Each supervisor, before entering upon his official duties as such, shall take and subscribe an oath before some officer authorized by law to administer oaths, that he will honestly, faithfully, and impartially demean himself in office as supervisor of the various districts in which he was elected, and that he will not neglect any of the duties imposed upon him by law. The president and secretary of the drainage board shall each enter into a bond in the sum of ten thousand dollars to the district for the faithful performance of his duties as such officer. Each of the remaining members of the board shall give bond in the sum of twenty-five hundred dollars, conditioned in the same manner. All of such bonds shall be approved by the clerk of the district court, and the expense of such bonds shall be paid by the drainage district; Provided, however, upon the completion of the drainage improvements of the district the amount of all of the bonds shall be reduced to the sum of fifteen hundred dollars each. All expenditures of the drainage district shall from time to time be audited by a committee of three elected by the landowners of each district at their annual meeting for the election of officers. Such auditing committee shall file written reports of its examinations with the secretary of the drainage district.