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Vinson Kurian

Thiruvananthapuram , Aug. 8

LATE than normal onsets, decreasing precipitation and other aberrations in monsoon behaviour have combined to lend an air of unpredictability to evolving weather pattern over Kerala, which has witnessed drought-like contingencies on more than one occasion in the recent past.

According to studies conducted in 2000 by the Water Technology Centre of the New Delhi-based Indian Agricultural Research Institute, less-than-optimum monsoons have created large rainfall deficiencies in various districts and large water deficits in an increasing number of representative stations leading to severe dry spells in the State.

Although blessed with what in meteorological parlance is called a `wet climate', the State has had to encounter severe dry spells and droughts in 1983, 1985, 1986, and 1987 in the immediate past. There have been dry spells extending to five and four weeks respectively in 1985 and 1986 during the southwest monsoon period.

This, when the State is normally known to average 3,800 mm of rain in the North and 1,800 mm in the extreme South during the season. The potential rainy season is the southwest monsoon, which contributes more than 80 per cent of the annual rainfall.

As has already been reported, the monsoon rain has been seen to be tapering off in intensity as one travelled from the North to the South. In recent years, the decreasing trend has been seen both in seasonal rainfall and 10-day extreme rainfall duration.

In the North, the northeast monsoon rainfall is now seen contributing to about 15 per cent of the annual rainfall. This may have adverse implications for the standing second rice crop in the area. Southwest monsoon rain, which contributes 82 per cent of the area's total rainfall, does not show any upward trend either. In the South, both the southwest and northeast monsoon rains have thinned to the extent of 5 per cent and 8.3 per cent respectively, the study notes. Mean annual rainfall also has also been on a decreasing trend.

As for drought events in the past, the damage was particularly significant in 1987. About 1,500 villages in 14 districts were affected, with 9.82 lakh hectares of cropped land and a six-lakh strong cattle population bearing the brunt. During January - May 1987, practically the entire State was under the grip of drought.

In the 1983 drought event, about paddy crop on an estimated 3.23 lakh hectares withered away leading to losses of Rs 106.86 crore. Cash crops such as coconut, rubber, coffee and tea were also badly affected. In the 1989 episode, as much as 60 per cent of the cropped area was exposed to the whims of the nature. Some three million kilograms of tea worth Rs 1 crore withered.

Read the clouds to gauge the drought

Cloud analysis and orographic rainfall patterns may give a good indication of the drought situation in a wet State such as Kerala.

Orography is the average height of land, measured in geopotential meters, over a certain domain. In geoscientific models, such as general circulation models, orography defines the lower boundary (except where there is ocean).

Understanding the effects of hills and mountains is of crucial importance for weather forecasting and climate prediction. Orography affects both local weather (e.g. summit-to-valley wind speed differences; enhancement of precipitation over mountains with an associated relatively dry `rain shadow' region downstream) and also the entire global circulation through the extra drag exerted on the atmospheric flow.

During weak monsoons and droughts in Kerala, the orographic contribution is almost nil, but this is not attributed to a weaker westerly component during the dry spell. The Nepha (or cloud) analysis from satellite pictures also gives good information about drought.

During 1966, a year of weak monsoons and drought, satellite pictures showed a zone of cloudiness shifting far into southern India. During drought situations over the State, there is no high-level moving system of waves in the upper tropospheric easterlies.

During the drought of 1966, high-level wave flows were more or less straight easterly flows with less speed variation than in a good strong monsoon season.

Cloud analysis during active and strong monsoons as in 1967 showed at least seven `oktas' of cloudiness on any given day over the State, extending from the interior of the southern peninsula southward and westward (1,200 km from the Kerala coast of the Arabian Sea). The average of the entire period was equal to 4.3 oktas.

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