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Mausam, (1989), 40, 1, 85·90

551 . 577. 2 ; 55 I . 50 1.7 77 (547 . I )

Statistical distribution of extremes of rainfall atColaba, Bombay

NITYAN A ND SI G H

indian Ills /illite of Tropical M eteorology, Pune

i Received 13 January 1987)

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ABSTRACT . Frequency distributio n o f ann ual maximum 1, 2. J, 6, 12. 2-4. 36 and 48-hr rainfall recordedat Col aha observa tory. Bombay from 1924 to 19H4 is studied using Gumbel and gam ma dis tributions v-bo th havebeen fille d using mom ent s and maximum likelihood cstirnatcs separately. Gumbel dist ributio n filled with met hodof moments is round to he most suitable as indicated by minimum values of the a bsolut e relative devia tion :1111.1quadratic deviation hut the fit of Gumbel di-rribunon using max imum likelihood estimates was. by and large,equally satisfactory. The results arc presented on an extreme-value probability paper giving rainfall depth­dura tion-frequency (D OH relationships for Bombay. Relationship between mea n annual max imum rainfalland the dura tion has been obtained in ordcr to simplify the understanding o f ex treme rainfal l amount and du ration .The rela tion ship is found valid for S. 10. 15 and 30-min extreme rainfall also.

I. Inlrn::l lK1ion

Intense rainfall causes dislocation of normal lifein the city of Bombay. The city with a populationof about 9 million is situated a long the west coastof Indian Peninsula (the geographical coo rdinatesarc 18° 54'N and 72' 49'E). Th e mean annua l rai nfallor Bombay is 1805 mrn , 94 % of which is receivedduring southwest monsoon period from June throughSeptember. The problem or floodin g in the city co uldbe avoided by designing suita ble drainage system andfor thi s purpose knowledge or distribution of shortduration extreme rainfall events is essential. Thus, thestudy proposes to examine the frequency distributionof I, 2, 3. 6, 12. 24, 36 and 48·hr annua l maximumrainfall.

Frequency analysis of annual maximum rainfallis done to .predict the extreme rainfall amount for largere .urn periods-c-larger than the period of avai lab lerecords. T he major task of the problem is the choiceor suitable prob ability mode l whic h cou ld fit mostclcse 'y the freq uency distribution of historica l rainfalldata . The re arc num erou s theo retica l pro ba bility mod el'sugo.csted .lo be used for frequency analysis of ext remeprccrpitau on (Sevruk and Geiger 1981l, The choiceor . appropriate model for a parti cular ra infa llSC nc s depends largely on considerations of conventionand experience of unalysing the rainfall data. So in

this process choice of more than one model i~ obvious.In the presen t study Gu mbel and gam ma distrib urio nmodels have been ut ilized. T he results arc presentedon a diagra m as rainfall de pth-durat ion -frequ ency(DD F) relationships for Bom bay. An a tte mpt hasalso been made to understand distr ibut ion of5 , 10. 15and 3D-min extreme rainfall from hourly extr eme rain­fall distr ibutions.

2. Data used

Ii ourly (clock-hour) rain fall da te for Co laba obscrva t­o,y, Bombay fro m 1924 to 1966 has been ohta ined fro mthe publication entitled' Magnetic meteorological and 0 1­mospheric electrici ty observations made at the governmen tobserva tories, Bombayand Alibilg 'and fro m 1967 to '84, itis collected par tly from I.M.D.. Poona and partly fro mRegiona l Meteorologica l Cent re. Bombay. Thus, thestudy is based on 61 years ( 1924-84) contin uou s records ofhou rly ra infal l which is longest (as it is know n 10 author)record of hou rly ra infall for an y sta tion in India .Ann ual maxim um rainfall series for I. 2, 3, 6. 12, 24,36,and 48-hr du rations arc formed by picking highestrain fa ll or different dura tions each year and their

lrrequency distr ibuti on have been studied. Th e duration s

.are any continuous clock hours and not sticking tocalendar dates. Hereafter, annual rnaxirnurn rainfallseries will be referred to as rainfall series.

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