The cost and expense of purchasing and acquiring property and constructing the works and improvements provided for in the Irrigation District Act shall be wholly paid out of the construction fund, or in the bonds of the irrigation district at their par value, after having first advertised the same for sale as provided in section 46-1,100, and having received no bids therefor of ninety-five percent or upwards of their face value. In case such bonds or the money raised by their sale is insufficient for the purposes for which the bonds were issued, additional bonds may be issued, after submission of the question at a general or special election to the electors of the district. In case of the issuance of additional bonds, the lien for taxes for the payment of the interest and principal of such issue shall be a subsequent lien to any prior bond issue. However, the provisions of this section shall not apply where the cost and expense of purchasing and acquiring property and constructing the works and improvements provided for in the Irrigation District Act are covered by contract between the district and the United States. In lieu of the issuance of additional bonds, the board of directors may provide for the completion of the irrigation system of the district by the levy of an assessment therefor in the same manner in which levy of an assessment is made for the other purposes provided in the Irrigation District Act.