To levy and collect taxes as herein provided, the board shall, in each year, determine the amount of money necessary to be raised by taxation, taking into consideration other sources of revenue of the district, and shall fix a rate of levy which, when levied upon every dollar of assessed valuation of taxable property within the district, and with other revenues will raise the amount required by the district annually, to supply funds for paying expenses of organization and the costs of construction, operating and maintaining the works and equipment of the district, and promptly to pay in full, when due, all interest on the principal of bonds and other obligations of the district, and in the event of accruing defaults or deficiencies, an additional levy may be made as provided in section 42-3215. The board shall, on or before the first day of September of each year, certify to the board of county commissioners of each county within the district, or having a portion of its territory within the district, the rate so fixed with directions that at the time and in the manner required by law for levying taxes for county purposes, such board of county commissioners shall levy such tax upon the assessed valuation of all taxable property within the district, in addition to such other taxes as may be levied by such board of county commissioners at the rate so fixed and determined.