Assessment of Potential Future Streamflow Scenarios for Long-Term Planning in the Colorado River Basin

Projecting the future amount of water available in the Colorado River Basin is one of the significant challenges in long-term planning. Given that the precise characteristics of future hydrology are unknown, assumptions about future hydrology are inevitable in planning. A variety of hydrology scenarios have been developed by different researchers and agencies to provide streamflow sequences as inputs to the Colorado River Simulation System (CRSS), which is the major planning tool in the Colorado River Basin that implements compacts and agreements that collectively are known as the "Law of the River". Streamflow scenarios may be based on historical, tree-ring-reconstructed, or climate-informed flow projections, or a combination of them. Each scenario has unique statistical attributes and represents a set of assumptions about the uncertain future hydrology. Each hydrology scenario also has different effects on the system performance and thus may lead to different planning outcomes. These hydrology scenarios have not all been thoroughly characterized or compared, and there is a need to evaluate and categorize them for use in addressing different water management purposes in the Colorado River Basin. In this study, we use a broad range of metrics to provide an objective and quantitative way to interpret and analyze the similarities and differences between hydrology scenarios and determine the suitability of the scenarios in different planning applications. This is achieved by either evaluating the characteristics of the streamflow scenarios directly or by testing the impact of the scenarios on the performance of the water resources system through model simulations. By establishing performance criteria, the results show to what extent the metrics are met for the various hydrology scenarios. These outcomes are then used to catalog the hydrology scenarios and characterize their alignment with various planning purposes in the Colorado River Basin. The findings are expected to be helpful for researchers, engineers, and managers at various agencies involved in Colorado River Management, such as the U.S. Bureau of Reclamation, when deciding which hydrology scenarios should be used for particular planning purposes.