The Colorado River is a lifeline for the American Southwest, supporting over 5.5 million irrigated acres of agricultural land and a population of 40 million people. The state of Colorado’s West Slope basins - six subbasins of the Colorado River that lie on the western side of the continental divide - are critical headwaters of the Colorado River, delivering nearly 70% of inflows to the Colorado River Basin’s (CRB) Lower Basin in an average year. The West Slope basins also play a vital role in supporting the state of Colorado’s local economy and natural environment. Agricultural production and recreational activities supported by water resources within the West Slope basins are estimated to contribute over $6 billion annually to the state’s economy. Streamflow in the West Slope basins sustains populations of endangered fish not found outside the CRB. Balancing the multisectoral water demands in the West Slope basins is an increasing challenge for water managers. Droughts in the 20th and early 21st centuries have reduced reservoir levels, lowered environmental flows, and threatened agricultural production. Internal variability - irreducible uncertainty stemming from interactions across non-linear processes within the hydroclimate system - complicates future vulnerability assessments. The historical streamflow record in the West Slope represents a single realization of an inherently stochastic process, which does not capture the full extent of internal variability and plausible hydroclimatic extremes. Climate change further exacerbates drought vulnerability in the West Slope basins with the potential for significant declines in streamflows projected by mid-century.  

This work contributes a detailed analysis of multisectoral drought vulnerabilities in the West Slope basins that systematically accounts for both internal variability and climate change. We contribute a novel multi-site Hidden Markov Model (HMM)-based synthetic streamflow generator to create streamflow across the six West Slope basins that better characterizes the region’s hydroclimate and drought extremes. We then route a large ensemble of streamflows generated by the HMM generator through StateMod, the state of Colorado’s water allocation model, to evaluate spatially compounding drought impacts across the West Slope basins. We capture the effects of climate change by perturbing the HMM to generate a climate-adjusted ensemble of streamflows that reflects a middle-of-the-road, mid-century 7% reduction in streamflows to Lake Powell relative to the 1970-2000 historical baseline. Our results show that drought events emerging from the system’s stationary internal variability in absence of climate change can have significant impacts that exceed extreme conditions in the historical record, including unprecedented lows in deliveries to the Lower basin (e.g., Lake Powell), reduced environmental flows, low reservoir levels, and significant agricultural shortages. Our results further illustrate that even relatively modest levels of plausible climate changes can cause a major regime shift where extreme drought impacts become routine. These results have relevance to future Colorado River planning efforts, and our methodology can be expanded to other snow-dominated regions that face persistent droughts.