California Code, Public Resources Code - PRC § 5569

For the purpose of paying all sums coming due for principal and interest on all bonds of the district, there shall be levied and collected each year a tax sufficient to pay the annual interest on such bonds and, also, that part of the principal which will become due before the time for fixing the next general tax levy.  If the maturity of the indebtedness created by the issue of bonds is made to begin more than one year after the date of the issuance of the bonds, the tax shall be levied and collected each year, to pay the interest on the indebtedness as it falls due, and, also, to constitute a sinking fund for the payment of the principal on or before maturity.

The taxes in this section required to be levied and collected, shall be in addition to all other taxes levied for district purposes, and shall be collected at the same time and in the same manner as other district taxes are collected, and shall be used for no purpose other than the payment of said bonds and accruing interest.