Where and When Does Streamflow Regulation Significantly Affect Climate Change Outcomes in the Columbia River Basin?

Abstract:

The Columbia River basin includes hundreds of tributaries across the Pacific Northwest, straddling the U.S. – Canada border. The transboundary river network supports a vast water-food-energy nexus that sustains irrigation and urban water supply and generates nearly half of the hydroelectricity in the U.S.. The basin encompasses a diverse range of hydroclimates from arid lowlands to glaciated mountain regions. Complex topography and strong seasonality, combined with the prominent role of snow in the hydrologic cycle, give rise to spatially heterogeneous climate impacts on natural streamflow. Streamflow in the U.S. is heavily regulated by the U.S. Army Corps of Engineers, the U.S. Bureau of Reclamation, and other agencies for flood mitigation, water supply, hydropower, and ecosystem function. While climate is a primary driver of basin hydrology and climate change is projected to have significant impacts on natural streamflow patterns, extensive regulation also modulates streamflow. These effects could lead to dramatically different climate change projections between studies that rely on unregulated or natural flows and studies that explicitly account for regulation. This study investigates where and when regulation significantly affects the climate change projection by comparing climate outcomes for unregulated and regulated streamflow at 90 sites across the Columbia River basin. Unregulated streamflow projections are taken from an existing dataset developed by the University of Washington that consists of an 80-member ensemble of 150 years of daily streamflow. The ensemble was constructed by selecting ten global climate models (GCMs) from the Coupled Model Intercomparison Project version 5 (CMIP5), two statistical downscaling methods, and four hydrologic model implementations. Regulated streamflow is simulated by the U.S. Army Corps of Engineers WAT-ResSim hydroregulation modeling framework using an operational rules-based approach. We focus our analysis on the times, locations, and conditions under which future flows differ meaningfully between the unregulated and regulated projections. The findings of this study identify the importance of accounting for regulation in climate change projections and highlight areas of potential system vulnerability and resiliency.