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com/loi/thsj19 Rainfall distribution over Indian subdivisions during the wettest and

travers les sous-divisions indiennes pendant les moussons les plus humides et les

GHOSE To cite this article: O. N. DHAR, A. K. KULKARNI & G. C.

plus secs de la période 1901 1960, Hydrological Sciences Journal, 23:2, 213-221, DOI:

1080/02626667809491790 Published online: 25 Dec 2009.

articles: 1 View citing articles Full Terms & Conditions of access and use can be

1 Hydrological Sciences Journal ISSN: (Print) (Online) Journal homepage: Rainfall distribution over Indian subdivisions during the wettest and the driest monsoons of the period / Mémoire sur la distribution de pluie à travers les sous-divisions indiennes pendant les moussons les plus humides et les plus secs de la période O. N. DHAR, A. K. KULKARNI & G. C. GHOSE To cite this article: O. N. DHAR, A. K. KULKARNI & G. C. GHOSE (1978) Rainfall distribution over Indian subdivisions during the wettest and the driest monsoons of the period / Mémoire sur la distribution de pluie à travers les sous-divisions indiennes pendant les moussons les plus humides et les plus secs de la période , Hydrological Sciences Journal, 23:2, , DOI: / To link to this article: Published online: 25 Dec Submit your article to this journal Article views: 267 View related articles Citing articles: 1 View citing articles Full Terms & Conditions of access and use can be found at

2 HydrologicalScicnces-Bulletin-des Sciences Hydrologiques, 23, 2, 6/1978 Rainfall distribution over Indian subdivisions during the wettest and the driest monsoons of the period O.N. DHAR, A. K. KULKARNI andg. C. GHOSE Indian Institute of Tropical Meteorology, Ramdurg House, University Road, Poona , India Received 17 August 1977 Abstract. An attempt has been made in this note to study the rainfall for the sub-divisions of India for the wettest and the driest monsoons (i.e. June-September) during the period All the rainfall stations in each sub-division, have been taken into consideration to work out the average rainfall of each sub-division for individual months, seasons and years. The sub-divisions of the country that existed on 1 January 1971 have been considered in this note. This study has shown that during the 60-year period, 1917 was the wettest year and 1918 the driest year. The rainfall distribution in the monsoon periods (i.e. June to September) of the wettest and driest years, 1917 and 1918 respectively, has been studied. Possible meteorological causes for the exceptional rainfall in 1917 and the drought in 1918 also have been briefly mentioned. Mémoire sur la distribution de pluie à travers les sous-divisions indiennes pendant les moussons les plus humides et les plus secs de la période Résumé. Dans cette mémoire on a essayé d'étudier la pluie pour les sous-divisions de l'inde pour les moussons les plus secs et les plus humides (juin-septembre) pendant la période On a considéré toutes les stations pluviométriqués dans chaque sous-division pour les mois, les saisions et les années particulières. On a tenu compte des sous-divisions qui existaient le premier janvier 1971 dans cette mémoire. Cette étude a montré que pendant la période de soixante ans, 1917 était l'année la plus humide et 1918 l'année la plus sèche. Dans cette e' tude on a considéré la distribution de pluie pour les périodes de mousson (juin-septembre) des années les plus humides et les plus sèches, 1917 et 1918 respectivement. On a aussi fait mention des causes possibles de la précipitation exceptionnelle de 1917 et de la sécheresse de INTRODUCTION It has been said that India has more wealth in water resources than Arabia has in oil. Every year, more than 75 per cent of this water is received during the four monsoon months from June to September. Dharef al. (1974), taking into consideration the rainfall data of about 3000 stations from all over the country during the 60-year period from 1901 to 1960, found that the mean annual and the mean monsoon rainfall of the country as a whole (outside the island sub-divisions of the Bay of Bengal and the Arabian Sea) are about 1190 mm and 900 mm respectively /78/ $ Blackwell Scientific Publications 213

3 214 O. N. Dhar et al. While examining the rainfall distribution over different sub-divisions of the country (Fig. 1) outside the island sub-divisions, it was noticed that there were two worst years during the 60-year period from the point of view of excess and deficient rainfall. In the wettest year, 1917, the country received the highest annual rainfall of about 1450 mm which was in excess of the mean annual rainfall by about 22 per cent. The succeeding year was found to be the driest year during this period, as the average annual rainfall received in this year was about 960 mm, about 19 per cent less than the mean annual rainfall. In the succeeding sections of this note, an attempt has been made to study the distribution of rainfall over the different contiguous sub-divisions of the country during the monsoon periods for the two extreme years. FIG. 1. Meteorological sub-divisions of India at 1 January MEAN MONSOON RAINFALL OF INDIVIDUAL SUBDIVISIONS OF INDIA On the basis of 60 years of rainfall data, the mean annual and mean monsoon rainfall of the 31 contiguous meteorological sub-divisions (Fig. 1) are given in Table 1. In this table the mean monsoon rainfall of each of the meteorological sub-divisions is also shown as a percentage of the mean annual rainfall of the respective sub-divisions. Perusal of Table 1 shows that the mean monsoon rainfall of coastal Mysore (now called

