Cal. WAT § 1241

If the person entitled to the use of water fails to use beneficially all or any part of the water claimed by him or her, for which a right of use has vested, for the purpose for which it was appropriated or adjudicated, for a period of five years, that unused water may revert to the public and shall, if reverted, be regarded as unappropriated public water. That reversion shall occur upon a finding by the board following notice to the permittee, licensee, or person holding a livestock stockpond certificate or small domestic use, small irrigation use, or livestock stockpond use registration under this part and a public hearing if requested by the permittee, licensee, certificate holder, or registration holder.