India’s rainfall patterns changing drastically, say Stanford scientists

Longer dry spells and wetter wet spells could spell doom for the Indian agriculture, they warn

By Anushka Kaushik
Last Updated: Monday 17 August 2015



Tough times are ahead for the Indian agriculture which is highly dependent on the summer monsoon. According to a study by scientists from Stanford University in the United States, there has been a consistent drop in the average seasonal rainfall India receives during the summer monsoon months of July-August. The study also warns of extreme weather patterns in future.

The scientists analysed temperature and precipitation data from 1951 to 2011 and found a significant decreasing trend in the mean rainfall during the summer monsoon months. In contrast, there has been an increase in the daily rainfall variability during July-August by a 5 per cent level.

Changes in atmospheric conditions have led to such higher frequency of dry spells and increasing intensity of wet spells, notes the study, which defines wet and dry spells as three or more consecutive days of extremely high or low rainfall. Such extreme weather events are increasing the risk of drought and flood in central India.   

Stronger convective activity over southern and eastern India leads to increased intensity of wet spells. Similarly, the decreased intensity of dry spells occurs due to reduced extent of horizontal cold air transfer (which occurs after reduction in upper-level cyclonic anomaly). 

Additionally, the frequency of occurrence of dry and wet spells is influenced by high vertical wind shears. The study posits that between 1981 and 2011, reduced levels of upper and lower-level winds weakened wind shears, increasing the frequency of dry spells and reducing the intensity of wet spells. 

The study observed atmospheric conditions from 1951 to 2011. They measured precipitation during the period by using a new statistical method that accounts for spatial and temporal relationships between rainfall levels, temperature, and other geophysical phenomena. These parameters have been ignored in previous statistical tools. Such statistical tools are crucial while investigating factors, such as temperature and rainfall that can change overnight. 

With 60 per cent of employment in the agriculture industry in India and more than 56 per cent of agricultural area being rain-fed, studying the modifying patterns of the monsoon is crucial. It’s vital for the stability of Indian agriculture to study monsoon levels, particularly in the summer months as it accounts for 85 per cent of the annual rainfall received in the country. The study was published in the June issue of Nature Climate Change. The findings improve the understanding of impact of climate change on rainfall extremes and would help manage risks associated with irrigation and agriculture.  


Research: Observed changes in extreme wet and dry spells during the South Asian summer monsoon season

Research: Analysis of variability and trends of extreme rainfall events over India using 104 years of gridded daily rainfall data

Research: The basic causes of the large-scale deficiency in the South-West Monsoon rainfall over India in 1965 and 1966

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  • When we use truncated data

    When we use truncated data series of rhythmic variation we get misleading conclusions. Three years back, the Central Minister informed to Parliament with reference to question raised in the house. He informed the house rainfall is decreasing. He used the period between 1927 to 1986. This is a part of Sine curve wherein the first 30 year part of All-India Southwest Monsoon rainfall form the above the average part of 60-year cycle. Next 30 years part comes under the below the average part of the 60-year cycle. In the sine curve upper peak [+ peak] to lower peak [- peak] show drastic decreasing trend in the precipitation. If the data period considered was from 1897 to 1956 then it shows drastic increasing trend in the precipitation.

    1 2 3*
    1867-1896 873 mm 2/7 years
    1897-1926 828 mm 7/3 years
    1927-1956 868 mm 2/5 years
    1957-1986 837 mm 10/5 years
    1987-2016 860 mm 2/5 years

    * 1 = 30-year Period; 2 = mean of 30 year period Precipitation (mm); 3 = number of years with < 90%/> 110% [deficit/surplus conditions] of the average precipitation of the data series

    Note 1: the data upto 1994 was taken from the IITM homogenized data series. from 1995 onwards used the data used in the IMD long-range rainfall forecast data set. Thus, there is some discrepancy after 1994. -- Scientific groups have used in the past different data sets in the long-range forecast of precipitation and arrived at different predictions but all failed in the same way.

    Note 2: Up to 1956 the data was recorded in inches and from 1957 onwards it is being recorded in millimeters.

    The research by Stanford used 1951 to 2011 data series. That is it covered 1957 to 2016 period cycle with a shift backward by 6 years. 1987-2010 the average is 860 mm and before that period it is 837 mm and thus it shows rainfall is not decreasing but increased as per the cycle characteristic.

    Suggestion: there is a need that IITM must up date the series following the same procedure as adopted earlier -- who is now retired -- then we get the correct figure of 60-year cyclic pattern.

    Do not use truncated data series.

    Dr. S. Jeevananda Reddy

    Posted by: Anonymous | 6 years ago | Reply