(1)By such a date as the Secretary of State may specify, a water development board shall prepare and submit to him a scheme for his approval as an apportionment scheme.
(2)Such a scheme as aforesaid shall provide for an apportionment between the constituent boards of a water development board of the aggregate amount to be requisitioned from those boards and shall have regard to the general principle that the amount to be requisitioned from each constituent board is to be proportionate to the quantities of water supplied and expected to be supplied to that board by the water development board.
(3)When such a scheme as aforesaid has been approved, with or without modifications, by the Secretary of State as an apportionment scheme, it shall come into operation on such a date and for such a period (if any) as may be specified in the apportionment scheme, and, where any period is so specified, the Secretary of State may approve an application for an extension of that period.