Trends and Variability on Regional Scale
Due to the presence high spatial variability of rainfall, significant trends have been observed in annual as well as the seasonal rainfall on subdivision scale. This has been observed in the data for the period 1901-2003 by Guhathakurta and Rajeevan (2008). The analysis of district rainfall data for the period 1901-2011 by Guhathakurta and Saji (2013) revealed variability and trends in monthly and seasonal rainfall of further smaller regions. In order to study the secular variations of regional rainfall, a trend analysis was carried out for the monthly rainfall series of June, July, August and September and also for the four seasons and annual rainfall for all the 36 subdivisions (Figs. 5 and 6).
Figure 5 shows the spatial variation of trends in rainfall for the four monsoon months for the period 1901-2010. In June, eight subdivisions all along the western parts show increasing trends in rainfall while the subdivisions, viz. Kerala, Uttarakhand, Sub-Himalayan West Bengal and Sikkim and all three subdivisions of north-east India showed decreasing trends. In July, six subdivisions, viz. Himachal Pradesh, east Madhya Pradesh, Jharkhand, Arunachal Pradesh, NMMT and Kerala have shown decreasing trends. Major changes are observed in August rainfall where eleven subdivisions have reported significant trends of increasing rainfall. Rainfall over eight subdivisions mostly from eastern, north-eastern and northern parts of the India shows significant trends of decreasing rainfall. Significant decrease was observed over the two subdivisions from north-eastern region, viz. Arunachal Pradesh, Nagaland, Manipur, Mizoram and Tripura in all the four months and the season.
Spatial variations of trend in rainfall for the four seasons and annual time scales are shown in Fig. 6. Winter rainfall decreased significantly in 21 subdivisions while increasing trend has been noticed only over Jammu and Kashmir and sub-Himalayan West Bengal and Sikkim. In the pre-monsoon season, when rainfall is mainly dominated by convective activities, five subdivisions showed decreasing trends while two subdivisions showed increasing trends. Rainfall increased significantly in eight subdivisions, viz. Madhya Maharashtra, Saurashtra and Kutch, south interior Karnataka, coastal Karnataka, Konkan and Goa, coastal Andhra Pradesh, Lakshadweep and Gangetic West Bengal during southwest monsoon. Ten subdivisions viz. Chhattisgarh, Jharkhand, Uttarakhand, Himachal Pradesh, Arunachal Pradesh, NMMT (Nagaland, Manipur, Mizoram and Tripura), sub-Himalayan West Bengal, Kerala, east Uttar Pradesh and east Madhya Pradesh have shown significant decreasing trends in monsoon rainfall. No significant change in post-monsoon rainfall has been observed over any subdivisions except over Arunachal Pradesh. The annual pattern is almost the same as the monsoon pattern where eight subdivisions (out of which five in peninsular India) reported increasing trend and also eight (mostly from central and eastern parts) subdivisions reported decreasing trends.
Fig. 5 Trends in the monthly rainfall (June, July, August and September) for the 36 meteorological subdivisions of India for the period 1901-2010. Trends shown here are qualitative in nature