One of the most important problems faced in hydrology is the estimation of flood magnitudes and frequencies in ungauged basins. Hydrological regionalisation is used to transfer information from gauged watersheds to ungauged watersheds. However, to obtain reliable results, the watersheds involved must have a similar hydrological behaviour. In this study, two different clustering approaches are used and compared to identify the hydrologically homogeneous regions. Fuzzy C-Means algorithm (FCM), which is widely used for regionalisation studies, needs the calculation of cluster validity indices in order to determine the optimal number of clusters. Fuzzy Minimals algorithm (FM), which presents an advantage compared with others fuzzy clustering algorithms, does not need to know a priori the number of clusters, so cluster validity indices are not used. Regional homogeneity test based on L-moments approach is used to check homogeneity of regions identified by both cluster analysis approaches. The validation of the FM algorithm in deriving homogeneous regions for flood frequency analysis is illustrated through its application to data from the watersheds in Alto Genil (South Spain). According to the results, FM algorithm is recommended for identifying the hydrologically homogeneous regions for regional frequency analysis.
This paper analyses the bivariate relationship between flood peaks and corresponding flood event volumes modelled by empirical and theoretical copulas in a regional context, with a focus on flood generation processes in general, the regional differentiation of these and the effect of the sample size on reliable discrimination among models. A total of 72 catchments in North-West of Austria are analysed for the period 1976-2007. From the hourly runoff data set, 25 697 flood events were isolated and assigned to one of three flood process types: synoptic floods (including long- and short-rain floods), flash floods or snowmelt floods (both rain-on-snow and snowmelt floods). The first step of the analysis examines whether the empirical peak-volume copulas of different flood process types are regionally statistically distinguishable, separately for each catchment and the role of the sample size on the strength of the statements. The results indicate that the empirical copulas of flash floods tend to be different from those of the synoptic and snowmelt floods. The second step examines how similar are the empirical flood peak-volume copulas between catchments for a given flood type across the region. Empirical copulas of synoptic floods are the least similar between the catchments, however with the decrease of the sample size the difference between the performances of the process types becomes small. The third step examines the goodness-of-fit of different commonly used copula types to the data samples that represent the annual maxima of flood peaks and the respective volumes both regardless of flood generating processes (the traditional engineering approach) and also considering the three process-based classes. Extreme value copulas (Galambos, Gumbel and Hüsler-Reiss) show the best performance both for synoptic and flash floods, while the Frank copula shows the best performance for snowmelt floods. It is concluded that there is merit in treating flood types separately when analysing and estimating flood peak-volume dependence copulas; however, even the enlarged dataset gained by the process-based analysis in this study does not give sufficient information for a reliable model choice for multivariate statistical analysis of flood peaks and volumes.
Regional design flood computation formulae in Slovakia were traditionally based on the regionalisation of the 100-year flood discharge. Floods with shorter return periods have usually been computed by regional frequency factors from the 100-year flood discharge. Several previous studies have indicated that a rather high safety factor is included in some of the traditional regional formulae. In this paper therefore an alternative method for the determination of the flood frequency curve in ungauged catchments in the high core mountain region of Slovakia has been investigated. Floods from rainfall and snowmelt were treated separately; the study was focused on summer floods. For the division catchments into pooling groups subjective and objective methods e.g. cluster analysis were used. The methodology was based on the regional estimation of the index flood and its standard deviation from catchment characteristics. Flood quantiles were determined from a two-parameter distribution. The performance of several regional methods was compared and the applicability of the tested methods for various hydraulic and hydro-ecological design tasks was also discussed. and Odhad N-ročných maximálnych prietokov bol na Slovensku tradične založený na regionalizácii maximálnych storočných prietokov. V príspevku sme preto navrhli a testovali alternatívne regionálne metódy nepriameho odhadu štatistických charakteristík maximálnych prietokov v letnej sezóne v oblasti Tatier. Skúmali sme viaceré možnosti rozdeliť tento región na oblasti s rovnakým vzťahom medzi podmieňujúcimi činiteľmi tvorby odtoku a priemernou hodnotou a smerodajnou odchýlkou maximálnych letných prietokov. Subregióny boli konštruované použitím subjektívnych úvah, logickým delením, ako aj pomocou objektívnych metód využívajúc princípy zhlukovej analýzy. Regionálne vzťahy na určovanie charakteristík maximálnych letných prietokov boli odvodené použitím viacnásobnej regresie. V závere sme porovnali hodnoty N-ročných maximálnych letných prietokov určené použitím viacerých metód a diskutovali sme vhodnosť použitia aplikovaných postupov pre inžiniersku prax.