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 general obligation bonds and warrants of said district for such improvement district as they become due and also for the payment of operating expenses of said 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 general obligation bonds and warrants of the district for said improvement district and the interest thereon as the same becomes due and also to pay operating expenses for said improvement district.