An irrigation district, on account of expenses of operation and maintenance and to pay the current expenses of the district, in any year may not borrow in excess of ninety percent of the levy of assessments for that year. In case of due and outstanding obligations of the district on account of current expenses and expenses of operation and maintenance contracted before the year in which the levy is made, the district board may make an additional levy, not to exceed twenty dollars per acre [.40 hectare], upon all irrigable lands within the district, to create a    special fund for the payment of the past-due obligations. Whenever the claims or obligations against any fund for any year are fully paid, the board may transfer any unused balance to any fund for any preceding or succeeding year.