All the net income of the gas transmission system or systems, for each fiscal year, of a gas district incorporated under this article shall, unless otherwise provided in the certificate of incorporation of such district, be divided, within a reasonable time after the close of each fiscal year, among its member municipalities in proportion to the amount of gas sold to or within each such municipality. All the net income of each gas distribution system, for each fiscal year, of any such district shall, unless otherwise provided in its certificate of incorporation, be distributed and paid, within a reasonable time after the close of each fiscal year, to the municipality or municipalities within which such distribution system is located. The term “net income” as used in the preceding two paragraphs of this section shall mean, with respect to the system or systems and for the fiscal year in question, the net income thereof computed in accordance with generally accepted accounting principles plus depreciation and amortization less the sum of the following: (Acts 1951, No. 762, p. 1319, §15; Acts 1983, No. 83-613, p. 950; Acts 1989, No. 89-705, p. 1410, §1.)