64
3.4.2 Seasonal Rainfall Trend
3.4.2.1 Pre-monsoon Rainfall Trend
The ITA slope of rainfall in pre-monsoon season is presented in Table 3.3 and
Fig. 3.4a and the results obtained from MK/mMK and Sen’s slope are presented in
Table 3.4, Fig. 3.4i and e. Although the obtained results showed few discrepancies
between the methods, the results of all the methods showed that rainfall in most of
the stations (total number stations = 571) had an increasing tendency in Pre-monsoon
season (Fig. 3.4). ITA showed that increasing trends in rainfall in 74% stations and
MK/mMK also showed similar results for 67% of stations. Furthermore, Z-statistics
of MK/mMK test revealed that identified increasing trends were statistically significant at different confidence intervals (Table 3.4) for almost half (43.86%) of the
stations, and only 27.65% of stations had significant decreasing trends. The spatial
distribution of ITA slope is shown in Fig. 3.4a. The whole study area showed an
increasing trend in rainfall except south-western and some pocket areas which are
distributed discretely over the study area (Fig. 3.4a). This increasing trend may
amplify the vulnerability of Landslides and flash flood in mountain region, riverbank erosion and waterlogging in Gangetic West Bengal, coastal Orissa and few
pocket area of Bangladesh. The highest increasing rate (6.32 mm/year) was found
in Teknaf station in Bangladesh and the highest decreasing rate (−8.83 mm/year)
was found in Balaghat station under the state of Madhya Pradesh in India. Increasing
sea surface temperature (SST) due to global warming is expected to be one of the
reasons for such significant increasing trend of rainfall in these parts (Shahid and
Khairulmaini 2009). A positive correlation was noticed by Salahuddin et al. (2006)
between sea surface temperature (SST) and the rainfall of Bangladesh. However,
Sen’s slope analysis revealed that amount of rainfall decreases on average at a rate
of −1.46 mm/year over the study area in pre-monsoon. Similar to significant trend
(Z-statistics) distribution, the higher magnitude of changes (>4 mm/year) were
found in the northern, north-eastern and south-western parts of the area
(Fig. 3.4e and i).
3.4.2.2 Monsoon Rainfall Trend
The results of ITA slope of rainfall in the monsoon season showed the dominance of
decreasing trends throughout the study area during 1951–2015 (Table 3.3). The
obtained ITA and Z statistics test indicate almost similar results for decreasing
(73%) and increasing (26%) tendency of rainfall in monsoon season (Table 3.3 and
3.4). The significant decreasing trends (MK/mMK) of rainfall were mostly found in
the south-western, central and extreme eastern part of the study area (Fig. 3.4b),
whereas it was distributed over whole study except south-eastern and few pocket
region for ITA trend (Fig. 3.4b). The significant decreasing trend in the extreme
eastern and western part of the study area is simplified as the influence of climate
T. Mandal et al.
3.4.2 Seasonal Rainfall Trend
3.4.2.1 Pre-monsoon Rainfall Trend
The ITA slope of rainfall in pre-monsoon season is presented in Table 3.3 and
Fig. 3.4a and the results obtained from MK/mMK and Sen’s slope are presented in
Table 3.4, Fig. 3.4i and e. Although the obtained results showed few discrepancies
between the methods, the results of all the methods showed that rainfall in most of
the stations (total number stations = 571) had an increasing tendency in Pre-monsoon
season (Fig. 3.4). ITA showed that increasing trends in rainfall in 74% stations and
MK/mMK also showed similar results for 67% of stations. Furthermore, Z-statistics
of MK/mMK test revealed that identified increasing trends were statistically significant at different confidence intervals (Table 3.4) for almost half (43.86%) of the
stations, and only 27.65% of stations had significant decreasing trends. The spatial
distribution of ITA slope is shown in Fig. 3.4a. The whole study area showed an
increasing trend in rainfall except south-western and some pocket areas which are
distributed discretely over the study area (Fig. 3.4a). This increasing trend may
amplify the vulnerability of Landslides and flash flood in mountain region, riverbank erosion and waterlogging in Gangetic West Bengal, coastal Orissa and few
pocket area of Bangladesh. The highest increasing rate (6.32 mm/year) was found
in Teknaf station in Bangladesh and the highest decreasing rate (−8.83 mm/year)
was found in Balaghat station under the state of Madhya Pradesh in India. Increasing
sea surface temperature (SST) due to global warming is expected to be one of the
reasons for such significant increasing trend of rainfall in these parts (Shahid and
Khairulmaini 2009). A positive correlation was noticed by Salahuddin et al. (2006)
between sea surface temperature (SST) and the rainfall of Bangladesh. However,
Sen’s slope analysis revealed that amount of rainfall decreases on average at a rate
of −1.46 mm/year over the study area in pre-monsoon. Similar to significant trend
(Z-statistics) distribution, the higher magnitude of changes (>4 mm/year) were
found in the northern, north-eastern and south-western parts of the area
(Fig. 3.4e and i).
3.4.2.2 Monsoon Rainfall Trend
The results of ITA slope of rainfall in the monsoon season showed the dominance of
decreasing trends throughout the study area during 1951–2015 (Table 3.3). The
obtained ITA and Z statistics test indicate almost similar results for decreasing
(73%) and increasing (26%) tendency of rainfall in monsoon season (Table 3.3 and
3.4). The significant decreasing trends (MK/mMK) of rainfall were mostly found in
the south-western, central and extreme eastern part of the study area (Fig. 3.4b),
whereas it was distributed over whole study except south-eastern and few pocket
region for ITA trend (Fig. 3.4b). The significant decreasing trend in the extreme
eastern and western part of the study area is simplified as the influence of climate
T. Mandal et al.