For the purpose of defraying the expenses of the organization of the district, and the care, operation, management, repair and improvement of such portions of such canal and works as are completed and in use, including salaries of officers and employees, or repayment of any contract for construction by and between the United States of America and any irrigation district as provided in section 46-126, the board may either fix rates of tolls and charges, and collect the same from all persons using such canal for irrigation or other purposes, or may provide for the payment of such expenditures by a levy of assessments therefor, or by both tolls and assessments; if by the latter method, such levy shall be made upon the completion and equalization of the assessment roll; and the board shall have the same powers and functions for the purposes of such levy as are now possessed by boards of supervisors in this state, and such assessment shall be collected as provided in section 46-136. If, after the annual assessment for the current year, the funds provided are for some unusual or unforeseen cause insufficient for the proper maintenance and operation of the district, the board of directors shall have the power to borrow additional funds needed, to an amount not to exceed fifty cents per acre for the land embraced in the district, pledging credit of the district for the payment of the same, and shall include in the estimate for the levy for the ensuing year for the general fund the amount so borrowed, and provide for the payment of the same.