Hidrology Case Studie

Abstract
Probable maximum precipitation (PMP) is a characteristic of rainfall at a particular location that can be used in designing water
impounding structures. The PMP for rainfall stations in Malaysia by Hershfield statistical method in earlier studies was estimated
using a frequency factor of 15 which is the highest value in the world. The value of 15 as frequency was found to be too highfor a
humid country like Malaysia. The objective of this study therefore was to provide a fresh and reliable estimates of PMP in Malaysia
using historical rainfall data. In this updating study, annual maximum 1-day rainfall data from 39 rainfall stations with records
longer than 15 years found in the state of Johor, Malaysia, were analysed to obtain estimates of 24-h point PMP using anappropriate frequency factor calculated. Results of the 24-h PMP are presented in the mapped form. It was found that 24-h PMP
over Johor varied from about 400 mm to over 1000 mm and there is a tendency for PMP to be higher on the east coast while
decreasing westwards. The average ratio of the 24-h PMP to the highest observed rainfall was found to be about 2.0.
© 2006 Elsevier B.V. All rights reserved.Keywords: Malaysia; Hydrometeorology; Monsoon; Probable maximum precipitation

1. Introduction
One of the most important problems in Hydrometeorology is the estimation of reliable probable maximum
precipitation (PMP) for different durations that are expected to occur over a point or an area. These estimates
are of considerable importance to hydrologists for calculating the probable maximumflood (PMF) for spillways on large earth and rock filled dams. The main
objective of designing spillways using the PMF is to
avoid the overtopping of dams as a result of river

⁎ Corresponding author.
E-mail address: drmohdnor@water.gov.my (M.N. Desa M.).
0169-8095/$ - see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.atmosres.2006.06.005

flooding. WMO (1986)defines PMP as the greatest depth
of precipitation for a given duration that is meteorologically possible over a given station or a specified area.
There are two major methods available to estimate value
of PMP. The first is the meteorological method in which
the PMP for different durations over an area is determined by transposition and maximisation of major historical rainstorms. The second is astatistical method where
the estimates of PMP are derived from frequency analysis
of the annual maximum rainfall series for different durations. The statistical method is useful when there is
insufficient meteorological data to apply storm maximisation method.
A statistical method for estimating PMP for small
areas has been developed by Hershfield (1961, 1965)

M.N. Desa M., P.R. Rakhecha/ Atmospheric Research 84 (2007) 84–90

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in the previous study. As a consequence, the PMP estimates were considerably high and were therefore, not
regarded for flood calculations.
In view of the above mentioned problem, Desa et al.
(2001) and M.N. Desa et al. (2003) employed the
Hershfield method to find out the appropriate frequency
factor that can give reliable PMP values forstations in
Malaysia for practical application. They analysed the
series of the annual daily maximum rainfall amounts for
stations in Selangor and Kelantan states and obtained
the appropriate frequency factors of 8.7 and 7.4 respectively. These values are applied to calculate the PMP
values in these two study areas.
It is recognised that the PMP estimates for other
regions in Malaysia are verymuch needed indeed more
so for designing new dams as well as for future review
of PMP used for dams ever constructed in the country. In
view of this an attempt has been made in this study to
estimate PMP for 24-h duration at different rainfall
stations in the state of Johor, Malaysia using the

based on a general frequency equation given by Chow
(1951). The use of the Hershfield method in...