Cal. Water Code § 51871

When an assessment has been levied upon district lands and any installment of the assessment called by the board or by the county treasurer is adjudged invalid by any court of competent jurisdiction, payments of the installment, including penalties and interest, made by any person before the call was adjudged invalid, may be credited by the treasurer of the county where the assessment list is filed on the assessment on which the invalid call was paid, and shall be applied to the payment of any subsequently called installment of the assessment on the parcel or upon demand, the county treasurer shall refund the amount of the installment adjudged invalid to the person who made the payment.