Mich. Comp. Laws § 280.626
When 2 or more public corporations other than the state or a county or counties are to be assessed, the drainage board, or if appropriate in chapter 21 proceedings, the drain commissioner, may determine that costs to be assessed to the public corporations, in excess of those apportioned for drainage to state or county highways, shall not be apportioned by the establishment of a fixed percentage of costs to be borne by each public corporation, but that the costs shall be assessed at large to all of the public corporations in the aggregate and apportioned annually between the public corporations on the basis of the relative valuations, as equalized, of each within the area served by the drain, if the drainage board or drain commisssioner shall determine that this method of apportionment will more fairly result in the costs of the drain being apportioned in accordance with the benefits to be derived therefrom. Notice of tentative apportionment in the foregoing manner shall include a calculation of the apportionment of costs between public corporations assessed in the aggregate based on the latest available valuations.
MCL 280.626