Cal. Wat. Code § 1775
If at any time it appears to the board, after a hearing of the persons interested and an investigation, that the full capacity of the works built or constructed, or being built or constructed, under an appropriation of water made pursuant to the Water Commission Act or this code has not developed or cannot develop the full capacity of the stream at the point where the works have been or are being built or constructed, and that the holder of the appropriation will not or cannot, within a period deemed to be reasonable by the board, develop the stream at that point to such capacity as the board deems to be required by the public good, the board may permit the joint occupancy and use, with the holder of the appropriation, to the extent necessary to develop the stream to its full capacity or to such portion of its capacity as may appear to the board to be advisable, by any persons applying therefor, of any such works.
Ca. Water Code § 1775