The assessment valuations shall be used as a basis for levying assessments against the parcels described in the operation and maintenance assessment roll for the purpose of raising funds for the maintenance, repair and operation of the district reclamation works, the payment of incidental expenses thereof, and for the construction of reclamation works supplemental to, or in replacement of, those already possessed, when in the opinion of the board, such works are necessary for carrying out the original or any supplemental plan of the district.