Rainfall distribution in Indian monsoons 215 coastal Karnataka), along the west coast of India, is of the order of 2890 mm which is the highest monsoon rainfall received by any sub-division in this

4 Rainfall distribution in Indian monsoons 215 coastal Karnataka), along the west coast of India, is of the order of 2890 mm which is the highest monsoon rainfall received by any sub-division in this country. It is seen that this mean monsoon rainfall is about 87 per cent of the mean annual rainfall for this sub-division. The next highest sub-divisional mean monsoon rainfall is received by the Konkan sub-division, along the west coast, which received about 2740 mm of rainfall which is nearly 94 per cent of the mean annual rainfall of this sub-division. The sub-divisional percentage ratios of mean monsoon to mean annual rainfall vary from 34 per cent in the case of Tamil Nadu to 95 per cent in the case of Gujarat. The coefficient of variability of the monsoon rainfall for these sub-divisions is given in Table 1. It may be observed from Table 1 that the lowest values of the coefficient of variability have been obtained for the sub-divisions of northeastern India, viz. northern Assam, southern Assam, sub-himalayan West Bengal, etc., and the highest values for the sub-divisions of western India, viz. Saurashtra-Kutch, West Rajasthan, Punjab, etc. It is, however, seen from Table 1 that of all the contiguous sub-divisions of this country, the Tamil Nadu sub-division receives the lowest amount of monsoon rainfall (i.e. about 350 mm) and its coefficient of variability is of the order of 19 per cent. This sub-division, however, received a higher amount of rainfall, (i.e. about 470 mm) during the post-monsoon period of October-December, but surprisingly the coefficient of variability of rainfall is higher (i.e. about 27 per cent) when compared to the monsoon rainfall. Probably, this is due to the fact that the post-monsoon rainfall of Tamil Nadu is mostly associated with cyclonic storms and depressions whose frequency of occurrence in this season is rather irregular. RAINFALL DISTRIBUTION OVER THE DIFFERENT SUB-DIVISIONS DURING THE WETTEST MONSOON The study of the 60-year rainfall of the country has shown that 1917 was the wettest year during this period. During the monsoon season of 1917, the country received rainfall nearly 19 per cent in excess of the mean monsoon rainfall of the country which is of the order of 900 mm. The average monsoon sub-divisional rainfall for the year 1917 as a percentage of the mean monsoon rainfall of individual sub-divisions has been worked out. Figure 2 shows sub-divisional percentage departures in the wettest monsoon with respect to the mean monsoon rainfall of individual sub-divisions. It is seen from Fig. 2 that the sub-divisional percentage departures are the highest in northwestern India, the region which normally received the lowest amount of monsoon rainfall (Table 1). The highest percentage departure of +156 per cent was obtained in western Rajasthan. In eastern Rajasthan and Haryana percentage departures were as high as +83 per cent and +81 per cent respectively. Excepting the sub-divisions of lower and upper Assam, sub-himalayan West Bengal, Gangetic West Bengal, the Bihar plains and Orissa in northeastern Indian and Rayalaseema, Kerala, coastal Mysore and Madhya Maharashtra in the Indian peninsula, the rest of the sub-divisions of the country received moderate to heavy rainfalls during the monsoon season of Even in the case of the Tamil Nadu sub-division, the percentage departure was as high as +29 per cent. It is thus evident from Fig. 2 that during the monsoon period of 1917 those sub-divisions of the country which normally receive fairly moderate to heavy rainfall during this season, like the sub-divisions of northeastern India and west coast sub-divisions, did not receive the very heavy rainfall experienced by the rest of the sub-divisions. Exception-

