The distribution of rainfall in any region is mostly influenced by the relief features on the surface of the Earth and the direction of the rain-bearing winds in that region. Another important factor which influences the distribution of annual rainfall in India is the path followed by the cyclonic depressions. The region located on the windward side of mountains, hills or plateaus receives comparatively more rainfall than the leeward side.
The average annual rainfall in India is about 120 cm. Most of this rainfall is received during four months (June, July, August and September) of the Southwest Monsoon season. The amount of annual rainfall varies not only from place to place, but also from season to season. In general, the monsoon rainfall is highly erratic.
Many areas of the north-eastern part of India receive more than 500 cm of annual rainfall, while many parts in the Thar Desert receive less than 10 cm of annual rainfall. There are yearly variations in the distribution of annual rainfall and they are the root cause of floods, droughts and famines in India.
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Broadly, India can be divided into the following four regions, depending upon the average annual rainfall received by those regions.
Areas of Very Heavy Rainfall:
The areas receiving an average rainfall of more than 200 cm are included in this region. The highest rainfall occurs along the Western Coastal plain, extending from Mumbai to Thiruvananthapuram, the western slopes of the Western Ghats, the southern slopes of the Eastern Himalayas, the sub-Himalayan areas in the northeast, the hills of Meghalaya.
In certain parts of the Khasi and Jaintia Hills, in north-eastern India, the average annual rainfall generally exceeds 1000 cm, but drops to about 200 cm or even less in the Brahmaputra valley and the adjoining hills, which are located in the rain-shadow area. Meghalaya is the wettest part of India with Mawsynram and Cherrapunji getting about 1220 cm and 1100 cm of average annual rainfall respectively. This region does not experience drought conditions, but is subjected to frequent floods.
Areas of Heavy Rainfall:
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The areas receiving an average annual rainfall between 100 cm and 200 cm are included in this region. The eastern slopes of the Western Ghats, northern and middle Ganga valley, north-eastern peninsula, Manipur, coastal areas of Andhra Pradesh and Tamil Nadu are included in this region. Here irrigation is needed as a protective measure against drought.
Areas of Low Rainfall:
The areas receiving an average annual rainfall between 50 cm and 100 cm are included in this region. The Upper Ganga Valley, eastern Rajasthan, Punjab, Haryajia, Kashmir and large parts of Gujarat, Maharashtra, Western Madhya Pradesh, Andhra Pradesh, Karnataka, and Tamil Nadu, are included in this region. Large irrigation projects have been developed to overcome the danger of droughts and famines.
Areas of Very Low Rainfall:
The areas receiving an average annual rainfall of less than 50 cm are included in this region. Here the desert and semi- desert conditions prevail. The areas included are the rain-shadow areas in the Deccan Plateau, north Kashmir, southern Punjab, western Rajasthan and Kachchh. There is an acute shortage of water in these areas.