Future floods using hydroclimatic simulations and peaks over threshold: An alternative to nonstationary analysis inferred from trend tests

Abstract

Nonstationary flood frequency analysis (NFFA) has increasingly been applied to predict future floods under climate change. The inference of nonstationarity from trend tests (INTT) on historical floods is a widespread practice used to justify the application of NFFA; however, its reliability has seldom been investigated. This research examines future floods from interpretations of INTT compared to those obtained from cause-and-effect processes by a hydrological model (INCE). The study uses INCE to quantify the changes in the regime and magnitude of floods due to potential climate change in the mid-21st century using multi-model ensemble simulations under two greenhouse gas emissions scenarios (i.e., RCP 2.6 and RCP 8.5). Independent peaks over threshold from simulated streamflow were used to estimate the changes in future (2040–2064) flood regimes and magnitudes compared to their historical (1983–2007) counterparts for 29 unregulated catchments across Alberta, Canada. Separation of extreme events and their fittings to generalized Pareto distributions (GPD) were based on a hybrid approach that combined two developed automated threshold selection methods and four estimators for the parameters of the GPD. Based on comparing the results of INTT and INCE, we show that future changes in floods contradict. We also show that the future frequency curves shifted differently at different return periods compared to historical curves, while in some instances, future climate tended to decrease small floods and increase larger floods or vice versa. Finally, flood magnitudes in 2/3 of the studied catchments in Alberta are predicted to intensify, accompanied by increases in the rate of occurrence and earlier shifts in the timing of floods for both climate scenarios, whereas no considerable change in the duration was recognized.