Cal. Water Code § 8164

The proceeds of the collection of such drainage district taxes and assessments shall be applied solely and exclusively to the payment of the principal and interest for which they were levied and the surplus, if any, of the proceeds of such collections shall be deposited in the county treasury to the credit of the interest and sinking fund of such flood control district and used solely and exclusively for the payment of principal and interest of the bonds of such flood control district.