Southwest Airlines, the second-largest carrier operating at Denver International Airport, informed the state Thursday that it would furlough 556 of its Colorado employees unless it can reach agreements with four of its unions on cost-cutting measures or obtain more federal assistance from Congress. “The involuntary furlough will take place next year unless we reach cost-saving agreements,” Julie Weber, chief people officer at the Dallas-based carriers said in a letter sent to the Colorado Department of Labor and Employment under the Worker Adjustment and Retraining Notification Act. The Denver cuts are part of a larger national reduction of 6,800 workers that Southwest announced Thursday.