A district, so far as practicable, shall fix such rates for water in the district, and in each improvement district therein, as will result in revenues which will:

1. (a) Pay the operating expenses of the district and the improvement district.
2. (b) Provide for repairs and depreciation of works.
3. (c) Provide a reasonable surplus for improvements, extensions, and enlargements.
4. (d) Pay the interest on any bonded debt.
5. (e) Provide a sinking or other fund for the payment of the principal of such bonded debt as it becomes due.
6. (f) Repay advances, together with interest at a rate not to exceed the interest value of money to the district, made from the district to an improvement district.