There are four major reasons for drought in India- delay in the onset of monsoon/ failure of monsoon, variability of monsoon rainfall, long break in monsoon and areal difference in the persistence of monsoon.
Variability of monsoon rainfall
The variability in rainfall is a major cause of drought. The percentage of variability is inversely related to the total rainfall.
Areas like northeastern region and the western coastal plains and windward side of the Western Ghats, which receive the highest amount of rainfall in India, have low variability of rainfall. On the other hand, areas of low rainfall have a higher variability of rainfall. This covers the most of the interiors.
The areas most affected are interior Orissa, western Jharkhand, Chattisgarh, Madhya Pradesh, Rajasthan, Punjab, Haryana, Kathiawar and Kachchh in Gujarat, Vidharbha and Marathwada in Maharashtra, northern and eastern Karnataka and western Andhra Pradesh.
Delay in the onset of monsoon
Traditional farming and lifestyle patterns place heavy reliance on the timely arrival of monsoon. So even if the monsoon is delayed by a week, it causes a major upset in the farming communities and causes drought and famine.
These delays however are a fairly common occurrence in areas that get marginal rainfall.
The areas most affected are Punjab, Haryana, the Indo-Gangetic plains, Bundelkhand, eastern Rajasthan, eastern Gujarat, Madhya Pradesh, south Bihar, Jharkhand, interior West Bengal, Orissa, Andhra Pradesh, interior Karnataka and eastern Tamil Nadu.
Breaks in monsoon
Even within the monsoon if there are long dry periods, that too can cause drought in affected areas. This may happen due to a reduction in the speed of the rain-laden winds, or a diversion in the route. The damage to crops is usually extensive.
The areas most affected are the areas in the core monsoon affected areas.
Areal difference in monsoon
A deviation in the route of monsoon winds, or an early withdrawal of the monsoon too can have similar repercussions on an area. If the monsoon hangs over east Uttar Pradesh for a longer period of time, then west UP or Haryana are adversely affected.