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6 Rainfall distribution in Indian monsoons ally heavy rainfall was experienced by the sub-divisions of northwestern India from Kashmir in the north to Saurashtra-Kutch in the west, a region which normally receives comparatively little monsoon rainfall. The other sub-divisions which also received excess (i.e. +20 per cent or more) monsoon rainfall in the 1917 monsoon were the sub-divisions of northern and central India, viz. western Uttar Pradesh, western Madhya Pradesh, Marthawada and other major sub-divisions of the peninsula, viz. coastal Andhra Pradesh, Telangana, Rayalaseema and Tamil Nadu. in <?.*' # ~\ 1 my ip /flhto '+83.J-11TP ~ V *'^"QHUTAfir+ I ^j"^ ARABIAN SEA 10 65" IHIIIII Excess rainfall + 20% or more.! j Normal rainfall + 19% to -19%. FIG. 2. Sub-divisional percentage departures of monsoon rainfall in 1917 from the average monsoon rainfall. The meteorological situations responsible for making the 1917 monsoon the wettest monsoon of the 60-year period have been examined. The Arabian Sea current of monsoon arrived on schedule along the west coast and extended over the country with the usual rapidity but the Bay of Bengal current appeared in northeast India some 2 weeks ahead of schedule and its influence penetrated inland with great speed and by 3 June rainfall occurred in northwestern India, nearly 3 weeks before the normal date. The number of depressions which moved through the country during the monsoon period of 1917 was five as compared to an average of seven. The monthly frequency of these disturbances during the different monsoon months of 1917 is given in Table 2. It was observed, however, that during the 1917 monsoon season a number of low

7 218 O.N.Dharetal. TABLE 2. Month No. of monsoon depressions June 2 July 1 August 1 September 1 Total 5 pressure areas moved through the country which gave well distributed rainfalls in different regions, particularly in northwestern India. July, however, was the only month when the monsoon was of average strength. This was largely due to the fact that in this month there were two 'breaks' in the monsoon, one was from 5 to 11 July and the other from 25 to 27 July (Ramamurthy, 1969). Monsoon rainfall during the rest of the monsoon was phenomenally heavy and no long breaks in rainfall occurred over any other part of the country. This was possibly due to the fact that both branches of the monsoon, the Arabian Sea and Bay of Bengal currents, were quite active during most of the monsoon season. It is also seen that the month of September was the wettest month during this monsoon period although in this month only one depression moved through the country. The monthly distribution of average rainfall over the country as a whole during each of the monsoon months of 1917 is given in Table 3. TABLE 3. Month Rainfall (mm) Percentage : departures from the mean monthly rainfall June July August September ^ It is also observed that during the monsoon period of 1917 rainfall over a number of sub-divisions of the country was in excess yet there were no major floods (Dhar et ah, 1975). Probably, this was due to the fact that the rainfall was more or less evenly distributed and there were no localized heavy speus of rain which normally occur in association with depressions/cyclonic storms, of which there were fewer than normal. RAINFALL DISTRIBUTION OVER THE DIFFERENT SUB-DIVISIONS DURING THE DRIEST MONSOON The year 1918 was the driest on record; the annual and monsoon rainfall of the country was in deficit by 19 per cent and 22 per cent respectively (Dhar et al., 1974) when

8 Rainfall distribution in Indian monsoons 219 compared to the long-term average. Figure 3 shows the sub-divisional percentage departures of the 1918 monsoon rainfall. It can be seen that, excepting the subdivisions in northeastern India where the percentage departures were positive, in the remaining sub-divisions of the country, percentage departures were negative. Scanty (i.e. 60 per cent or less) rainfall was experienced in the sub-divisions of Saurashtra- Kutch, Gujarat and western Rajasthan. The rainfall in the remaining sub-divisions was HUH Excess rainfall + 20% or more. Ë~~j Normal rainfall + 19% to -19%. \///A Deficient rainfall - 20% to -59%. KSS^ Scanty rainfall -60% or more FIG. 3. Sub-divisional percentage departures of 1918 monsoon rainfall from the average monsoon rainfall. deficient by 20 to 59 per cent except in the sub-divisions of eastern Madhya Pradesh, Orissa, Bihar Plateau, Bihar Plains and other sub-divisions of northeastern India. In all the sub-divisions of the peninsula of India including the west coast subdivisions, rainfall was deficient (i.e. 20 to 59 per cent). The Arabian Sea current set in along the Kerala coast on or about 11 May, nearly three weeks ahead of normal. It extended slowly northwards, but soon afterwards its activity began to decline. From the last week of June to about the middle of August it was extremely weak. There were two major 'breaks' during the monsoon period of this year (Ramamurthy, 1969). One was from 7 to 23 July and the other from 7 to 12 August. The monsoon started withdrawing by about the first week of September from

