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districts during the mid 1980s. To compare the changes in the location and
shape parameters of the rainfall distribution, the complete time series was
divided into two sub-series, i.e., Sub-series 1 (1960-1984) and Sub-series 2
(1985-2003). The basic statistics of annual rainfall presented in Table 11.1
show that an increase in average for the period 1985-2003 is associated with
an increase in the variability (standard deviation). This indicates that the wet
period is more prone to uncertainty compared to the corresponding dry period.
Studying the changes in percentiles and quantiles, however, is of interest to
understand the nature of extreme climatic events, and also has greater societal
impacts (Beniston et al., 2004; Ferro et al., 2005). The minimum and maximum
annual rainfall have moved towards opposite direction in the recent period in
comparison to the earlier period suggesting that both drought and flood have
become more severe. The negative value of the Yule-Kendall skewness ( S k )
was observed largely in the coastal zone and the northern plateau of Orissa
(Table 11.1), which indicates that the wet years have outnumbered the dry
years and a few unusually low rainfall years have influenced the shape of the
distribution. However, the Eastern Ghat region and part of the central table
land of the state have experienced more dry years and a few unusually high
rainfall years as evident from the positive S k .
The PPCC test of normality indicated (not shown) that the annual rainfall
for the whole time series is nearly normally distributed for most of the districts.
Hence, change in origin of the annual rainfall for different sub-periods was
examined using the t -test. Furthermore, the change in the linear relationship
between the sub-periods was tested using the Chow's F -test. Results of t -test
and F -test show that neither the shift in origin (average) nor the structural
changes have taken place for the sub-series of annual rainfall as both the
observed t- and F -statistics are less than their respective critical values. In
most of the districts, annual rainfall exhibited negative linear relationship
with the year for the sub-series indicating a decreasing trend. However, the
positive linear relationship between annual rainfall and year was observed for
most of the districts when the complete time series was considered. This
indicates that the severe anomalous years during second sub-series have
influenced the overall trend. In India, an increasing trend of extreme rainfall
events has been reported (Sen Roy and Balling Jr, 2004; Goswami et al.,
2006). The number of raining days, however, has decreased. Similarly, Liu et
al. (2005) observed that the increasing proportion of rainfall in China has been
contributed by the heavy rainfall events, and also noted a decreasing trend of
light rainfall events.
11.4.2 Trends in Seasonal Rainfall Time Series
The monsoon season comprising June, July, August and September months,
contributes about 80% of the mean annual rainfall of Orissa. Rainfall from the
non-monsoon months has been clubbed into pre-monsoon season (February,
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