Question

why is irrigation nessary in India how the cropping pattern of many regions

Answer 1

It is important to increase the area under irrigation because:
It is necessary to reduce the dependency on rainfall and stabilize the cropping pattern.
For example in South India without perennial rainfall, water supply replenishment is dependent on a cycle of dry seasons alternating with monsoon seasons.
Monsoons are not certain in nature. So, farming cannot entirely depend upon rains.
Irrigation also allows farmers to grow more crops in one year on the same land
The constant availability of water for irrigation provides a sense of stability to the farmer, and also encourages him to practise newer farming methods and patterns to maximise the productivity.