| Abstrakt: |
The US and Canada have entered negotiations to modernize the Columbia River Treaty, signed in 1961. Key priorities are balancing flood risk, hydropower production, and improving aquatic ecosystem function while incorporating projected effects of climate change. In support of the US effort, Chegwidden et al. (2017) developed a large-ensemble dataset of past and future daily flows at 396 sites throughout the Columbia River Basin (CRB) and select other watersheds in western Washington and Oregon, generating a large ensemble using state-of-the art climate and hydrologic models. In this study, we use that dataset - the largest now available - to present new analyses of the effects of future climate change on flooding using water year maximum daily flows. For each simulation, flood statistics are estimated from Generalized Extreme Value distributions fit to simulated water year maximum daily flows for 50-year windows of the past (1950-1999) and future (2050-2099) periods. Our results contrast with previous findings: we find that the vast majority of locations in the CRB are estimated to experience an increase in future discharge magnitudes. We show that on the Columbia and Willamette rivers, increases in discharge magnitudes are smallest downstream and grow larger moving upstream. For the Snake River, however, the pattern is reversed, with increases in discharge magnitudes growing larger moving downstream to the confluence with the Salmon River tributary, and then abruptly dropping. We decompose the variation in results attributable to climate and hydrologic factors, finding that climate contributes more variation in larger basins while hydrology contributes more in smaller basins. Equally important for practical applications like flood control rule curves, the seasonal timing of flooding shifts dramatically on some rivers (e.g., on the Snake, 20th century floods occur exclusively in late spring, but by the end of the 21st century some floods occur as early as December) and not at all on others (e.g. the Willamette). [ABSTRACT FROM AUTHOR] |
