That the net earnings of such power plant, and such plant and all property and property rights connected or used therewith, except property the title to which is vested in the United States pursuant to the act of congress of June 17, 1902 (32 Stat. 388) and acts amendatory thereof or supplementary thereto, shall be and remain as security for the payment of said bonds and the interest thereon; that in the event the net earnings from said power plant are not sufficient to pay said interest and provide for the refunding of said bonds, as hereinbefore provided, then the board of directors of said district shall, by levying annual assessments upon the lands of said district, raise the required funds, and all the real property in the district shall be made and remain liable to be assessed for such payments.