Cal. Water Code § 36425

In addition to the charges authorized to be fixed and collected pursuant to Article 4 (commencing with Section 35470) of Chapter 2 of Part 5 of this division, the board in lieu, in whole or in part, of raising money for the payment of the principal of and interest on bonds and warrants of the district issued for an improvement district or for the operating expenses of that improvement district, may fix and collect charges for the making of water available to the holders of title to land in that improvement district or the occupants thereon. The additional charges within an improvement district shall be fixed and collected in substantially the same manner and upon the same terms and conditions as is provided in Article 4 for charges to be fixed and collected within the entire district, except that the additional charges fixed and collected from an improvement district shall be expended only for the payment of the principal of, and interest upon, bonds and warrants of the district issued for the improvement district or for payment of the operating expenses of the improvement district.