10% decline in decadal monsoon rainfall across 11% of sub-districts in Indo-Gangetic plains, North-East, Himalayas

A monsoon variability assessment across a 40-year-period taken up by the Delhi-based Council on Energy, Environment and Water (CEEW) observes that decreasing rainfall trends during June-September (South-West monsoon) are not statistically significant over 40 years but nearly 11 per cent of sub-districts (tehsils, talukas, mandals, circles and sub-divisions) in the country across the rain-fed Indo-Gangetic plain, North-East India and the Himalayan region have witnessed a 10 per cent decrease in the past decade (2012-22) compared to the climatic baseline (1982– 2011).

June-September (South-West monsoon) rainfall trends

Agriculturally important

These sub-districts contribute to more than half of the country’s agricultural production as per the assessment, titled Decoding India’s Changing Monsoon Patterns: A Tehsil-level Assessment, and authored by Shravan Prabhu and Vishwas Chitale of CEEW. Of the tehsils that saw a decrease, an estimated 68 per cent saw monsoon rainfall take the hit in all months, while 87 per cent showed a decline during the crucial months of June and July. Nearly 64 per cent of all tehsils in the country experienced an increase in frequency of heavy rainfall days by 1-15 per year in the past decade. This pattern is prominent in States with the highest GDP – Maharashtra, Tamil Nadu, Gujarat and Karnataka.

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Explaining the context, Vishwas Chitale, also Senior Programme Lead, CEEW, said existing literature predicts intensifying monsoon seasons with increased rainfall in core monsoon regions in both the medium term (up to 2050) and long term (up to 2100) under different representative concentration pathways (RCPs). “Yet, capturing short-term granular spatial variabilities has been a challenge, especially beyond the district level. Existing assessments often focus on prolonged long-term trends at coarse resolutions and have not been able to account for intricate nuances within seasons spanning across months and days, or even variations within a single district.”

Vishwas Chitale, Senior Programme Lead, CEEW

‘First ever’ assessment

The CEEW analysis sought to fill this gap in enhanced analyses of short-term variabilities and to untangle intricacies of the South-West and North-East monsoon. Chitale calls this as the first sub-district-level monsoon variability assessment focusing on trends in the past four decades (1982–2022), with a specific emphasis on quantifying changes in monsoonal rainfall patterns in the past decade based on statistical criteria and indices endorsed by the World Meteorological Organisation (WMO) and the India Meteorological Department (IMD).

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Excess rain years

In the past 40 years, the country experienced 29 ‘normal’, nine ‘above-normal’, and three ‘below-normal’ monsoon years during June-September. An estimated 30 per cent of districts witnessed a high number of deficient rainfall years and 38 per cent witnessed a high number of excessive rainfall years. At least 23 per cent, including New Delhi, Bengaluru, Nilgiris, Jaipur, Kachchh and Indore, witnessed both a high number of deficient as well as excessive rainfall years. At an even more granular level, 55 per cent of tehsils witnessed a 10 per cent increase rain during the past decade compared to the climatic baseline. A statistically significant increase was observed in traditionally drier tehsils of Rajasthan, Gujarat, central Maharashtra and parts of Tamil Nadu.

N-E monsoon trends

October-December (North-East monsoon) rainfall trends

Meanwhile, North-East monsoon rainfall has increased by more than 10 per cent in 2012-2022 in approximately 80 per cent of tehsils in Tamil Nadu; 44 per cent in Telangana; and 39 per cent in Andhra Pradesh. A statistically significant increasing trend has been visible in the October-December rainfall along the tehsils of Maharashtra and Goa on the West Coast and Odisha and West Bengal on the East Coast, which could partially be attributed to the cyclonic activities in the Arabian Sea and Bay of Bengal. Analysis of monthly variability indicated that nearly 48 per cent of tehsils in the country saw an increased rainfall in October by more than 10 per cent, likely due to delayed withdrawal of the preceding South-West monsoon.

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