If a contract with the state, the department, any other district, or the United States requires the payment by the district of periodic service charges, the amount shall be levied from time to time as an assessment on land in the district, to the extent that funds are not otherwise available for the payment of such charges. Upon request of the board, assessment commissioners shall be appointed under Chapter 2 (commencing with Section 46150) of Part 9 of this division who shall determine the portion of the total benefits to accrue to the district under the contract which will be realized by each parcel of land in separate ownership in the district. The determination shall be stated in apportionment rolls substantially in form and manner as provided in Chapter 2 of Part 9 of this division, except that the apportionment of benefits shall be stated in percentages of benefits to the district as a whole, or in such other manner as may be determined upon by the commissioners. Thereafter all assessments levied in order to meet the payments due or to become due under the contract shall be apportioned among the respective tracts of land in accordance with the rolls. Any such assessment shall be collected in the same manner as an original assessment.