California Code, Public Utilities Code - PUC § 29121

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 general obligation bonds of the district 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 and payable, a tax sufficient to pay the annual interest on the bonds and such part of the principal thereof, including any sinking fund installments required by any of the district's agreements with its bondholders, as becomes due before the proceeds of a tax levied at the next general tax levy will be available for such purposes.