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March, April and May) and post-monsoon season (October, November,
December and January), as the non-monsoon months receive no rainfall or
very negligible rainfall. The descriptive statistics of the seasonal rainfall for
the study period includes: pre-monsoon season (mean 129 mm; standard
deviation 99 mm), June (mean 199 mm; standard deviation 96 mm), July
(mean 307 mm; standard deviation 110 mm), August (mean 334 mm; standard
deviation 116 mm), September (mean 220 mm; standard deviation 92 mm),
and post-monsoon season (mean 136 mm; standard deviation 119 mm),
respectively. The long-term normal rainfall for the monsoon season are 213
mm, 351 mm, 335 mm, 236 mm in June, July, August and September,
respectively. This indicates a reduction in monsoon season rainfall for the
study period. The seasonal hydroclimatic variables are not normally distributed
in this study, and therefore, the nonparametric statistical tests were employed
for trend identification and quantification. Both the Mann-Kendall (MK) and
the Spearman
rho
(SR)
tests were applied to the seasonal rainfall in two time
frames, i.e. 1985-2003 and 1960-2003. Before application of the Mann-Kendall
test, the presence of significant serial correlation was tested and the pre-
whitening was done.
Figure 11.3 shows the cumulative frequency distribution of MK and SR
trend test results of 78 season-stations rainfall time series (six seasons for
each of the 13 stations). The positive trend indicates wetting trend and the
negative trend indicates drying trend. Under the null hypothesis of no trend,
the frequency curve should have been around the axis resulting zero mean
trend. Further, under the assumption that both the drying and the wetting
trends have equal distribution, the frequency curve should divide the axis into
two halves with equal proportions of area above and below the axis. For the
time frame 1985-2003, 60% and 62% of the season-stations have experienced
the drying trends based on the MK and SR tests, respectively [Fig. 11.3(a)].
However, no significant trend was observed except one for the SR test. The
mean observed test-statistic value of MK test is -0.2 (standard deviation
0.88), and the mean observed test-statistic value of SR test is -0.20 (standard
deviation 0.79). However, for the time frame 1960-2003 [Fig. 11.3(b)], 44%
of the season-stations have experienced the drying trends based on the MK
and SR tests. The mean observed test-statistic value of MK test is 0.26 (standard
deviation 1.14), and the mean observed test-statistic value of SR test is 0.26
(standard deviation 1.12). To illustrate the seasonal distribution of the rainfall
trend, the MK test results were used. Figure 11.3(a
)
shows that the negative
drying trends are more conspicuous in the monsoon and the post-monsoon
seasons for the time frame 1985-2003. However, for the time frame 1960-
2003, positive (wetting) rainfall trends have been more in the pre-monsoon,
June, August seasons. The results of this study are in agreement with findings
of several other studies conducted in the study area or other parts of India
(e.g., Mohapatra and Mohanty, 2004; Ramanathan et al., 2005; Chung and
Ramanathan, 2006; Goswami et al., 2006; Dash et al., 2007; Ramesh and
Goswami, 2007).
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