Cal. Health & Safety Code § 6550.15

All taxes collected pursuant to this article shall be expended only for the payment of bond principal and interest, or only for the improvement district project for which levied, except that any surplus tax proceeds levied otherwise than for payment of bond principal and interest which remain after the completion of any improvement district project for which levied shall be transferred to the bond interest and redemption fund of such improvement district, if any, otherwise to the district’s general fund.