Cal. Water Code § 71687.1

Based on such findings the board shall, by resolution, determine what portion, if any, of the estimated cost of purchasing water for replenishment and makeup for the ensuing fiscal year shall be paid for by a replenishment assessment, and what portion, not exceeding 25 percent of the said portion, of the estimated cost of purchasing in the future that quantity of water which should be purchased during the ensuing water year, but which is estimated to be unavailable during that year, should be raised by a replenishment assessment.