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.

Such a scheme as aforesaid shall provide for an apportionment between the constituent water authorities of a water development board of the aggregate amount to be requisitioned from those authorities and shall have regard to the general principle that the amount to be requisitioned from each constituent authority is to be proportionate to the quantities of water supplied and expected to be supplied to that authority by the water development board.