dams) which don't seem to have political traction right now and even then could only account for an additional 1M increase.
On the other hand, of the four top potential water sources, NONE exploit new water. ALL are savings estimates based on conservation (though they call it efficiency in the case of ag).
Increased agricultural efficiency is the lowest of the top four in terms of savings and seems either hit or miss, with an order of magnitude difference between low and high estimates. That order of magnitude uncertainty is not something that encourages putting great faith in ag savings.
Groundwater storage is the next lowest potential payback of the top four. I believe that they are referring to natural infiltration rather than pumping excess water underground in times of excess. Interestingly, there's a current groundswell of support among gardeners, city planners, and environmentalists for better groundwater infiltration to prevent storm run off and one can find many examples of urban rain gardens designed to capture, use, and infiltrate rain water rather than sending it to the storm sewer system. This seems a feasible method to capture more water, since it is already gaining traction, requires no central planning other than infiltration standards, is distributed, can be incrementally implemented, and has other beneficial effects such as improving coastal water quality. A successful PR campaign might make even more headway here.
The two greatest potential areas of saving, recycled water (so called "toilet to tap" programs) and urban efficiency (low flow toilets, shorter showers, less garden watering, etc) are also both conservation measures. I'm not sure I need to say much about them other than to point at the graph to show how much more potential is there than in any other measure.
The important point here is that we have about 1 MAC more water that is feasible from new sources or better management of existing sources and more than 3.5 MAF available from conservation measures, assuming we meet the minimum in each category. So excuse the hyperbolic headline trumpeting "no" new sources, it's just that conservation trumps new sources in the two most critical categories of cost and impact.
See On the pulic record blog for more insighful commentary on this topic.
As a point of reference, I think that California's industry and population uses about 9 million acre feet (MAF) of water a year, a number I extrapolated from elsewhere on the web but which could be wrong since I'm was a bit careless. I'm not sure if this includes ag. In any case, this number is a convenient reference, since 1 MAF is about 10% of 9 MAF and the graphic breaks things down in bite sized 0.5 MAF increments.