Changes in extreme rainfall events and flood risk in India during the last century- A report by the India Meteorological Department

The occurrences of some exceptionally heavy rainfall during the recent years causing flash floods in many areas necessitated the study of long term changes in extreme rainfall over India.

The study includes the analysis of the frequency of rainy days, number of rainy days and heavy rainfall days as well as one-day extreme rainfall and return period analysis in order to observe the impact of climate change on extreme weather events and flood risk. It has been found that frequency of heavy rainfall events are decreasing in major parts of the central and north India while increasing in peninsular India, east and north east India.

The report provides interesting findings that are useful for hydrological planning and disaster managements such as –

• The study suggests that noticeable changes have occurred in the extreme rainfall events that happened in the past century over India.
• The country experienced large spatial variations in annual normal rainy days. Annual normal rainy days varied from 10 over extreme western parts of Rajasthan to the high frequency of 130 days over north eastern parts of the country.
• Both non parametric test and linear trend analysis identified decreasing trends in the frequency of wet days in most parts of the country.
• Trend analysis of frequency of rain days, rainy days and heavy rainfall days shows significant decreasing trends over central and many parts of north India and increasing trends over peninsular India. Also the great desert areas of the country have experienced increased number of wet days.
• Analysis of one-day extreme rainfall series has shown that the intensity of extreme rainfall has increased over Coastal Andhra Pradesh and adjoining areas, Saurashtra and Kutch, Orissa, West Bengal, parts of northeast India, east Rajasthan. Significant decrease both in intensity and frequency of extreme rainfall have been observed over Chhattisgarh, Jharkhand and some parts of north India.
• The flood risk also increased significantly over India. The flood risk was more in the decades 1981-90, 1971-80 and 1991-2000.
• The study indicates that the minimum temperature in the 2050s is expected to rise by 4°C all over India, with a further rise in temperature in the southern peninsula.
• At an all-India level, little change in monsoon rainfall is projected up to the 2050s. There is an overall decrease in the number of rainy days over a major part of the country. This decrease is greater in the western and central parts by more than 15 days. The decreases in the number of rainy days over major parts of the country were also observed in this study. However in the present case for the first time the analysis of frequency of wet days and the intensity of extreme rainfall was done by using maximum network of observing stations. This helps to bring out the smaller scales observed changes in frequency and intensity of rainfall.

Download the report here -