California Code, Water Code - WAT § 51521

(a) Any installment of an assessment for which no bond or time warrant has been issued, which is unpaid after 60 days from the date of the order, is delinquent with the accrued interest thereon, and a penalty of 10 percent of the amount of the installment and interest shall be added thereto and collected for the use of the district.

(b) Notwithstanding subdivision (a), the board may, for good cause, collect the unpaid installment described in subdivision (a) and waive the penalty or interest.