Variability and Trends of Extreme Rainfall and Rainstorms
Pulak Guhathakurta, D.S. Pai and M.N. Rajeevan
During the southwest monsoon season (June-September), low-pressure systems like monsoon depressions form over the Bay of Bengal and move across northern parts of India, causing widespread rainfall activity over the central region and the west coast. At some occasions, sustained heavy rainfall activity in 2-3 days leads to large-scale floods over different river basins. Generally, the worst floods occur in the Indo-Gangetic Plains. The other equally worst flood-hit region is the northeastern parts of the country. Floods occur in the Central India region by rivers such as the Mahanadi, the Naramada, the Godavari, and the Krishna. Floods cause several deaths and economic losses every year. Damages from floods take several forms, including destruction of footbridges that often provide the only link between remote villages, demolition of irrigation diversions, and damages to flood plain agricultural land by erosion and sedimentation. Warmer climates, owing to increased water vapor, lead to more intense precipitation events and therefore increase risks of floods.
Given the population density and economic conditions, occurrence of extreme rainfall events and associated floods over India are viewed very seriously. The heavy rainfall event over Mumbai on July 26, 2005 (Chang et al. 2009) and the recent floods over the hilly regions of northern Indian states of Uttarakhand (Joseph et al. 2014) and Jammu and Kashmir in September 2014 are just few examples of extreme hydrometeorological events which caused devastating damages. Dash et al. (2009)
P. Guhathakurta • D.S. Pai
India Meteorological Department, Earth System Science Organization (ESSO), Shivajinagar, Pune 411005, India
M.N. Rajeevan (H)
Earth System Science Organization (ESSO), Ministry of Earth Sciences,
© Springer Science+Business Media Singapore 2017
M.N. Rajeevan and S. Nayak (eds.), Observed Climate Variability and Change Over the Indian Region, Springer Geology, DOI 10.1007/978-981-10-2531-0_3
examined possible changes in the frequency of rain events in India for the period 1951-2004. The study suggested that long spells show a significant decreasing trend over India as a whole while short and dry spells indicate an increasing tendency. They attributed these changes to the weakening of the summer monsoon circulation over India. Dash et al. (2011) examined the changes in short and long spells of rainfall during the summer monsoon season (June to September) over India. The trend analysis suggested that short spell (less than 4 days) rain events with heavy intensity have increased over India as a whole. Recently, Guhathakurta et al. (2014) using the station data for the period 1901-2011 have shown significant decreasing trends in the frequency of moderate rain (24-h rainfall between 5 and 10 cm), no significant change in the frequency of heavy rainfall events (24-h rainfall greater than 10 cm), and significant increasing trend in the frequency of very heavy rainfall events (24-h rainfall greater than 15 cm) over the monsoon core region of India.
Revadekar and Preethi (2012) analyzed extreme precipitation events over India using the 1° x 1° gridded rainfall data over India (Rajeevan et al. 2006) and their relationship with agricultural output. They found that extreme precipitation events have an adverse effect on agricultural output over India. There are many other studies (Rakhecha and Pisharoty 1996; Rakhecha and Soman 1994; Stephenson et al. 1999; Sen Roy and Balling 2004; Rakhecha and Soman 1994; Guhathakurta et al. 2011) addressing heavy rainfall events during the southwest monsoon season. Sen Roy and Balling (2004) have considered parameters such as total annual precipitation, 5-day total precipitation, and 30-day total precipitations in their extreme rainfall analysis. In this chapter, we discuss the observed variability and long-term changes in heavy rainfall events and rainstorms over India.