S.D. Codified Laws § 9-40-23
The governing body may also set apart bonds hereunder authorized equal to the amount of any lien, secured debt, or charge then subsisting against any utility at the time of the purchase or acquisition of such utility pursuant to this chapter, and shall set aside for interest and debt service funds from the balance of the income and revenues of such utility, remaining after setting aside the necessary amount or funds for the payment of principal of and interest on the revenue bonds, a sum sufficient to comply with the requirements of the instrument or contract creating the underlying lien, debt, or charge; or, if such instrument or contract does not make any provision therefor, said governing body shall fix and determine the amount which shall be set aside into said interest and debt service funds for interest and principal payments on said underlying lien, debt, or charge at the maturity thereof. Any surplus after satisfying the underlying lien, debt, or charge may be transferred to the fund for the redemption of the principal and interest of bonds issued hereunder.
SDCL 9-40-23