At least one (1) year before the first principal maturity of said bonds (as fixed by the board of commissioners pursuant to section 42-2954[, Idaho Code,]) and thereafter annually, the drainage commissioners of such district issuing them are hereby authorized and required to levy an assessment sufficient to liquidate said bonds at their respective maturities. Such assessment shall be collected by the official whose duty it is to collect the county taxes, and kept as a separate fund for the sole purpose of liquidating said bonds in accordance with the provisions of the following section. The annual levies of assessments to pay bonds issued prior to the effective date of this amendment shall be made according to the law in existence at the time of the issuance of said prior bonds and according to the maturity needs thereof.