On seasonal approach to nonstationary flood frequency analysis

Annual peak flow series of Polish rivers are mixtures of summer and winter flows. Seasonal series are expected to be more homogeneous than annual series in respect of both the distribution and trend in time. Consequently, a seasonal maxima approach to stationary flood frequency analysis (FFA), which increases accuracy of quantile estimates in comparison to the annual maxima approach when the underlying distributions hold, has been introduced by Polish Hydrological Service. The set of Polish data of seasonal and annual maxima have been examined for their homogeneity regarding the distribution type and trend in mean and standard deviation. Large spatial variability of seasonal and annual distribution type as well as of linear trend estimates have been observed. Seasonal variation of trends requires seasonal approach for non-stationary FFA which is presented here and illustrated by a case study. Nonstationary FFA is performed for winter and summer peaks separately and time-dependent annual peak flow quantiles are computed by union formula of two independent events. Applied estimation techniques of NFFA and standard error evaluation by a bootstrap technique are briefly described.