The recent southwest megadrought appears to be driven by a combination of natural variability and anthropogenic climate change, leaving many stakeholders wondering how much permanent aridification should be expected. The Colorado River Basin has been a large focus of studies aiming to address this question due to the river’s importance to several major cities and agricultural areas. At the same time, the eastern side of the Colorado Rocky Mountains has been overlooked to some extent, despite water from these headwater regions being critical to cities such as Denver, CO and agricultural areas further East.

To fill this niche, we examine ten CMIP6-era single-model initial-condition large ensembles (SMILEs) over six regions largely contained within the state of Colorado. These regions are defined by the four-digit Hydrologic Unit Codes (HUC4s), with three lying on either side of the Continental Divide (CD).

From these SMILEs, we quantify the climate change signal in precipitation, potential evapotranspiration (PET) water vapor deficit (WVD), runoff, 10-cm soil moisture, and aridity index (AI). The AI is defined as precipitation/PET such that a lower AI indicates a more arid climate. We also compute the times of emergence (ToEs) for these changes relative to early industrial (water years 1851-1900) and modern (water years 1975-2024) means. Results for the individual HUC4 regions were similar across those West and East of the CD, so Fig. 1 summarizes the results across all ten SMILEs for the three western (blue) and three eastern (red) regions combined. The region East of the CD exhibits larger decreases in 10-cm soil moisture and AI than the West, with ToEs decades earlier. Several multi-model ToEs have already passed, especially with respect to the early industrial means. Most other multi-model ToEs suggest we can expect another regime shift by 2055 relative to even modern day means.

Figure 1: Thick colored lines are the multi-model signals relative to 1851-1900 means, shading shows the range of signals for individual SMILEs, colored vertical lines indicate the multi-model ToEs compared to 1851-1900 (solid) and 1975-2024 (dashed). Horizontal and vertical dashed black lines indicate 0 change and the present day (2024).