Sec. 3. The method of audit hereby provided for Navigation Districts, Water Improvement Districts, Water Control and Improvement Districts, and all other Districts created for improvement and conservation purposes in counties containing a population of not less than seventy-five thousand (75,000), nor more than eighty thousand (80,000), as shown by the last preceding Federal Census, and not directly administered by the Commissioners Court of such counties, shall supersede all other provisions for auditing the receipts and expenditures of such districts otherwise prescribed by law, and all laws and parts of laws in conflict herewith are hereby repealed.