The regional variations in the distribution of annual rainfall over India are well pronounced. The influence of the Western Ghats and the Himalayas, Meghalaya Plateau and North-eastern Hills in stimulating precipitation is clear, as arc also the rain shadow in the Deccan, and the diminution of total rainfall up the Ganga Valley.
It may be noted that the 1000 mm isohyets divides the country into two roughly equal parts, a division that carries into agricultural regionalisation as the boundary between rained rice cultivation and that of wheat or millets (Johnson, 1979, pp. 53-54.
The highest rainfall occurs along the west coast, on the Western Ghats, sub-Himalayan areas in the north-east and the hills of Meghalaya. Here the rainfall exceeds 200 cm in a year. In certain parts of the Meghalaya Plateau the rainfall exceeds 1000 cm. It, however, drops to 200 cm or even below in the Brahmaputra valley and the adjoining hills. The isohyets of 100 cm rainfall run southwards from the Gujarat coast, roughly parallel to the crest of the Western Ghats up to Kanniyakumari.
In northern India it includes hills of Himachal Pradesh, Uttaranchal, Chhattisgarh, eastern Maharashtra, and northern Andhra Pradesh. The region lying to the west, south-west of this line have generally deficient rainfall where agriculture largely depends on irrigation. The rainfall in the Peninsular region lying to the east of 100 cm isohyets drops abruptly to below 60 cm but the Coromandel coast enjoys more than 100 cm of rainfall. The rainfall over parts of Punjab, Haryana, northern and central Rajasthan and Gujarat is below 60 cm. It is less than 20 cm in the arid regions of western Rajasthan.
In general the distributional pattern of annual rainfall shows two main trends: (a) it steadily declines towards the west and the north-west from Bengal and Orissa coasts, and (b) from the west and the east coasts it exhibits a declining trend towards the interior parts of the Peninsula.
Figure 5.2l.B shows the rainfall incidence determined on the basis of the occurrence of the rainy months throughout the calendar year. In this map the country has been divided into 12 regions having similar rainfall regimes (though not similar rainfall totals). Here a rainy month has been regarded as one during which more than one-twelfth of the mean annual rainfall is normally received (Johnson, 1969, p. 23).
The Assam type includes whole of the northeast where the rainy season extends to six months, from April to September. In the Bengal-Orissa type which includes West Bengal, Orissa and Chhattisgarh the rainy season lasts for five months, from May to September.
The central Indian type, in which the rains begin in June and continue for four months up to September, is a typical -legion of monsoon climate. It incorporates the largest area of the country covering Bihar, eastern Uttar Pradesh, Madhya Pradesh, western Rajasthan, Gujarat, Maharashtra and northern Andhra Pradesh. In north-west India type the duration of rainy season is for two months of July and August. It includes western Uttar Pradesh, Haryana, Punjab, central Rajasthan and western Gujarat.
The desert type includes the western most parts of Rajasthan where slight rains occur druid July. In Kerala-Karnataka type the rainy se lasts for four months (from June to September occupying parts of Kerala and Karnataka. In Tamil Nadu and south Andhra type the duration of rains for 3 months, from August to October. In type along the Rameshwaram coast the rainy season falls in winter (from October to January). In the Himalayan region rains in the form of rainfall a snowfall are observed from 10 to 12 months excel in northern Kashmir which is an area of wink rainfall (January and February).
Figure 5.22 shows the rainfall dispersion grams for individual stations of the country. The length of the dry season gives a fair indication of need for irrigation if agriculture is to be carried outside the rainy season.
A characteristic feature of the monsoon fall is its variability. The actual rainfall of the pi; in a year may deviate from its mean rainfall by 20 50 per cent which seriously affects the prospects J agriculture. Here the variability of the annual rainfall has been calculated by using following formula.
The values, thus derived, are called coincident of variation. These indicate the amount fluctuations recorded by rainfall over a long peril of time from the mean values (Raza and Ahmadabad 1990, p. 53).
The coefficient of variation of annual rain: in India generally ranges between 15 and 30 per c except in the north-west. Places such as Mangalore on the west coast, the sub-Himalayan belt include Sikkim and Arunachal Pradesh, and the north-east’ ern Hilly regions of Nagaland, Manipur and Minoa have a variability coefficient of variation less 15 percent. The variability increases from the West ern Coast towards the interior parts of the Peninsula Plateau as well as from West Bengal and Orissa towards the north and north-west.
Over the interim regions of Maharashtra, Andhra Pradesh Karnataka it is as high as 30 per cent. The simple showing 30 per cent variability runs over soothe Gujarat, western Madhya Pradesh and central Pradesh. The entire area lying to the west and the north-west of this isopleths, excluding the Himalayan and the sub-Himalayan areas are characterised by high annual variability of more than 30 per cent. The variability over Gujarat and eastern Rajasthan is over 40 per cent which increases even to 80 per cent in desert areas of the western Rajasthan.
The variability of rainfall has significant role in the Indian agriculture. The areas showing high variability of rainfall have chronic deficiency of water and are prone to droughts and famines.