Yes and No.
What I mean by that is that it's sometimes illegal to store and collect rainwater in California and other times it's not illegal to collect and store the rainwater.
Collecting rainwater in California that runs off your roof and through your gutters or from your own ponds etc is not illegal to catch that rainwater and store that rainwater in barrels or tanks.
However what is illegal is damming up creeks and rivers that run through your property as you can collect some water from a creek or river that runs through your property.
But you cannot stop the water flow from the creek to another property to take and collect all the rainwater from the creek.
But you can store and collect rainwater from your roof, the creek or any water that flows into or collects into buckets or rain water barrels legally.