Non-stationary regional flood frequency analysis at ungauged sites

Regional flood frequency analysis (FFA) is commonly used to estimate flood risk at a particular site where little or no information is available on peak flows. This approach requires the assumption of flood stationarity. In this paper, we present a method to perform regional FFA at ungauged sites when the assumption of stationarity is not valid. Non-stationary at-site models are used to estimate flood risk at gauged sites. This information can then be used along with meteorological and drainage basin characteristics to define a hydrologic neighborhood of the ungauged site by means of canonical correlation analysis (CCA). A multiple regression model is then developed within the hydrologic neighborhood. Feasibility of regional FFA at ungauged sites is illustrated with a group of river flow gauging stations located in southeastern Canada and northeastern United States and which show a signal of non-stationarity. Results indicate that the development of a multiple regression model using 2-4 explanatory variables (including basin drainage area) leads to efficient estimation of the non-stationary regional flood quantiles Q(5)(t) and Q(100)(t) for the end of the historic observation period (relative root mean square error (RMSEr) of 38.2% and 60.8%, respectively). The use of CCA for the definition of a hydrologic neighborhood did not Lead to better results. This is explained by the fact that the total number of sites (29) is small and, consequently, the size of the hydrologic neighborhoods is too low to develop efficient regression models within each of them. Comparison with the stationary results show that ignoring a trend in the hydrologic regime of an ungauged site can Lead to serious under- or overestimation of the quantile estimates for that site. (C) 2007 Elsevier B.V. ALL rights reserved.