Spatial attribution of declining Colorado River streamflow under future warming

Studies of climate change impacts in the Colorado River Basin have focused on the Upper Basin, finding that warming has exacerbated streamflow declines since 2000. Presently, it is unclear if the predicted high degree of interannual precipitation variability across the basin could overwhelm impacts of future warming and how this might vary in space. To address this, we employed an updated Variable Infiltration Capacity model framework to generate streamflow projections across the entire basin. Meteorological data were drawn from downscaled historical and future climate projections of eight climate models that best represent climatology. Results showed a high confidence of Far-Future (2066-2095) mean annual streamflow declines (relative to the Baseline, 1976-2005) at the Upper Basin, Lower Basin, and basin-wide scales (ensemble median trends of -5 % to -25 %). Streamflow declines were attributed to warming that diminished snowfall and melt during April to September and year-round increased soil evaporation from the Upper Basin, and overall precipitation declines in the Lower Basin. Widespread reductions in annual runoff and baseflow efficiencies indicated that precipitation increases did not offset the noted warming effects. Spatial coherence tests indicated that streamflow correlations among subbasins remained relatively constant in the future scenarios, though correlations with the Green River increased. This spatial attribution analysis is providing actionable information for water management in the Colorado River Basin under current and future conditions.