Historical Warming and Drying in Colorado and Their Impact on Cool-Season Precipitation and Snow

Changes in water resources from the Colorado River are critical to quantify, given the contentious debates over allocating a dwindling water supply to a growing population that relies on it for agriculture, recreation, and life. Colorado state and a headwaters basin to the Colorado River__the Gunnison River basin__are analyzed to assess how thermodynamic changes over the past 40 years have altered precipitation and dry days, along with snow and ablation. High-resolution model data at a 4-km grid spacing are evaluated against observations from 1981 to 2021 and show high fidelity in capturing observed spatial and temporal distributions of cool-season precipitation. Warming and drying over Colorado over the past 40 years in the model data support a decreasing frequency of snow days up to 5 days per winter and an increasing frequency of dry days up to 6 days per winter. Such changes result in peak snow water equivalent amounts decreasing by-2.1 mm yr^-1 in the model and-3.4 mm yr^-1 in observations. Ablation days increase by up to 4 days per winter, particularly at altitudes over 3000 m, with dry days contributing 85% to ablation at high altitudes and 20% from rain days. The increasing frequency of dry days has contributed more to ablation from 2001 to 2021 compared to 1981–2001. These results suggest that water resources have already changed over the headwaters of the Colorado River basin and need careful planning for the future, due to climate-change-induced warming.