Exploring the Spatially Compounding Multi‐Sectoral Drought Vulnerabilities in Colorado's West Slope River Basins.

The state of Colorado's West Slope Basins are critical headwaters of the Colorado River and play a vital role in supporting Colorado's local economy and natural environment. However, balancing the multi‐sectoral water demands in the West Slope Basins while maintaining crucial downstream deliveries to Lake Powell is an increasing challenge for water managers. Internal variability of the hydroclimatic system and climate change complicate future vulnerability assessments. This work contributes a detailed accounting of multi‐sectoral drought vulnerability in the West Slope Basins and the impacts of drought on downstream deliveries. We first introduce a novel multi‐site Hidden Markov Model (HMM)‐based synthetic streamflow generator to create an ensemble of streamflows for all West Slope basins that better characterizes the region's drought extremes. We capture the effects of climate change by perturbing the HMM to generate an ensemble of streamflows reflecting plausible changes in climate. We then route both ensembles through StateMod, Colorado's water allocation model, to evaluate spatially compounding drought impacts across the West Slope Basins. Our results illustrate how drought events emerging from the system's stationary internal variability in the absence of climate change can significantly impact local water uses and deliveries to Lake Powell, exceeding extreme conditions in the historical record. Further, we find that even modest climate change can cause a regime shift where historically low downstream delivery volumes and extreme drought impacts become routine. These results can inform future Colorado River planning efforts, and our methodology can be expanded to other snow‐dominated regions that face persistent droughts. Plain Language Summary: The state of Colorado's West Slope Basins ‐ six watersheds within the Colorado River Basin on the western side of the continental divide ‐ are essential water sources for the Colorado River and play a vital role in supporting the state of Colorado's local economy and natural environment. Water managers in the region are challenged by competing water demands for uses such as agriculture, environmental flows, and downstream deliveries to Lake Powell, a critical regional reservoir. Evaluating drought vulnerability in the region is complicated by the inherent randomness of streamflows and reduced flows from climate change. This work contributes a new analysis of local drought vulnerability in the West Slope Basins and the consequences for water deliveries to Lake Powell. We introduce a method to generate streamflow scenarios and explore drought impacts in the West Slope Basins. We run the scenarios through a model of the human‐natural system to explore local and regional drought vulnerability. Our results reveal elevated drought risks to downstream water users, agriculture, and the environment, even without climate change. Further, we find that even optimistic climate change projections can exacerbate drought risk, leading to unprecedented challenges for local water users and the broader regional system. Key Points: We introduce a multi‐site two‐state Gaussian Hidden Markov model to explore drought impacts in Colorado's West Slope Basins and Lake PowellInternal variability in the hydroclimatic system can cause spatially compounding drought impacts exceeding historically extreme eventsStreamflow declines driven by an optimistic climate change scenario can transition the system to a drier regime and increase drought impacts [ABSTRACT FROM AUTHOR]