Pub. Util. Code, § 70212

The board shall, in addition to the general tax levy as set forth in Section 70210, levy and collect annually until the district’s bonds are paid, or until there is a sum in the treasury of the district set apart for that purpose to meet all sums coming due for principal and interest on the bonds as they become due a tax sufficient to pay the annual interest on the bonds and such part of the principal thereof as becomes due before the proceeds of a tax levied at the next general tax levy will be available. If the maturity of the indebtedness created by the issue of bonds begins more than one year after the date of the issuance thereof, the tax shall be levied and collected annually at the time and in the manner aforesaid, sufficient to pay the interest on the indebtedness as it falls due and to constitute a sinking fund for the payment of the principal on or before maturity.