C.R.S. § 2-2-503

As a part of its preliminary and final redistricting plans for state senatorial districts, the legislative commission shall designate those senatorial districts in which state senators are elected at the general election to be held in November of the next year ending in 2, and every four years thereafter, and those senatorial districts in which state senators are elected at the general election to be held in November of the next year ending in 4, and every four years thereafter. Such designation of senatorial districts must be filed with the secretary of state as a part of the approved redistricting plan required to be filed by section 48.3 (5) of article V of the state constitution.