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Recent research on the bivariate flood peak/volume frequency analysis has mainly focused on the statistical aspects of the use of various copula models. The interplay of climatic and catchment processes in discriminating among these models has attracted less interest. In the paper we analyse the influence of climatic and hydrological controls on flood peak and volume relationships and their models, which are based on the concept of comparative hydrology in the catchments of a selected region in Austria. Independent flood events have been isolated and assigned to one of the three types of flood processes: synoptic floods, flash floods and snowmelt floods. First, empirical copulas are regionally compared in order to verify whether any flood processes are discernible in terms of the corresponding bivariate flood-peak relationships. Next the types of copulas, which are frequently used in hydrology are fitted, and their goodness-of-fit is examined in a regional scope. The spatial similarity of copulas and their rejection rate, depending on the flood type, region, and sample size are examined, too. In particular, the most remarkable difference is observed between flash floods and the other two types of flood. It is concluded that treating flood processes separately in such an analysis is beneficial, both hydrologically and statistically, since flood processes and the relationships associated with them are discernible both locally and regionally in the pilot region. However, uncertainties inherent in the copula-based bivariate frequency analysis itself (caused, among others, also by the relatively small sample sizes for consistent copula model selection, upper tail dependence characterization and reliable predictions) may not be overcome in the scope of such a regional comparative analysis.

The estimation of design floods in ungauged small and medium basins in Slovakia was often performed by simple regional flood formulae derived for various geographical regions. Envelope curves were also applied in order to get an acceptable regional safety factor for design purposes. Usually the 100-year flood was estimated from the basin area; correction factors were used to account for deviations from the average of local runoff forming conditions in the region. Floods with shorter return periods were computed by regional frequency factors. Comparisons of design floods derived from such formulae with at site values in 260 small and mid-sized basins [1,2] showed a case-sensitive, highly variable safety factor in these schemes. They are not generally applicable for river training and restoration since they tend to overestimate the design discharges in almost all basins of a particular region. For these tasks other regional approaches, which do not include the regional safety introduced by the envelope curve concept, but allow for site specific over-and under-estimation resulting from the regional average concept, seem to be more appropriate. Some of the recently introduced concepts of regional homogeneity as e.g. in [3,4,5,6,7] were suggested to be used together with regional flood formulae composed of catchment characteristics other than the basin area only. To eliminate the heterogeneity in the annual flood data series due to the diverse genetic origin of flood events, it was proposed to consider annual maximum peak discharges separately for two seasons - the summer season with rainfall-induced discharges and the winter season with floods originating from snowmelt and mixed events.

This study addresses the question of the existence of a parent flood frequency distribution on a European scale and aims to better understand the effect of catchment scale and climate on the statistical properties of regional flood frequency distributions. A new database of L-moment ratios of annual maximum series (AMS) of peak discharges from 4105 catchments was compiled by joining 13 national datasets. Using this database and additional Monte Carlo simulations, the Generalised Extreme Value (GEV) distribution appears as a potential pan-European flood frequency distribution, being the 3-parameter statistical model with the closest resemblance to the estimated average of the sample L-moment ratios, but failing to represent the kurtosis dispersion, especially for high skewness values. A more detailed investigation performed on a subset of the database (Austria, Italy and Slovakia, involving a total of 813 catchments with more than 25 yr of record length) confirms that the GEV distribution provides a better representation of the averaged sample L-moment ratios compared to the other distributions considered, for catchments with medium to high values of mean annual precipitation (MAP) independently of catchment area, while the 3-parameter Lognormal distribution is probably a more appropriate choice for dry (low MAP) intermediate-sized catchments, which presented higher skewness values. Sample L-moment ratios do not follow systematically any of the theoretical 2-parameter distributions. In particular, the averaged values of L-coefficient of skewness (L-Cs) are always larger than Gumbel's fixed L-Cs. The results presented in this paper contribute to progress towards the definition of a set of pan-European flood frequency distributions and to assess possible effects of environmental change on its properties.