Wis. Stat. § 229.57: Budgets; rates and charges; audit.

A district shall adopt a calendar year as its fiscal year for accounting purposes. The board of directors shall annually prepare a budget for the district. Rates and other charges received by the district shall be used for the general expenses and capital expenditures of the district and to pay interest, amortization, and retirement charges on bonds. The district shall maintain an accounting system in accordance with generally accepted accounting principles and shall have its financial statements and debt covenants audited annually by an independent certified public accountant.