Whenever a contract of indebtedness has been created by the district, it shall be lawful for the board to make the annual levy of taxes and special assessments in such amount as will create a surplus of funds to meet the annual installments of indebtedness and interest and the necessary maintenance and operating charges, and the board shall cause such surplus funds to be placed in a sinking fund which may be used for the payments of contingencies, defaults and delinquencies, and to pay the future annual installments of indebtedness on contracts and interest.