The most obvious confounding factor is that California relies heavily on Sierra snowpack to provide water throughout the year. Snow that melted last year and ran out to sea is simply no longer available to us this year, no matter how much of it there was. There is no economically plausible way to capture enough of the excess in one year to last us through extended dry periods.

Further, we need the vast majority of this moisture to come as snowpack in the year it does fall, so it can be naturally distribute throughout the warmer months. When most of that water arrives in the spring and summer, it quickly runs off. As things get drier, this problem worsens since the ground becomes less able to absorb moisture in the short term.