In India, the temperature is high enough to promote the growth of crops at most of the places throughout the year. However, it is the availability of water that determines to a great extent the success of crops, zones of vegetation and crop patterns. In this country, around 75 per cent of the total area sown is with rainfall. Thus, the total yearly rainfall as well the number of rainy months of an area is very significant in the country. A rainy month is the one that receives more than one-twelfth of the mean annual rainfall. On the basis of total annual rainfall in the country, it is divided into five major regions. These five regions of the country are as follows -
Areas of Inadequate Rainfall (it implies rainfall less than 50 cm a year) - there are basically two belts that receive inadequate rainfall. These belts are of small extent and some parts of these belts receive exceptionally very low rainfall. Like for instance, the Karakoram Mountains and area lying to the north of the Zanskar Himalaya range. This region is situated in the north of the Himalayas. It, therefore, receives little rainfall during the summer monsoon rainy season. Further, a little rainfall is also caused in the months of January, February and March by the westerly depressions. Thus, it is evident that there are two maxima of rainfall in this region. Rainfall in late spring increases westward and August receives maximum rainfall. The rainy months are July and August and exceptionally January, February and March. This region is however very dry and nothing grows without irrigation.
The other region of India that receives extremely low rainfall is the western part of Kutch Districtand Rajasthan. These portions are lying to the west of a line starting from Kutch Coast and running northwards through the delta of the Luni Riverand touching Haryana near Hisar. Moreover, it also comprises small patch of Punjab, Andhra Pradesh, Maharashtra, Karnataka and Ladakh.
Besides, the narrow belt of mountainous country running in northwest-southeast direction. It comprises mainly the Zanskar range. Basically there are two maxima of rainfall, one in August and the other one in either March or April and the six rainy months are July and August and January to April. November is the driest month and it receives around 2.5 cm of rainfall.
Secondly, crescent shaped belt of around 150 to 300 km of width running through north-south direction. It comprises Punjab-Haryanaplain west of a line starting roughly from Amritsar, central belt of Rajasthan and the western Gujarat excluding the more arid western Kutch. This belt mainly has four rainy months (June to September) and other months are basically dry.
The third distinctive zone is a narrow belt of around 100 km width, running in the north-south direction. It is lying east of the Western Ghats. It also runs through the western part of the Maharashtra Plateau, extending further southward in Karnataka and south-east Andhra Pradesh. The belt runs southwards from the Satpura Mountain Range. The southern limit of the belt has five rainy months (June to October). July is the rainiest month in the northern belt and September is the rainiest month in the southern belt. Further, south the number of rainy months increases from five to six (May and June and August to November).
Areas of Low Rainfall (it implies rainfall around 50 to100 cm a year) - it is a long belt running from towards southward. It is interrupted by a narrow belt of low rainfall of the Western Ghats Mountain Range in Indiaand also by a few patches of low rainfall in the interior of Tamil Nadu. In this long belt almost crossing the country from north to south, the time of the onset of the rainy season and the number of the rainy months vary from one part of the region to another.
Medium Rainfall Zone (it implies rainfall in between 100 to 200 cm a year) - the different areas of high rainfall in India and these are widely separated from one another. Firstly, a narrow belt in the Western Ghats running throughout their length in the north-south direction. The number of rainy months increases towards the south. Like there are four rainy months (June to September) in the north and five rainy months (June to October) in its middle part. Himachal Pradesh, Uttar Pradesh, Bihar, West Bengal, Orissa and few districts of Maharashtra form the largest single block of high rainfall in India. Tripura is also an area of high rainfall.
High Rainfall Zone (it implies rainfall above 200 cm a year) - there are mainly two areas of very high rainfall. Firstly, a long belt running southwards. In the northern part of this belt, there are four rainy months (June to September). July is the rainiest month. Darjeeling District of West Bengal and some states of north eastern India form the other belt of the very high rainfall zone. In this zone, there are some areas of exceptionally high rainfall. Arunachal Pradesh is also an area of very high rainfall. January and December are dry months receiving less than 3 cm rainfall.
Thus, it can be said that in India the distribution of annual rainfall is uneven with several rainfall zones.