trend. Therefore, it can be inferred from the
analysis that the post-monsoon rainfall was
increasing. Rainfall during winter season has also
shown decreasing trend.
13.4.4 Monthly Rainfall Trends
Behavior of monthly rainfall has been studied for
individual months by the MK test and the results
are presented in Table 13.2. It is interesting to
note that rainfall in June and August showed a
decreasing trend. Similar decreasing trends were
noticed during May and March for rainfall contribution to the annual. Rainfall of March,
September and November showed an increasing
trend. Rainfall during March, July, September,
October and November showed an increasing
trend while remaining months showed no particular significant trend. There was a decline in
rainfall contribution of June to the annual rainfall
over a period of time. Unlike in June, the contribution of rainfall during July is stable though
variations were noticed. In contrast, the contribution of rainfall during September and October
is increasing. As a whole, the percentage rainfall
contribution during the southwest monsoon was
increasing while decreasing during pre-monsoon
and winter season over BHB. The above phenomenon was more significant in recent decades.
However, rainfall during the monsoon season is
stable while instable in remaining months. These
two contrasting phenomena in monthly rainfall
trends are the major concern across the basin.
13.4.5 Decadal Monthly Rainfall:
Short-Term Fluctuations
Since the rainfall series of BHB show annual and
seasonal variations, there is every possibility that
the monthly rainfall over the BHB is also characterized by multi-decade periods of excess
(above-average) and deficient (below-average)
rainfall. The decade-scale variability of monthly
rainfall based on Cramer’s t-statistic for BHB is
depicted in Fig. 13.4. Several interesting features
can be observed. First, the graphs are not exactly
parallel and indicate month-to-month variations
in the rainfall. Second, another remarkable feature that is evident from the plot is a striking
change in monsoon regime at the end of nineth
decade of the twentieth century and a prominent
period of excess rainfall in last decade of the
twentieth century in BHB. Third, the signatures
of southwest monsoon rainfall pattern are less
evident in the month of October. Four, the pattern displayed by Cramer’s t-statistic between
1920 and 1970s is multifaceted and it is not
possible to detect any systematic pattern, as
during the other decades. The recent decadal
change is attributed due to climate change associated with global warming. But clearly more
work is required before any positive conclusion
is drawn. In addition, the sub-period analysis
(Cramer’s test) also revealed a recent increase in
the post-monsoon rainfall series in the last decade. The rainfall of winter months was below the
overall mean during the three decades (1961–
1970, 1971–1980 and 1981–1990). There is one
10-year period, 1921–1930 which showed positive anomaly from the overall mean in winter
rainfall.
13.5 Discussion
The outcome of the study revealed various facts
about the spatial–temporal variation of rainfall in
this basin. The variations observed in different
data series such as mean annual, onset of monsoon, rainfall in the month of winter and in the
monsoon season. The overall characteristics of
the long term and decadal annual as well as
monsoon rainfall over the region are declining in
nature. But in the last decade of twentieth century, the monsoonal rainfall has been increased
(Fig. 13.4). However, it did not explain anything
specific about the recent changes in climatic
conditions. An undisputable fact about large
floods in BHB is that they occur during the
southwest monsoon months and are generated by
heavy to exceptionally heavy rainfall in the
catchment area. However, neither the exceptional
13 Changing Rainfall Patterns and Their Linkage to Floods …
177
analysis that the post-monsoon rainfall was
increasing. Rainfall during winter season has also
shown decreasing trend.
13.4.4 Monthly Rainfall Trends
Behavior of monthly rainfall has been studied for
individual months by the MK test and the results
are presented in Table 13.2. It is interesting to
note that rainfall in June and August showed a
decreasing trend. Similar decreasing trends were
noticed during May and March for rainfall contribution to the annual. Rainfall of March,
September and November showed an increasing
trend. Rainfall during March, July, September,
October and November showed an increasing
trend while remaining months showed no particular significant trend. There was a decline in
rainfall contribution of June to the annual rainfall
over a period of time. Unlike in June, the contribution of rainfall during July is stable though
variations were noticed. In contrast, the contribution of rainfall during September and October
is increasing. As a whole, the percentage rainfall
contribution during the southwest monsoon was
increasing while decreasing during pre-monsoon
and winter season over BHB. The above phenomenon was more significant in recent decades.
However, rainfall during the monsoon season is
stable while instable in remaining months. These
two contrasting phenomena in monthly rainfall
trends are the major concern across the basin.
13.4.5 Decadal Monthly Rainfall:
Short-Term Fluctuations
Since the rainfall series of BHB show annual and
seasonal variations, there is every possibility that
the monthly rainfall over the BHB is also characterized by multi-decade periods of excess
(above-average) and deficient (below-average)
rainfall. The decade-scale variability of monthly
rainfall based on Cramer’s t-statistic for BHB is
depicted in Fig. 13.4. Several interesting features
can be observed. First, the graphs are not exactly
parallel and indicate month-to-month variations
in the rainfall. Second, another remarkable feature that is evident from the plot is a striking
change in monsoon regime at the end of nineth
decade of the twentieth century and a prominent
period of excess rainfall in last decade of the
twentieth century in BHB. Third, the signatures
of southwest monsoon rainfall pattern are less
evident in the month of October. Four, the pattern displayed by Cramer’s t-statistic between
1920 and 1970s is multifaceted and it is not
possible to detect any systematic pattern, as
during the other decades. The recent decadal
change is attributed due to climate change associated with global warming. But clearly more
work is required before any positive conclusion
is drawn. In addition, the sub-period analysis
(Cramer’s test) also revealed a recent increase in
the post-monsoon rainfall series in the last decade. The rainfall of winter months was below the
overall mean during the three decades (1961–
1970, 1971–1980 and 1981–1990). There is one
10-year period, 1921–1930 which showed positive anomaly from the overall mean in winter
rainfall.
13.5 Discussion
The outcome of the study revealed various facts
about the spatial–temporal variation of rainfall in
this basin. The variations observed in different
data series such as mean annual, onset of monsoon, rainfall in the month of winter and in the
monsoon season. The overall characteristics of
the long term and decadal annual as well as
monsoon rainfall over the region are declining in
nature. But in the last decade of twentieth century, the monsoonal rainfall has been increased
(Fig. 13.4). However, it did not explain anything
specific about the recent changes in climatic
conditions. An undisputable fact about large
floods in BHB is that they occur during the
southwest monsoon months and are generated by
heavy to exceptionally heavy rainfall in the
catchment area. However, neither the exceptional
13 Changing Rainfall Patterns and Their Linkage to Floods …
177