The total amount provided in the budget to be raised by taxation shall in no instance exceed the amount of taxes authorized by law to be levied during that year, including the amounts necessary to meet outstanding indebtedness, as evidenced by bonds, coupons, or warrants regularly issued. No changes shall be made in the budget after its adoption, except as provided by sections 13-511 and 23-918. In arriving at the amounts required to be raised by taxation for each fund, the total requirements, the outstanding warrants, and the operating reserve shall be added and from such total shall be deducted the revenue from sources other than taxation and the cash on hand on June 30. The operating reserve in no event shall be more than fifty percent of the total expenditures for the fund during the last completed fiscal year. The income of the county, as estimated in the budget, shall be and become applicable in the amounts and according to the funds specified in the budget for the purpose of meeting the expenditures as contemplated and set forth in the budget.