Analysis of probability distribution of annual maximum, mean, and minimum stream flows in Sri Lanka

Abstract

Selection of a probability distribution for discharge series is very important in estimating floods accurately. Since the introduction of L-moments (Hosking 1990), numerous investigations have been carried out to assess the goodness of fit of various probability distributions to regional samples. L-moment diagrams were constructed for annual maximum flood flows, annual average daily stream flows, and annual minimum low flows at 46 catchments in Sri Lanka. The data were screened to eliminate gross errors and inconsistencies. For this purpose the Discordancy measure Di introduced by Hosking and Wallis in 1993 was used. The Discordancy measure allows for an objective determination of which sites, i, in a region, are grossly discordant with the group as a whole. For each flow series, sites were dropped with discordancy measures Di,> 3. Suitable distributions for each site were selected by comparing the observed and theoretical relations between L-cv and L-skewness of the flood flows. The goodness-of-fit test was then applied in order to examine the descriptive performance of screened distributions.