(2) In irrigation districts which enter into a contract with the United States providing for the payment of charges to the United States upon the basis authorized by the Act of Congress of December 5, 1924, entitled "An act making appropriations to supply deficiencies in certain appropriations for the fiscal year ending June 30, 1924, and prior fiscal years, to provide supplemental appropriations for the fiscal year ending June 30, 1925, and for other purposes," annual levies of assessments for the purpose of making payment to the United States under the contract may be made by the board on the basis provided for in that Act and the contract, either before or after judicial confirmation of apportionment of benefits. In such districts annual levies for the purpose of making construction payments to the United States may be made on the basis of the gross average annual acre-income of the lands of the district or divisions of the district, or classes of lands in the district, as such gross average annual acre-income is determined by the Secretary of the Interior, until the amount apportioned against each tract has been fully paid. [Formerly 545.438]