Cal. Water Code § 137

The Department of Water Resources shall keep a record of all expenditures chargeable against each specific portion of the revolving fund, and any unused balance in any portion of the fund shall on approval by the Department of Finance be withdrawn from the revolving fund and transferred to the credit of the appropriation from which it was transferred or, as to funds from other than state appropriations, be paid out or refunded as provided in the agreement relating to the contributions.