Colo. Rev. Stat. § 37-42-130
If bonds are issued which are subject to redemption prior to maturity, and if, after the payment of all coupons and bonds due in any fiscal year, it is found that the bond fund of an irrigation district contains an amount of money sufficient therefor, it is the duty of the district treasurer to call such bonds as first become due and payable and to retire such indebtedness. Call shall be made by registered mail, addressed to the holder of such bonds so called at his address of record, giving the number of the bonds called, and notifying the holder thereof, that upon presentation of such bonds with all future due coupons attached, they shall be redeemed at their face value, with interest to date of call. When any bond has been so called, such fact shall be noted upon the bond register of the district treasurer, and money in the amount of its face, with interest to date of call, shall be set aside for its payment from the bond fund, and no coupons upon such bond maturing at a date later than the date of such call shall be paid. If money remains in or is paid into the bond fund after final liquidation of all bonded indebtedness, such money shall be transferred to the general fund by the treasurer of the district.
C.R.S. § 37-42-130