The recent Integrated Hydro-Terrestrial Modeling (IHTM) 2.0 workshop highlighted a growing focus on advancing regional water availability assessments and stress testing frameworks. However, there are at present several major challenges that the IHTM community faces. For example, there is a need to advance how we project climate change and its regionally specific effects in institutionally complex regional water systems. This study introduces a water availability stress testing framework for Colorado's West Slope River Basins. These basins are critically important to water availability in the broader Colorado River, contributing nearly 70-percent of the inflows to Lake Powell in a typical year. Median mid-century projections suggest a 7-percent reduction in the region's annual streamflows. However, standard climate projections do not fully capture the full extent of the potential impacts that the relatively optimistic median 7-percent reduction in annual flows could have given the system's broader internal variability and persistence of drought extremes.

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 an 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 plausible changes in climate. Our results show that drought events emerging from the system’s stationary internal variability in absence of climate change can have significant multisectoral impacts that exceed extreme conditions in the historical record. Our results further illustrate that even relatively modest median levels of plausible climate changes can cause a major regime shift where extreme drought impacts become routine.