Uneven rainfall

Print edition : November 03, 2006

THE total seasonal rainfall during the year's southwest monsoon (June 1 to September 30) for the country as a whole was 99 per cent of its long-period average (LPA), which, being within plus or minus 10 per cent of the LPA, can be termed as a `normal' monsoon. However, both spatial and temporal distributions have been skewed this year, and as a result the country is faced with the unusual situation of a large deficiency of as much as 17 per cent in the northeastern region (see "Changing patterns", Frontline, October 6). The appearance of `normal' rainfall was owing to the excess rainfall (116 per cent of LPA) over central India (see table). Are these part of changing trends in the rainfall pattern over the country? Monsoon performance in the next few years might bear this out.

Interestingly, this year's pattern is a repeat of last year's: a total rainfall of 99 per cent of the LPA accompanied by deficit rainfall in the northeastern region. However, indications are that the meteorological conditions creating this were more similar to those observed in 1994 than 2005. That is, the usual `break condition' in August that brings copious rains to the northeastern States was absent. The `monsoon trough' was anchored south off its normal position; this resulted in excess rainfall over central India and deficit rainfall over States at higher latitudes, especially Uttar Pradesh, Bihar and the northeastern States.

Of the 36 meteorological subdivisions, seasonal rainfall was deficient in 10, with five of them (Andaman and Nicobar Islands, Arunachal Pradesh, Assam and Meghalaya, western Uttar Pradesh and Haryana) experiencing `moderate drought' conditions (deficit of 25-50 per cent). It was in excess (more than 120 per cent of the subdivisional LPA) in six and normal (minus 119 per cent to plus 119 per cent of the LPA) in 20. The excess subdivisions (western Rajasthan, the Gujarat region, Saurashtra and Kutch and central Maharashtra, Orissa and Jammu and Kashmir) were mainly in the west, which points to the uneven spatial distribution this year. Of the 533 districts, rainfall was deficient (more than 19 per cent deficit) in 209, of which 130 faced moderate drought conditions (26-50 per cent deficit) and 30 experienced `severe drought' conditions (51 per cent or more deficit).

The temporal or monthly distribution for the country as a whole was somewhat abnormal. Though the June rainfall only contributes about one-fifth of the monsoon rainfall, this year it fell short of the LPA by 13 per cent, in spite of the good rainfall in the first week (see bar chart on facing page). The week-to-week progression shows that a large deficiency occurred from the second to the fourth week of June as well as July (the latter contributes about one-third of the monsoon rainfall). The excess during the first two weeks of August brought rains only to central India, robbing the northern and northeastern regions of their usual quantum. In fact, during the entire season, the cumulative rainfall remained below its LPA except for the first week of June. By the fourth week of July, the cumulative deficit was 14 per cent, which subsequently improved to give an overall deficit of 1 per cent at the end of the season.

The long-range forecast of the India Meteorological Department (IMD) in April had put the seasonal rainfall at 93 per cent of the LPA with plus or minus 5 per cent model error. This was updated in June to be 92 per cent of the LPA with a possible error of plus or minus 4 per cent. The actual rainfall of 99 per cent was significantly beyond the error bar.

The predictions with regard to the four homogeneous rainfall regions of the country were similarly off the mark. Rainfall was expected to be 91 per cent over northwestern India, 90 per cent over central India, 94 per cent over northeastern India and 97 per cent over the southern peninsula. The regional predictions had a model error of plus or minus 8 per cent. In terms of the actual rainfall of 94 per cent, 116 per cent, 83 per cent and 95 per cent respectively in the four regions, the forecast for Central and northeastern India were off.

R. Ramachandran

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