If California could encourage more people to invest a few hundred bucks in rain catchment storage..that would put a dent in the problem.
As it stands now I doubt more than 1% of the homes are so equipped.
Years ago I found 50 gallon plastic barrels for $15 each that were used by a brewery for liquid malt extract.
I found plastic fittings and hose [at Lowes] to "gang the barrels together" so that after the first barrel fills, it over flows into the next barrel, then into the next. By the time you get to the last barrel the contents are pretty pure.
Before I send the rain gutter runoff into the barrels, it is sent into a 5 gallon bucket that has a paint strainer fitted to it..then the output is sent to the barrels. Lowes has these paint strainers also.