Cal. PUC § 12893

The board shall, at the time of fixing the general tax levy and in the manner provided for the general tax levy, 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, a tax sufficient to pay the annual interest on the bonds and such part of the principal thereof as becomes due before the time for fixing the next general tax levy. 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.