The use of nonsystematic flood data forstatistical purposes depends on reliability of assessment both flood magnitudesand their return period. The earliest known extreme flood year is usually thebeginning of the historical record. Even though the magnitudes of historicfloods are properly assessed, a problem of their retun periods remainsunsolved. Only largest flood (XM) is known during whole historical period andits occurrence carves the mark of the beginning of the historical period anddefines its length (L). So, it is a common practice of using the earliest knownflood year as the beginning of the record. It means that the L value selectedis an empirical estimate of the lower bound on the effective historical lengthM. The estimation of the return period of XM based on its occurrence, i.e. , gives the severe upward bias. Problem is to estimate thetime period (M) representative of the largest observed flood XM. From the discreteuniform distribution with support of the probability of the L position of XMone gets ?which has been takenas the return period of XM and as the effective historical record length. Theefficiency of using the largest historical flood (XM) for large quantileestimation (i.e. one with return period T = 100 years) has been assessed usingmaximum likelihood (ML) method with various length of systematic record (N) andvarious estimates of historical period length ?com- paring accuracywith the case when only systematic records alone (N) are used. The i-th simula-tion procedure incorporates systematic record and one largest historic flood(XMi) in the period M which appeared in the Li year backward from the end ofhistorical period. The simulation result for selected distributions, values oftheir parameters, different N and M values are presented in terms of bias (B)and root mean square error (RMSE) of the quantile of interest and widelydiscussed.
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