Except to the extent that provision has otherwise been made by the board, as may be permitted by law, for the payment of the principal of and interest upon bonds of the district for the improvement district as they become due and also for the payment of the operating expenses of such improvement district, the annual estimate made by the board of the sum required by the district to discharge all of its obligations shall, in addition to the other amounts required to be included therein, include an amount sufficient to pay the principal of bonds of the district for such improvement district and the interest thereon as the same becomes due and also to pay operating expenses for the improvement district.