Legal Question in Real Estate Law in California

That is a surprisingly complex question. You will need to consult a knowledgable real estate attorney in person and go over all the details. Water discharge rights are too complex for an internet Q&A.

While I agree that complicating side issues frequently arise in water-discharge cases, I think you can get an appreciation for the basic law from reading a case or two. Perhaps start with what the California Supreme Court said in the landmark case of Keys v. Romley, decided in 1966 and reported as 64 Cal.2d 396. You can probably find it on line by using the search terms "Keys" and "Romley." I don't think this is the last and final word, but you can get a feel for whether the neighbor's discharge of water might be basis for a lawsuit or a demand that he make corrections.