For the purposes of a requisition referred to in section 85(1), the aggregate amount required to be raised from their constituent water authorities 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 and referred to as an “apportionment scheme”, and such aggregate amount shall be the sum by which the estimated expenditure of the water development board chargeable to the revenue account of their general fund exceeds the estimated income of that account, and there shall be added to, or as the case may be, deducted from that sum any sum required to be brought forward either as a debit or as a credit from a previous financial year.