9 220 O. N. Dhar et al. northwestern India and by the middle of this month, the withdrawal was practically complete from northwestern, northern and central India. This monsoon which advanced over the country nearly 3 weeks early, became weak thereafter and withdrew from northern India 2-3 weeks early. The number of depressions which traversed the country during the monsoon period of 1918 was five. Their monthly frequency is given in Table 4. TABLE 4. Month No. of monsoon depressions June 1 July 1 August 2 September 1 Total 5 Although there were depressions in each of the four months of the monsoon period, this did not help very much in augmenting rainfall distribution over the different sub-divisions of the country. On the whole, the failure of the 1918 monsoon to a large extent was due to the fact that the Arabian Sea current was weak over most of the sub-divisions of the country which normally come under its sway. Practically, al the sub-divisions of the country which are located to the west of latitude 85 E, excepting the sub-divisions of Bihar Plains, Bihar Plateau, eastern Madhya Pradesh and Orissa, received deficient to scanty (i.e. 20 to 76 per cent) rainfall. Only the two sub-divisions of northern and southern Assam received excess (i.e. +20 per cent or more) rainfall during the monsoon period due to the fairly active Bay of Bengal current. The monthly distribution of rainfall over the country as a whole during each of the monsoon months of 1918 is given below. TABLE 5. Percentage departures from Month Rainfall (mm) the mean monthly rainfall June July August September From the above it is seen that except for the month of June, the monsoon rainfall was low in all the three remaining months of the monsoon period and July and September were the worst affected months. Perhaps setting in of a long 'break' in the monsoon in the month of July and early withdrawal of the monsoon from the country in September, were the main causes which made the rainfall deficiency in these 2 months quite severe.

CONCLUSIONS Rainfall distribution in Indian monsoons 221 The main conclusions of this study are that the year 1917 was the wettest year during the period 1901-1960 and the year 1918 was the driest

10 CONCLUSIONS Rainfall distribution in Indian monsoons 221 The main conclusions of this study are that the year 1917 was the wettest year during the period and the year 1918 was the driest year during the same period. During the wettest monsoon period of 1917, the sub-division of western Rajasthan received +156 per cent excess rainfall which is a record for this sub-division. During the driest monsoon of 1918, rainfall of western Rajasthan sub-divisions was in deficit by about 76 per cent. Although during the monsoon period of 1917 and 1918 the number of depressions which traversed the country was equal (i.e. five in all), there was a great disparity in the amount of rainfall received during the monsoon seasons of these two years. In the 1917 and the 1918 monsoons the country as a whole received 1060 mm and 700 mm of rainfall respectively, while the mean monsoon rainfall of the country as a whole is about 900 mm. This clearly shows that the number of depressions/cyclonic storms alone does not control the rainfall distribution during the monsoon months. In 1917, a number of low pressure areas moved through the country which strengthened the monsoon activity over different parts of the country especially in the northwestern region during the month of September. In 1917 two short breaks in monsoon occurred in the month of July while in 1918 one long and one short break in monsoon occurred in the months of July and August respectively. Moreover, in 1918 the Arabian Sea current of monsoon which appeared over the Kerala coast nearly 3 weeks in advance of the scheduled date of arrival became weak towards the end of June and thereafter it remained weak over the country. The Bay of Bengal current was active over the sub-divisions of northeastern India where rainfall was in excess. The monsoon completely withdrew from northwestern, northern and central India nearly 3 weeks earlier. All these factors were responsible in making the monsoon of 1918 the driest monsoon of the 60-year period. On the other hand, during the 1917 monsoon the Bay of Bengal current of monsoon advanced over northeastern and northern India nearly 2 weeks before the normal date and for almost the entire monsoon season, both branches of the monsoon current were equally active. During this year, the monsoon withdrew from northwestern India towards the end of September nearly 2 weeks after the scheduled date. ACKNOWLEDGEMENTS The authors are thankful to Shri B. N. Mandai of the Hydrometeorological Research Project of the Institute for his help in the preparation of this paper and to Miss P. Premia for typing the manuscript. REFERENCES Dhar, O.N., Parthasarathy, B. & Ghose, G.C. (1974) A study of mean monthly and annual rainfall of contiguous Indian area. 'Vayu Mandai', Bull. Ind. Met. Soc, 4, no. 2. Dhar, O.N., Bhattacharya, B.K. & Ghose, G.C. (1975) A catalogue of the highest ever recorded floods in Indian rivers: a preliminary appraisal. Indian J. Pwr Riv. Vail. Dev. 25, no. 12. Ramamurfhy, K. (1969) Some aspects of the 'break' in the Indian southwest monsoon during July and August. Forecasting Manual, Part IV, no IMD, Poona-5.

INDIA METEOROLOGICAL DEPARTMENT (MINISTRY OF EARTH SCIENCES) SOUTHWEST MONSOON-2010 END OF SEASON REPORT

INDIA METEOROLOGICAL DEPARTMENT (MINISTRY OF EARTH SCIENCES) SOUTHWEST MONSOON-2010 END OF SEASON REPORT HIGHLIGHTS For the country as a whole, the rainfall for the season (June-September) was 102% of