Whenever a development of power is necessary for the irrigation of lands under the Salt River reclamation project, Arizona, or an opportunity is afforded for the development of power under said project, the Secretary of the Interior is authorized, giving preference to municipal purposes, to enter into contracts for a period not exceeding fifty years for the sale of any surplus power so developed, and the money derived from such sales shall be placed to the credit of said project for disposal as provided in the contract between the United States of America and the Salt River Valley Water Users' Association, approved September 6, 1917: Provided, That no contract shall be made for the sale of such surplus power which will impair the efficiency of said project: Provided, however, That no such contract shall be made without the approval of the legally organized water-users' association or irrigation district which has contracted with the United States to repay the cost of said project: Provided further, That the charge for power may be readjusted at the end of five-, ten-, or twenty-year periods after the beginning of any contract for the sale of power in a manner to be described in the contract.