Globally increasing demand for water services and risks of water scarcity are causing river basin managers to struggle with how to best manage their water resources. These challenges have motivated rapid growth in model development and simulation to aid evaluations of the performance of alternative management policies and project future conditions. In recent efforts focused on modeling water availability, a divide has emerged between the research community of large-scale hydrologic modelers emphasizing hydro-climatological processes and that of watershed-system modelers emphasizing the environmental, infrastructural, and institutional features that shape water scarcity at the local basin level. These two research foci each rely on dynamics at different scales but have been increasingly converging towards one another, with basin-scale models directing efforts toward improved incorporation of climatic processes, and global- and regional-scale hydrologic models directing efforts toward improving their representation of how humans convey, store, use, and govern water.

This study assesses if convergent insights on water scarcity assessment can be drawn from two representative state-of-the-art models from each of these communities. The comparison is focused on the Upper Colorado Basin within the state of Colorado and evaluates whether the conclusions one would draw about scarcity for water users in the basin would be consistent between the two models. Results suggest that the regional-scale model (MOSART-WM) can overestimate basin-scale water scarcity vulnerability while underestimating its sub-basin scale variability. The basin-scale model (StateMod) suggests a larger variance of scarcity across the basin’s water users. This could be due to several factors, including the simplified reservoir operations and the coarse representation of human elements in the basin by MOSART-WM, as well as the simplified institutional mechanisms of allocation. These outcomes can be used to inform the interpretation of large-scale studies on water scarcity, as well as how the two scales of analysis can be bridged to maintain both large-scale theoretical soundness and decision-scale relevance.