3. Variability and Trends of Extreme Rainfall and Rainstorms

In this chapter, variability and long-term trends of heavy rainfall events and rainstorms over India during the monsoon season (June to September) are documented. For analyzing extreme rainfall events, rain gauge station and gridded data for the period 1901–2010 have been used. For studying rainstorms, another gridded rainfall data set of 1951–2010 also has been used. The trend analyses revealed increasing trends in the frequency of dry days in most parts of the country during the winter, pre-monsoon, and southwest monsoon seasons. Frequency of very light rain and light-to-moderate rain events has decreased significantly over most of the country. Both the station and gridded data have shown significant increasing trends of very heavy to extremely heavy rainfall events over most parts of the country. Over Central India, extreme rainfall events show significant decadal variations which could be related to variations in sea surface temperatures over the tropical oceans. Over the period 1901–2010, heavy rainfall events show an increasing trend of six percent per decade. Further, an analysis is made on rainstorms over North India where majority of rainstorms cause floods over North India. Frequency of rainstorms has shown an increasing trend of 4 rainstorms (50 % increase) in 65 years (1951–2015). Similarly, the duration of rainstorms has shown an increase of about 15 days (80 % increase) during the period 1951–2015, which is also significant. Both the increases are statistically significant at the 99 % confidence level.