As discussed in a recent post, rainwater harvesting is indeed gathering momentum in Western USA.
The Legal Planet Blog has a post on this water supply augmentation method.
Basically, urban rainwater harvesting transforms water proof city areas (paved roads, roofed perimeters,...) into small artificial watersheds superimposed on natural watersheds. Rainwater that runs off in the artificial watershed ends up in a cistern and gets disconnected from the environment.
Rainwater harvesting is certinaly a good idea and an improvement on wasting water through unitary sewers. However, water is a limited resource: there is only a limited volume of it in one place at one point in time. The implementation of large scale rainwater harvesting should be coupled with residual environmental flow protection (as in Australia) to avoid further water deprivation for environmental uses.
Given the American penchant for big absolute solutions, it would seem adequate to remain extra cautious about implementing rainwater harvesting on a huge scale before the safeguards are there to protect environmental water uses. Is there legal environmental flow protection in California?