Before April 1st of each year after the warrants are issued the board shall fix charges for the use of water in the improvement district sufficient, when added to other allocated revenue that will be received during the current year, to provide funds adequate to pay the amount of the improvement district warrants and interest thereon then due and that will fall due during the current year plus an amount equal to 10 percent more than such amount if the warrants provide that such additional amount shall be charged to establish a reserve fund to further assure payment of the warrants and interest.