California politically tends to be completely opposed to building new reservoirs, and a large amount of seasonal northern California rainfall ultimately gets routed out to the San Francisco Bay through storm drainage.
The other barrel of this particular footgun is that agriculture is still California's biggest industry and they require an absolutely enormous amount of water. This is already causing strain on state politics, because the central valley keeps demanding more water from northern California, which doesn't want to give up more of it, and southern California has just about drained the Colorado river, and restrictions on water rights are the single biggest driving force behind all the "State of Jefferson" signs you see in the rural northern counties.
I love California but the water situation is about to bite them really hard. "About to", of course, still being a decade or two out, but there don't seem to be any reasonable solutions to this intractable situation on the horizon.
That alone would fix most of California's water problem.
Climate change doesn't mean warmer temperatures everywhere or a dearth of freshwater. The Hudson Valley area referred to here has plenty of fresh water, is projected to become wetter according to most climate change models, and has become wetter recently (significantly so the last few years).
http://www.cnyweather.com/wxrainsummary.php