The board of directors of any irrigation district in this state sustaining contractual relations with the United States shall have the power to borrow funds for the purpose of making any necessary payments thereon and to pledge the credit of the district for the payment of the same. The board of directors of any irrigation district in this state shall have the power to borrow funds to meet the necessities of any unforeseen or unusual conditions arising in the operation and maintenance of the irrigation system of such district and to pledge the credit of such district for the payment thereof. The total sum borrowed by any district under the provisions of this section shall at no time exceed two-thirds the amount of the general fund levy of such district for the preceding year. If the levy for the then current year shall be insufficient to provide for the payment of the sum or sums so borrowed, then such payment shall be provided for in the levy for the year next ensuing.