The board shall then levy an assessment sufficient to raise the annual interest on the outstanding bonds, and all payments due or to become due the ensuing year to the United States under any contract between the district and the United States, accompanying which bonds of the district have not been deposited with the United States as in section 46-126 provided, which when collected, shall be called the bond and United States contract fund of .......... Irrigation District. At the expiration of ten years after the issuing of the bonds the board must increase such assessment for the ensuing years in a percentage of the whole amount of bonds outstanding, as follows: For the eleventh year, five percent; for the twelfth year, six percent; for the thirteenth year, seven percent; for the fourteenth year, eight percent; for the fifteenth year, nine percent; for the sixteenth year, ten percent; for the seventeenth year, eleven percent; for the eighteenth year, thirteen percent; for the nineteenth year, fifteen percent; and for the twentieth year, a percentage sufficient to pay off such bonds.