The following are hereby declared to constitute unappropriated water:
(a) All water which has never been appropriated.
(b) All water appropriated prior to December 19, 1914, which has not been in process, from the date of the initial act of appropriation, of being put, with due diligence in proportion to the magnitude of the work necessary properly to utilize it for the purpose of the appropriation, or which has not been put, or which has ceased to be put to some useful or beneficial purpose.
(c) All water appropriated pursuant to the Water Commission Act or this code which has ceased to be put to the useful or beneficial purpose for which it was appropriated, or which has been or may be or may have been appropriated and is not or has not been in the process of being put, from the date of the initial act of appropriation, to the useful or beneficial purpose for which it was appropriated, with due diligence in proportion to the magnitude of the work necessary properly to utilize it for the purpose of the appropriation.
(d) Water which having been appropriated or used flows back into a stream, lake or other body of water.