List of Tables
Chapter 2 Review of Literature
2.3 Changes in rainfall
Detection of trends in rainfall time series attracts growing attention due to widespread concerns that the human induced increase in greenhouse gases is altering the Earth’s climate and ecosystems (Letcher 2009). The increased atmospheric moisture content associated with warming (Trenberth et al. 2005) might be expected to lead to increased global mean precipitation. Information on spatial and temporal variations of rainfall is important in understanding the hydrological balance on a global/regional scale. Rainfall variability and changes are predicted to have a major impact on the water and agricultural sectors in the Asia–Pacific region (Cruz et al. 2007).
Global annual land mean precipitation showed a small, but uncertain, upward trend over the 20th century of approximately 1.1 mm per decade (IPCC 2007a). Though the trend was statistically insignificant, the pattern of precipitation change was spatially and seasonally variable and characterized by large inter-decadal variability. Precipitation has very likely
been increased during the 20th century by 5% to 10% over most mid– and high latitudes of Northern Hemisphere (NH) continents, but in contrast rainfall has likely decreased by 3%
on average over much of the subtropical land areas (IPCC 2007a). On the regional spatial scale, it has become significantly wetter in eastern parts of North and South America, northern Europe, and northern and central Asia, but drier in the Sahel, the Mediterranean, southern Africa, and parts of southern Asia (Dai et al. 2004). On the temporal scale, global annual land precipitation had increased overall before the 1950s, declined until the early 1990s, and has since then recovered (Peterson and Vose 1997; Mitchell and Jones 2005).
As to the attribution of precipitation change, anthropogenic factors appear to have influenced the latitudinal pattern of land precipitation and heavy precipitation over the twentieth century, and volcanic forcing was also detectable as having a role in global mean land precipitation (Zhang et al. 2007).
The changes in rainfall on sub-regions can be far more dramatic than changes in global and regional averages. Available data suggest that there may be different results even on the land regions. For example, decreasing trends of annual rainfall and seasonal changes were already noticed in Indonesia (WWF 2007), Italy (Cislaghi et al. 2005), South–Eastern Australia (Murphy and Timbal 2008), Sri Lanka (Zubair et al. 2008), Turkey (Partal and Kucuk 2006) and UK (Palmer and Raisanen 2002). On the other hand, trends in European annual precipitation revealed an increase in rainfall in northern Europe by 10% to 40% in the twentieth century, but little change or drying in southern Europe (Parry 2000).
Precipitation in the Great Plains of the United States of America also showed significant increase since the late 60’s – the last two decades being the wettest of the 20th century (Garbrecht and Rossel 2002). The Brazilian Amazon basin precipitation records revealed a shift around 1975, a downward trend in the northern area and upward trend in the southern part (Marengo 1999). Norway (Benestad and Haugen 2007), Spain (Mosmann et al. 2004) and the United States (Groisman et al. 2001) showed increasing trends of rainfall during summer and for Canada (Groleau et al. 2007) in winter season. Vincent et al. (2011) found significant decreasing trends in annual total rainfall in the countries of Western Indian Ocean during 1961–2008. In the Tibetan plateau, rainfall has increased in the eastern and central parts, while in the western part, it exhibited a decreasing trend during 1961–2001
(Xu et al. 2008). A significant increase of annual and pre-monsoon rainfall was also observed in Bangladesh during 1958–2007 (Shahid 2012).
In India, long-term trends of southwest monsoon rainfall for the country as a whole as well as for smaller spatial regions were studied by several researchers. Most of these studies during the last four decades had pointed out that monsoon rainfall is trendless and is often random in nature over a long period of time, particularly on an all-India scale (Mooley and Parthasarthy 1984; Thapliyal and Kulshrestha 1991; Sinha Ray and De 2003; Kripalani et al. 2003; Guhathakutra and Rajeevan 2008; Attri and Tyagi 2010). Though the all-India annual and monsoon rainfall during the last 100 years exhibited no significant trend, significant long-term rainfall changes were identified at different spatial and temporal scales in some studies (Rupa Kumar et al. 2002; Sinha Ray and Srivastava 2000; Dash et al. 2007; Kumar et al. 2010; Guhathakurta et al. 2011). Analysis of rainfall data for the period 1871–2002 indicated a decreasing trend in monsoon rainfall and increasing trend in the pre-monsoon and post-monsoon seasons (Dash et al. 2007). During recent decades (1979–2006), the monsoon rainfall over India was less by 4.5% compared to the period 1949–1978 (Ranade et al. 2008). Summer monsoon rainfall showed decreasing trend over northeast India, east Madhya Pradesh and adjoining areas, and parts of Gujarat and Kerala (Rupa Kumar et al. 2002; Dash et al. 2007). Analyzing Indian summer monsoon rainfall data during 1871–2010, Kulkarni (2012) observed decreasing tendency of rainfall during the last three decades of the 20th century. During 1976–2004, the deficit (excess) monsoons have become more (less) frequent due to weakening of monsoon circulation compared to 1901–1975. In a study, Kothawale and Kulkarni (2012) showed that in the recent warming period 1971–2010, monsoon rainfall of North India (north of 20°N) showed negative trend while South India (south of 20°N) showed positive trend. The occurrence of excess years were more over intense warm area (South India) than the mild warm area (North India) indicating enhancement of monsoon rainfall in a warming environment.
Indian summer monsoon rainfall (ISMR) showed epochal variability of approximately 30 years since 1870s (Kripalani and Kulkarni 1997; Kripalani et al. 2003). Kripalani et al.
(2003) examined the inter-annual and decadal variability in summer monsoon rainfall over India by using observed data for a 131-year period (1871–2001). They found random
fluctuations in annual rainfall and distinct alternate epochs (lasting approximately three decades) of above- and below-normal decadal rainfall. Earlier, Kripalani and Kulkarni (1997) characterized the ISMR series used by Kripalani et al. (2003) as below normal epochs (1901–1930 and 1961–1990) and the above normal epochs (1871–1900 and 1931–
1960) of approximately 30 years each. If this epochal variability is assumed to be continued, ISMR should have entered into the above normal epoch around 1990, which did not happen. After 1990, the epochal variability of ISMR has been disturbed due to weakening of low level monsoon circulation (Kulkarni 2012).