At the election at which the plan of taxation is determined or at any other time before the bonds are issued, the voters of a district which is not operating under a contract with the United States may vote on the proposition of whether or not benefits for tax purposes shall be fixed as an equal sum on each acre of land that is irrigated or to be irrigated by gravity flow from the canal system of the district. The benefit per acre shall be voted on as it is applied to land in the district that can be irrigated by gravity flow from the irrigation system and also the benefit to land in the district that cannot be irrigated by gravity flow.