Cal. Water Code § 1086

No witness shall be compelled to attend as a witness before the board under this division out of the county in which he resides, unless the distance is less than 150 miles from his place of residence to the place of hearing, except that the board, upon affidavit of any party showing that the testimony of such witness is material and necessary, may indorse on the subpoena an order requiring the attendance of such witness.