Plant growth and development are primarily governed by the environmental conditions of the soil & climate.The success or failure of farming is intimately related to the prevailing weather conditions. The modification of weather,except on a very limited scale, is yet in the realm of experimentation.It is, nevertheless, possible to optimise farm production by adjusting cropping patterns & agronomic practices to suit the climate of a locality.
Weather assumes significance in nearly every phase of agricultural activity from the preparatory tillage to harvesting and storage.Even after the produce is stored, weather continues to affect the fortunes of the farmers, as the reports of good or bad weather elsewhere tend to upset the price trends.
As weather is the single major limiting factor in crop production successfull farming calls for appropriate decisions in the light of weather conditions in the matter of the time of sowing, transplanting, scheduling of irigation, timing of fertiliser application, using of pesticides,etc.
A sound knowledge of the climatic factors and an understanding of the complex processes of interaction between the climate and the biological processes of thePlants are essential to a scientific approach to farming, based on planned cropping patterns and improved land-and-water-management practices.
PHYSICAL FEATURES AND CLIMATE
India is a land of many climates and varieties of soils affording scope for much diversity in agriculture. The geographic location and the physical features largely determine the climate of the country.
The lofty Himalayas run along its entire length in the north. To the south of this barrier are the alluvial plains watered by great rivers. In the farther south ,there lies the plateau of peninsular India skirted by narrow coastal strips, the Arabian Sea to the west, the Bay of Bengal to the east and the Indian Ocean to the south.
The pattern of rainfall all over India reflects the climatic variations in the different parts of the country. IT varies from per-humid in north-east India to arid in Rajasthan. A belt of arid or semi-arid climates extends from the north to the south, dividing the humid climates of the west coast & the central & eastern parts of the country, where the annual rainfall is generally less than 1,000 mm. This belt is the dry farming tract of India where new crops to augment food production can be introduced.
Rainfall. Rainfall is the most important of the climatic factors. The areas of very heavy rainfall exist on the windward side of the Western Ghats, the Khasi Hills & the Himalayas. These are the source regions for many of the major river systems of the country, particularly the Himalayan ranges. The north-western parts of India are the driest, Rajasthan receiving less than 500 mm of rain annually (table 1). A large part of the country receives, on an average, rainfall less than 1000 mm per annum. Moreover, the average annual rainfall of 1050 mm is the highest in any part of the world. The irrigation in the semi-arid or arid belt is not sufficient & the water balance is precarious. The agricultural potential of these dry tracts can be increased in the near future only by adopting a suitable package of practices aimed at the optimum utilisation of available moisture potential through improved soil-and-water management.
South-west monsoon. With the exception of Jammu & Kashmir in the extreme north & in Tamil Nadu in the south, 80-90 percent of the rainfall over the country occurs mostly during the south-west monsoon season. The success of agriculture in India, therefore, depends primarily on the timely onset, the proper amount & the distribution of rains in a season. The dates of the onset of the monsoon in different parts of the country & the intensity & the distribution of rain display large variations in time & space & from one year to another. Normally, the south-west monsoon reaches the Kerala coast by the end of May, advances along the Konkan coast in early June & extends over the entire country by the end of July. The rains continue up to the end of September, when the south-west monsoon recedes. In November & December, the north-east monsoon is the main contributor to the amount of rainfall over the south-eastern portion of the Peninsula.
Monsoon depressions. The activity of the south-west monsoon is not uniform in time & space during the whole season. The intensity & distribution of rainfall are controlled by a series of tropical depressions or low-pressure systems which originate near the head of the Bay of Bengal & travel across the country in a west-north-westerly direction. Heavy rainfall occurs mainly to the south of the tracks of these depressions. Three or four depressions form in a month during the monsoon. When in some years they are scarce, the rainfall will be confined to the Western Ghats & the mountain ranges of Assam & the foothills of the Himalayas, with the interior parts of the country not getting their usual share. And there is drought in these areas.
Even when the depressions are of normal frequency & intensity, their tracks determine the distribution of rainfall over northern & central India. When they follow a north-westerly track across the plains of the Ganga, there will be floods in northern India & drought in the Peninsula. The reverse is true when they take a westerly track across central India when the northern states are affected by drought, The conditions in the states, where the tracks of depressions terminate, e.g. in Gujarat & Rajasthan, tend to be erratic. When a depression reaches these states, they get abundant rains; otherwise, they are subject to a prolonged drought.
The monsoon depressions can be said to be the single factor that controls the distribution of rainfall over the whole of India. These, in turn, affect both irrigation & agriculture.
Breaks in the monsoon. The activity of the monsoon is brought to a halt for a week or two almost all over the country, occasionally during the monsoon. These are called ‘breaks’ in the monsoon & may occur in any of the monsoon months. The more prolonged breaks are likely during the mid-monsoon month, i.e. August. When a long break occurs during July at the beginning of the cropping season, its effect on the growing crops is quite harmful. The break in the monsoon is generally heralded by the shift of the monsoon trough from the plains of northern India to the hills. Simultaneously, a few low-pressure waves move westwards from the southern part of the Bay of Bengal across the southern part of the Peninsula. Consequently rains are confined to the hills & the submontane districts of the eastern Himalayas & to the extreme south of the Peninsula. Owing to very heavy rains in the catchments of the Brahmaputra & its tributaries & in the catchments of some of the rivers of northern Bengal & Bihar, floods in these rivers are possible during the ‘breaks’ season. For the rest of the country , it is a rainless period & may occasionally lead to disastrous consequences owing to crop failures over wide areas.