No contractor shall secure a right to the use of water from American Falls Reservoir, Minidoka project, except under a contract containing the provision that the contractor shall, as a part of the construction cost, pay interest at the rate of six per centum per annum upon the contractor's proper proportionate share, as found by the Secretary of the Interior, of the moneys advanced by the United States on account of the construction of said reservoir prior to the date of the contract.