Annually, at least 15 days before the first day of the month in which the board of supervisors of the county in which the district is located is required by law to levy the amount of taxes required by law for county purposes, the district board shall furnish to the board of supervisors a written statement of the amount necessary to pay the interest on bonds for that year, and the portion of the principal that is to become due before the time for making the next general tax levy.