Cal. Water Code § 72687

The taxable property in the annexed areas shall be subject to taxation after the annexation thereof for the purposes of the improvement district, including the payment of the principal of and interest on bonds and other obligations of the improvement district authorized and outstanding at the time of the annexation. If the terms and conditions established by the board specifically so provide, the taxable property in the annexed area shall be subject to taxation as if the annexed property had always been a part of the improvement district.