Legal Question in Real Estate Law in California

Probably not. California has quite a lot of case law (law based upon decisions of its Court of Appeal and Supreme Court) about the rights abd responsibilities of neighbors with respect to artifically-concentrated surface waters, whether ponded up and causing flooding or running and creating artificial streams and gullies.

Most of this law reflects the principle that one neighbor can concentrate and/or divert surface water reasonably, but not in a way that causes unreasonable harm to another neighbor. For example, an uphill neighbor is behaving reasonably when he develops his land by building a house, even if the runoff hits the downhill parcel in a different spot or in a slightly increased volume, if no harm is done. On the other hand, paving acres of uphill land and preventing natural absorption then directing all the runoff into the downhill neighbor's front yard would be quite unreasonable and could be stopped by injunction. The downhill neighbor can also act unreasonably by building dikes along the boundary to cause water to back up onto the uphill neighbor's land.

Unless your neighbor obtains a very substantial benefit from what he's doing, and there is no other practical way to do what he needs to do that causes you less harm, you probably have a pretty good case for getting money damages, an injunction, or both.