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Result and Analysis

Changes in the Frequency of Rainfall Events of Different Intensities in States

Monthly/seasonal/annual frequency of events in six categories, viz. wet days (24-h rainfall > 0.1 mm), light-to-moderate rain (2.5 mm < 24-h rainfall <

64.4 mm), heavy rain (64.5 mm < 24-h rainfall < 124.4 mm), very heavy rain (124.5 mm < 24-h rainfall < 244.4 mm), and extremely heavy (24-h rainfall > 244.5 mm), for each of the states and each year for the period 1901-2010 are computed. To remove the effect of missing data or variable number of stations in each state, the frequencies in each category are calculated in percentage of total frequency of all the events in the state.

Figure 1 gives the trends in the percentage of frequencies of all the six indices for the states during the winter season (January-February). The trends have been calculated for the period 1901-2010. Frequencies of wet days, very light, and light-to-moderate rain have decreased over most of the states except a few in South India and Jammu and Kashmir. Frequencies of very heavy rainfall have increased over Jammu and Kashmir and Uttaranchal during the winter season. Frequencies of wet days have decreased in many states over Central and Western India (Fig. 2) during the pre-monsoon season. Almost the similar pattern is observed in the frequencies of very light rain and light-to-moderate rain events. Increase in heavy/very heavy/extremely heavy rainfall events is observed over Jammu and Kashmir, West Bengal, Bihar, Tamil Nadu, Uttarakhand, Assam, and Manipur.

Trends in the frequencies of different rainfall events over the states during the winter season (January-February)

Fig. 1 Trends in the frequencies of different rainfall events over the states during the winter season (January-February)

Trends in the frequencies of different rainfall events over the states during the pre-monsoon season (March-May)

Fig. 2 Trends in the frequencies of different rainfall events over the states during the pre-monsoon season (March-May)

Figure 3 gives the trends in the percentage of the frequencies of all the six events during the southwest monsoon season (June-September). The general inference is the decrease in frequencies of lower intensities and increase in higher intensities (like heavy to very heavy rainfall) during the southwest monsoon season. Frequencies of very light-to-moderate rain have decreased in most of the states in Central India, Peninsular and northeast India. Significant increase in heavy rainfall event is observed over the nine states (West Bengal, Tripura, Haryana, Orissa, Andhra Pradesh, Telangana, Karnataka, Goa, and Tamil Nadu). Frequency of very heavy rainfall events has increased over West Bengal, Assam, Tripura, Sikkim, Punjab, Jammu and Kashmir, Gujarat, Maharashtra, Orissa, Andhra Pradesh, Telangana, Karnataka, Goa, and Tamil Nadu and extreme heavy rainfall events over West Bengal, Assam, Tripura, Uttar Pradesh, Punjab, Jammu and Kashmir, Maharashtra, Orissa, Andhra Pradesh, Karnataka, Goa, and Tamil Nadu.

During the post-monsoon season, no much changes are observed in the states except Jammu and Kashmir (increasing), West Bengal (increasing), and Karnataka and Arunachal Pradesh (decreasing) (Fig. 4). However, significant decreasing trend is noticed in the states such as Karnataka (both very light and light-to-moderate rain), Telengana (very light rain), and Kerala and Tamil Nadu (light-to-moderate rain) which are affected by the northeast monsoon during this season.

The annual pattern (Fig. 5) is almost similar to that of the southwest monsoon season. Frequencies of very light, light-to-moderate and wet days have decreased in most of the states. Frequencies of dry days have also increased significantly during

Trends in the frequencies of different rainfall events over the states during the southwest monsoon season (June-September)

Fig. 3 Trends in the frequencies of different rainfall events over the states during the southwest monsoon season (June-September)

Trends in the frequencies of different rainfall events over the states during the postmonsoon season (October-December)

Fig. 4 Trends in the frequencies of different rainfall events over the states during the postmonsoon season (October-December)

Trends in the annual frequencies of different rainfall events over the states

Fig. 5 Trends in the annual frequencies of different rainfall events over the states (January- December) the period 1910-2010. Significant increase in heavy rainfall event is observed over the eight states (West Bengal, Tripura, Manipur, Andhra Pradesh, Telengana, Karnataka, Goa, and Orissa. Frequency of very heavy events has increased over West Bengal, Tripura, Manipur, Sikkim, Andhra Pradesh, Telengana, Karnataka, Goa, Jammu and Kashmir, and Orissa and extreme rainfall events over West Bengal, Assam, Punjab, Jammu and Kashmir, Chhattisgarh, Goa, and Telengana.

Figure 6 gives the decadal variability of annual frequencies of rainfall events of different intensities. Trend analysis shows significant increase in the frequency of dry days during the period 1901-2010 while significant decrease in the frequencies of very light and light-to-moderate rainfall events. No significant changes in the frequency of heavy rainfall events are observed. However, significant increase in the annual frequency of very heavy and extremely heavy rainfall events is observed over the country as a whole. Decadal variability shows the decade 1981-1990 has the highest percentage of extremely heavy rainfall event, while the decade 19912000 has the highest percentage frequencies for both heavy and very heavy rainfall events among all the decades. * S

Decadal variability of annual frequencies of different rainfall events over India

Fig. 6 Decadal variability of annual frequencies of different rainfall events over India

 
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