ORIGINAL RESEARCH
Finding Probability Distributions for Annual
Daily Maximum Rainfall in Pakistan Using Linear
Moments and Variants
Ishfaq Ahmad1, Aamar Abbas2, Aamir Saghir3, Muhammad Fawad1
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1Department of Mathematics and Statistics, International Islamic University, Islamabad, Pakistan
2Department of Mathematics, University of Poonch Rawalakot, Pakistan
3Mir Pur University of Science and Technology, AJK, Pakistan
Submission date: 2015-12-22
Final revision date: 2016-02-07
Acceptance date: 2016-02-07
Publication date: 2016-05-25
Pol. J. Environ. Stud. 2016;25(3):925-937
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ABSTRACT
In this study, at-site frequency analysis (AFA) of an annual daily maximum rainfall (ADMR) series
was carried out using the method of linear moments (L-moments) and their variants such as trimmed
linear moments (TL-moments) and higher order linear moments (LH-moments). The ADMR series we
investigated was observed at 28 meteorological observatories across Pakistan as retrieved from the Pakistan
Meteorological Department (PMD). The basic aim of the study was to find best-fit (i.e., the most suitable)
probability distribution among the class of various probability distributions. Initially different goodness-offit (GOF) measures such as the Kolmogorov-Smirnov test (KST), Anderson-Darling test (ADT), root mean
square error (RMSE) and L-moments ratio diagram (LRD) were applied to determine not only the best-fit
distributions but also the best linear estimation method for AFA. We observed that no single probability
distribution could be declared as the best-fit distribution for all the stations. Five distributions were found
to be the most appropriate: generalized extreme value (GEV), three parameter lognormal (LN3), Pearson
type III (P3), generalized logistic (GLO), and generalized pareto (GPA). The TL-moments method was also
applied for parameter estimation to mitigate the effect of outliers on final estimates. LH-moments were used
for estimating the upper part of probability distributions and larger events in the data samples. LH moments
alleviate the unwanted affects due to small sample values that may be obvious during estimation of events
related to larger return periods. Using different GOF tests, we observed that the L-moments method was
best for eight stations, TL-moments with trimming (1, 0), and LH-moments with level η =2, 3, 4 were best
for six and 14 stations, respectively. A theoretical relationship between TL-moments and LH-moments was
also revisited, which revealed that LH-moments are special cases of TL-moments when we are motivated to
make trimming only from the lower side.