If after the levy of the annual assessment for the current year, the board finds that because of some unusual or unforeseen cause funds raised through the collection of the assessment, and from other sources, will not be sufficient for the proper maintenance and operation of the district, and the irrigation works of the district, the board may borrow additional funds needed to an amount not to exceed twenty dollars per acre [.40 hectare] for the irrigable lands within the district and may pledge the credit of the district for the payment of the loan, or the board may issue bonds in anticipation of further collections. The board shall include in the levy for the ensuing year the amount required to pay the loan or to retire the bonds.