If in any budget year the receipts of revenues of the system should from some unforeseen cause become insufficient to pay current expenses of the operation, maintenance, or debt service of the system, or if any calamity or other public emergency should cause the necessity of making extraordinary expenditures, the board may make an emergency appropriation of an amount sufficient to meet the deficiency and may authorize the issuance, negotiation, and sale of certificates of indebtedness in this amount. The board shall forthwith levy on all taxable property subject to special assessment for the system a tax sufficient to pay the certificates and interest thereon, and shall appropriate all collections of such tax to a special fund created for that purpose. The certificates may mature not later than April in the year following the year in which the tax is collectible. Emergency certificates of indebtedness shall be a general obligation of the county, and may be refunded by the issuance of bonds pursuant to section 116A.20, if the board determines that payment thereof in one year would place an undue burden upon property assessable for the system and that net revenues of the system will be sufficient to pay the refunding bonds as well as outstanding bonds and interest thereon when due.