Cal. Gov. Code § 50615

Any unexpended balance remaining in the special fund for the district after the payment of the costs and expenses of the maintenance for which the assessment was levied shall be credited to the fund to be raised for the next ensuing period of maintenance for the district. Whenever the legislative body does not order or reorder maintenance as authorized in this chapter, any such unexpended balance may be retained in the fund to be used, until exhausted, to defray the costs and expenses of maintaining and caring for the open spaces benefiting the district for which the annual assessment had been levied.