Cal. HNC § 6939.16

Whenever the revenues of the district are, or in the opinion of the board will be insufficient to provide for the payment in full of the principal or interest on the bonds, the board may, and if the revenues in the interest fund are insufficient to meet the next ensuing fiscal year’s interest on the bonds issued and then outstanding or if the moneys in the sinking fund are insufficient to pay or provide for the payment of any bonds maturing during the next ensuing fiscal year, or if any reserve fund has been drawn upon and not replenished it shall cause a tax to be levied and collected, upon all of the taxable property in the district, sufficient to meet all sums coming due for principal and interest on the bonds during the next ensuing fiscal year and before the time for fixing the next general tax levy in the county in which the district is located or to replenish such reserve fund.

The tax shall be levied and collected annually in the manner provided by Chapter 5 (commencing with Section 6940) of this part, and shall be in addition to all other taxes authorized to be levied and collected. When collected, the tax shall not be used for any purpose other than the payment of the bonds and the accruing interest thereon or the replenishment of such reserve fund.