S.D. Codified Laws § 9-40-30
If any surplus shall be accumulated at any time in the operation and maintenance fund, in excess of a reasonable reserve for current and anticipated expenses of operation and maintenance and in excess of the amount currently required to be set aside for the payment of any outstanding revenue bonds and interest thereon and for the accumulation of a reserve securing such bond and interest payments, in accordance with the ordinances or resolutions authorizing said bonds, any such surplus or any part thereof may be transferred at any time by the governing body to the depreciation account to be used for any improvements, extensions, or additions to such utility as provided in § 9-40-31, or for the redemption of any bonds which constitute a lien upon the earnings of the utility, as and when the same become redeemable by their terms.
SDCL 9-40-30