ABSTRACTA mathematical model was developed and used to simulate the stream‐aquifer system in the Arkansas River valley in southeastern Colorado, from Pueblo to the Colorado‐Kansas State line. The model simulates the interrelations among ground water and surface water including reservoirs, losses, and transmountain diversions, utilizing various water‐distribution rules. The model was used to analyze 24 water‐management plans designed to reduce shortages in the irrigation supply. One management plan simulated salvage of water from phreatophyte evapotranspiration, different reservoir operation regulations, use of imported ground and surface water, a new reservoir, additional ground‐water use, and application of excess streamflow. The resulting annual dependable supply was increased from 610,000 acre‐feet to 870,000 acre‐feet in relation to an annual demand of 1,100,000 acre‐feet. The model can be used as a tool to analyze other water