For the purposes of a requisition referred to in section 11(2) of this Act, the aggregate amount required to be raised from their constituent boards by a water development board for any financial year shall be apportioned in accordance with a scheme prepared and approved in accordance with the provisions of this Part of this Act and referred to as an " apportionment scheme"; and for the purposes of ascertaining the said aggregate amount section 12(1) of this Act shall apply as it applies to the aggregate amount required to be raised by a regional water board.