Mousam; (1989), 40, 2. 207·214

Variability in drought incidence over Indiaapproach

551. 577 .3R : 519.24 (540)

A statistical

A. CHOWDHU RY, M . M. DANDEKAR a nd P. S. RAUT

Meteorological Office, PIIIIC

( Recci ,,,,,I 24 October 1984)

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ABSTR ACT. Based on the monsoon rainfall for J13 years (1875·1987), an attempt has been made to qua ntifydrough t for the country as a who le and identifv drought years by develo ping a drought Index. Fo r thispurpose total area of India recei ... ing monsoon rainfall IbS than 75 % of the normal was obta ined foreach o f the 11 3 years. h om this, mean drought affected area and its standard de viat ion were worked o ut. lhcdifference between area a ffected by drought in any year and the 113 years' mean drought area divided by the stan­dard deviat ion was defined as drought index. Time interval between successive drought yea rs has been subjectedto statistical analysis and the distribution determined and tested for randomness. The series o f drought years. wasalso subjected to return periodanalysis.

w ithin the analvsed lime frame. 191 8 is fou nd 10 be the worst drought in I ndia. It is obse rved that India canso metimes even experience spells of three successive years of drought of varying intensity and extent. The: timeinterva l between success ive drought years can be ap proximat ely described by Poisson's distribution .

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I. Introduction

Study of rainfall deficiency in India has a tt rac ted theat ten tion of Indian meteorolog ists towards the begin­ning of the present cen tury. Pion eering work in thi sfi eld was do ne by Sir Gi lbert Walker in 19 19, who wor­ked out rainfall depar tu res from long-term means a ndidentified bad monsoon years . Subsequently until 1950,there was hardly an y attempt to study droughts in Indi a.Interest in drought studies revived aft er 1950, and parti­cularly after 1970, when drou ght incidence became a fre­quently recurre nt feature of Indian ra infall.

Walker (1919 ) used seasona l rainfall dat a d ur ing theperiod 184 1 to 1908 in his study ; however, he had somedoubts about the deg ree of accuracy of the data . Dr oughtco ncept was applied, per haps, for the first t ime to agri ­culture by Rarndas (1950) who defined drought to haveoccurred if the weekly ra infall is less than Iwice the meandeviat ion. He ident ified 1877, 1899 and 1913 as yearsof outstandi ng agr icult ural drought, while in 1920 thedrought was only partial , affect ing northwest and central

parts. Ramdas a nd Mallik (1948) defined drought tohave occurred when the act ual rai nfall during a weekwas less than or equ al to hal f the normal rainfall. Usingth is defin ition, Mallik (1958) examined the occurrence ofdroughts in Bihar and Utta r Pradesh. Surprisingly,he could no t ide ntify 1918 as a drought year in eastUttar Pradesh and Bihar, from the point of view ofshortening of wet season. William s (1958) studied thefrequency of drought in so uth Ind ia a nd concluded thatKarnataka has the least and Andhra Pradesh highestliability to dr ought. Mallik ( 1966) presented d istr ict­wise incidence of dry periods in west Rajasthan onweekl y basis and developed a dr ought index based onnumber of dry spells and their duration, He found thateither good or deficient rainfall cond ition s tend to occursimultaneously in many districts, lending support to theidea that west Raja sthan is a homogeneous cl im aticarea.

During the past decade a number of stud ies have beenreported on the identification of drought and its charact­eris tics, Chowdhury ,'I al. ( 1979) applied logarithmic

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