S.D. Codified Laws § 9-40-32
If any surplus, as defined in § 9-40-30, remains in the operation and maintenance fund upon the expiration of any fiscal year, and the governing body determines that such surplus is not needed for the maintenance of the depreciation account, said surplus may be transferred to any other fund of the municipality in accordance with the provisions of § 9-21-26.1, and may be used for any proper municipal purpose as directed by the governing body, or may be transferred to a public school district as provided in § 9-21-28, or a hospital corporation as provided in chapter 34-9; provided that all such surplus funds on hand at any time prior to such transfer shall be available and shall be used to such extent as may be required for the payment of principal and interest on any outstanding revenue bonds, if other net revenues of the utility are insufficient for such purpose, and no such transfer shall at any time be made in contravention of any covenant of the municipality contained in the ordinances of resolutions authorizing said bonds.
SDCL 9-40-32