It only seems simple if you're the kind of imbecile they put on the staff at the National Review. California has several reservoirs that aren't fed by gravity runoff, but by pumping ("off stream" reservoirs). All of the newly proposed reservoirs are of this type, such as the Sites Reservoir. To fill these you need excess water elsewhere. We don't have excess water elsewhere, thus we could not fill them. This isn't a simple matter of time-shifting water from one year to the next because as the OP is pointing out this is a long-term phenomenon that is permanently decreasing our steady-state precipitation.