Detection of Change in Flood Return Levels under Global Warming

Using recent advancements in the statistical extreme value theory, this study proposes a methodology for detection of change in flood return levels under climate change. Nonstationary scaling of regional projected peak flows with global warming is first tested by a likelihood ratio test. For nonstationary possible future realizations, the authors then investigate how long the stationary historical design magnitudes or return levels of floods will remain valid, taking into account the uncertainties in the estimation of observed and projected return levels. Although some flood projections are found to be nonstationary, many are stationary in nature. No coherent change in flood return level across the projections is detected in the case study of floods in the Columbia River using available streamflow projections. Most projections yield flood quantiles that are not likely to be critical in the coming century. However, for some simulations detection is achieved, with earlier detection in design magnitudes of lower return periods. A possible worst-case scenario considering the maximum of all the projections shows detection of change in floods of higher return periods in the 21st century.