Low flow frequency analysis by L-moments method (Case study: Iranian Central Plateau River Basin)

Document Type : Research Paper


Desert management Dept., International Desert Research Center (IDRC), University of Tehran


Knowledge about low flow statistics is essential for effective water resource planning and management in ungauged or
poorly gauged catchment areas, especially in arid and semi-arid regions such as Iran. We employed a data set of 20 river
flow time-series from the Iranian Central Plateau River Basin, Iran to evaluate the low-flow series using several frequency
analysis methods and compared the result of these methods in their ability to set low flows for the conservation of
instream water uses. Theoretical frequency distributions including the log-normal, three-parameter lognormal, Gumbel,
Pearson type III, and log Pearson type III were applied with the low-flow series. Goodness-of-fit tests including Lmoment
and conventional moment methods for the observed data were applied to identify the best distributions. For each
distribution, the calculated values of the residual sum of squares (RSS) was applied to compare between the conventional
moment and L-moment methods, and the best method was selected to determine the most appropriate probability
distribution. The lowest RSS values were used to select the best distribution for each station. Then, T-year low-flow series
was estimated using the best probability distribution. Our results suggested that, for annual low flows, based on the
computed RSS, Pearson type III, log Pearson type III, Gumbel distributions, and L-moment method were suitably
distinguished for 85%, 10%, and 5% of the stations, respectively. Finally, Compared to the conventional moment method,
L-moments method was found to be more adequate to identify low-flow series probability distributions in the Iranian
Central Plateau River Basin, while Pearson type III was found to be the best probability distribution for modeling
minimum flow series in the study area.


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