Water Code § 51502

At least 90 days before any installment of interest is due on any time warrants the board shall estimate the amount necessary to pay the interest, after crediting thereon the funds in the “warrant interest fund” applicable to the payment thereof, and shall add thereto 15 percent of the aggregate sum to cover possible delinquencies, and make a call on interest due on any unpaid unbonded assessment equivalent to the amount so estimated.