The Secretary of the Interior is authorized, in his discretion, to amend any existing water-right contract to the extent necessary to carry out the provisions of sections 423 to 423g and 610 of this title, upon request of the holder of such contract. The Secretary of the Interior, as a condition precedent to the amendment of any existing water-right contract, shall require the execution of a contract by a water-users' association or irrigation district whereby such association or irrigation district shall be required to pay to the United States, without regard to default in the payment of charges against any individual farm unit or tract of irrigable land, the entire charges against all productive lands remaining in the project after the permanently unproductive lands shall have been eliminated and the charges against temporarily unproductive areas shall have been suspended in the manner and to the extent authorized and directed by sections 423 to 423g and 610 of this title.

The Secretary is authorized, in his discretion, upon request of individual water users or districts, and upon performance of the condition precedent above set forth, to amend any existing water-right contract to provide for increase in the time for payment of construction charges, which have not been accrued, to the extent that may be necessary under the conditions in each case, subject to the limitation that there shall be allowed for repayment not more than forty years from the date the first payment matured under the original contract, and also to extend the time for payment of operation and maintenance or water-rental charges due and unpaid for such period as in his judgment may be necessary not exceeding five years, the charges so extended to bear interest payable annually at the rate of 6 per centum per annum until paid, and to contract for the payment of the construction charges then due and unpaid within such term of years as the Secretary may find to be necessary, with interest payable annually at the rate of 6 per centum per annum until paid.

The Secretary of the Interior is authorized to complete and execute the supplemental contract, being negotiated on May 25, 1926, and which had, on that date, been approved as to form by the Secretary, between the United States and the Belle Fourche Irrigation District and at the expiration of said supplemental contract to enter into a permanent contract on behalf of the United States with said District in accordance with the terms of said supplemental contract.