Cal. Pub. Util. Code § 13174
Money set aside and placed in any such separate fund shall remain therein until from time to time expended for the purposes for which such bonds were issued, including the reimbursement of other funds of the district for expenditures therefrom for purposes for which such bonds were issued, made after the adoption by the board of the preliminary resolution of intention provided for in Section 13101, and shall not be used for any other purpose whatsoever, except for temporary investment thereof as provided in this division; provided, however, that money may be paid or transferred from any such separate fund, in furtherance of the purpose of its establishment, to any other such separate fund established for a like purpose in connection with the same issue of bonds.
Ca. Pub. Util. Code § 13174