On and after December 22, 1944, whenever the Secretary of the Army determines, upon recommendation by the Secretary of the Interior that any dam and reservoir project operated under the direction of the Secretary of the Army may be utilized for irrigation purposes, the Secretary of the Interior is authorized to construct, operate, and maintain, under the provisions of the Federal reclamation laws (Act of June 17, 1902, 32 Stat. 388, and Acts amendatory thereof or supplementary thereto), such additional works in connection therewith as he may deem necessary for irrigation purposes. Such irrigation works may be undertaken only after a report and findings thereon have been made by the Secretary of the Interior as provided in said Federal reclamation laws and after subsequent specific authorization of the Congress by an authorization Act; and, within the limits of the water users’ repayment ability such report may be predicated on the allocation to irrigation of an appropriate portion of the cost of structures and facilities used for irrigation and other purposes. Dams and reservoirs operated under the direction of the Secretary of the Army may be utilized after December 22, 1944, for irrigation purposes only in conformity with the provisions of this section, but the foregoing requirement shall not prejudice lawful uses now existing: Provided, That this section shall not apply to any dam or reservoir heretofore constructed in whole or in part by the Army engineers, which provides conservation storage of water for irrigation purposes. In the case of any reservoir project constructed and operated by the Corps of Engineers, the Secretary of the Army is authorized to allocate water which was allocated in the project purpose for municipal and industrial water supply and which is not under contract for delivery, for such periods as he may deem reasonable, for the interim use for irrigation purposes of such storage until such storage is required for municipal and industrial water supply. No contracts for the interim use of such storage shall be entered into which would significantly affect then-existing uses of such storage.