Number Crunch: Measuring monsoon

Monsoon this year is 22 per cent below the long period average. . If rains do not pick up, India may witness its second drought year in four years. What is a bad monsoon and how do we measure it?

IMD’s classification

According to India Meteorological Department, there are four classifications based on rainfall received. Normal monsoon year is one where rain received is 96 to 104 per cent of the long period average. Rainfall above 104 per cent of LPA is considered excess rainfall. Below normal monsoon occurs when the country receives rain less than 96 per cent but more than 90 per cent of the LPA. The country is said to have received deficient rainfall if the rainfall is less than 90 per cent of the LPA.

The long period average for the 50-year period (1951-2000) is 89 cm.

When the rainfall is less than 90 per cent of the LPA and 20 to 40 per cent of the country receives deficient rainfall (20 per cent below normal), then it is termed as a drought year for the country.

For instance, though the year 2004 had rainfall less than 13 per cent below LPA it wasn’t an ‘all India drought year’ as only 18 per cent of the country had deficient rainfall.

If we consider the last three decades, while 1981-1990 witnessed three years of deficient rainfall, there were no instances of failed monsoon in the 1991-2000 decade. That pattern shifted in the last decade following the 2002 drought. The 2001-2010 decade had seen three instances of deficient rain with 2009 year drought being the worst since 1972. In 2009, the rainfall was 78 per cent below normal with 22 of the 36 centres witnessing deficient rainfall.

Distribution of rainfall

Even in years of normal rainfall, the rainfall distribution may not be even. There could be some regions that receive deficient rains and other excess. While the south-west monsoon in 2006 was normal, rainfall was deficient in around 10 out of the 36 centres. That is 59 per cent of the districts which received excess or normal monsoon made up for the deficient districts (41 per cent).

Similarly, in the year 2000, when India received rains that were 8 per cent below LPA, Central India witnessed severe drought. This affected commodities such as pulses and oilseeds which grow in Madhya Pradesh.

If one considers last six drought years, spanning three decades, North-West India was the worst hit during droughts. The region includes Jammu and Kashmir, Himachal Pradesh, Punjab, Rajasthan, Haryana, Chandigarh, Delhi, Uttaranchal and Uttar Pradesh.

Even during the current year, North-West India received rainfall 36 per cent below LPA in the first 40 days. Deficit in these regions may affect production of food grains, sugarcane, rapeseed and mustard.