California Code, Water Code - WAT § 6428

(a) Any owner who fails to pay any further fee or any part of a further fee required to be paid pursuant to Section 6305 within the time required shall pay a penalty of 10 percent of the further fee or part of the further fee, plus interest at the rate of one-half of 1 percent per month, or fraction of a month, from the date on which the further fee or the part of the further fee became due and payable to the state until the date of payment.

(b) Any owner who fails to pay any annual fee or any part of any annual fee required to be paid pursuant to Section 6307 within the time required shall pay a penalty of 10 percent of the annual fee or part of the annual fee, in addition to the annual fee or part of the annual fee, plus interest at the rate of one-half of 1 percent per month, or fraction of a month, from the date on which the annual fee or the part of the annual fee became due and payable to the state until the date of payment.