Chapter 3: Snow cover area variation study of the Brahmaputra River Basin and
3.3 Results and Discussions
3.3.1 Variation of snow cover area with respect to temperature
The snow cover maps for January, April, July and October from 2002 to 2015 have been prepared in the Erdas Imagine software using the methodology discussed above. Fig.3.5 shows an example of the final snow cover map created using the presented methodology. In the image, the white pixels show the snow cover area while black pixels show the non-snow area. All the snow cover area maps from 2002 to 2015 for January, April, July and October are shown in Fig.3.6, 3.7, 3.8 and 3.9 respectively.The total snow cover area for each selected month from 2002 to 2015 along with the temperature of HadCM3 for A2 scenario are given in Table 3.2. It is observed from the table that maximum snow cover area is found either in the month of January or in the month of April.
Fig.3.5: Map showing snow cover area of the Brahmaputra River Basin
Table 3.2: Snow Cover Area and temperature for different months
Year
January April July October
Snow cover area (sq.km) Temperature (˚C) Snow cover area (sq.km) Temperature (˚C) Snow cover area (sq.km) Temperature (˚C) Snow cover area (sq.km) Temperature (˚C)
2002 30474 -14.92 114588 -2.70 14536 7.91 84551 -0.67 2003 98414 -12.26 99972 -1.62 25851 8.79 21069 -1.05 2004 64472 -14.22 56236 -1.09 23695 7.20 63894 0.93 2005 74004 -11.66 83658 -0.92 11262 7.63 28355 -0.28 2006 37285 -13.87 80162 -3.81 12678 8.85 16157 -2.28 2007 49892 -14.43 87833 0.81 7881 8.16 12347 1.12 2008 29822 -10.20 92263 0.16 15540 7.84 32572 -2.00 2009 112736 -15.55 89539 -1.74 14328 7.75 25427 1.38 2010 66618 -11.43 68848 -0.03 12401 7.96 30162 -3.25 2011 62421 -12.88 74142 -1.69 13038 7.91 13129 1.43 2012 138555 -11.65 89930 -0.82 12236 8.24 20526 0.25 2013 126798 -15.70 52121 0.43 9088 7.33 16915 -1.06 2014 112355 -14.71 50245 -2.88 4503 8.22 9180 0.57 2015 102833 -13.56 51137 -2.13 6584 8.97 9982 0.04
2002 2003 2004
2005 2006 2007
2008 2009 2010
Fig.3.6: Snow Cover Maps of the Brahmaputra basin for January from 2002 to 2015
Fig.3.7: Snow Cover Maps of the Brahmaputra basin for April from 2002 to 2015
2002 2004
2005 2006 2007
2008 2009 2010
2011
2002
2012 2013
2014 2015
2003
2014 2015
2013 2012
2011
Fig.3.8: Snow Cover Maps of the Brahmaputra basin for July from 2002 to 2015
2002 2004
2005 2006
2003
2007
2008 2009 2010
2011 2012
2013
2014 2015
2002
2005
2004
2006 2007
2003
Fig.3.9: Snow Cover Maps of the Brahmaputra basin for October from 2002 to 2015
From Table 3.2, it is observed that the maximum snow cover area has been observed in 2012 for the month of January with a value of 138555sq.km. and the minimum has been observed in 2014 for the month of July with a value of 4503sq.km. The maximum temperature for the entire period from 2002 to 2015 has been observed in 2015 for the month of July (8.97˚C) and the minimum has been observed in 2013for the month of January (-15.70˚C). The increasing and decreasing trend of snow cover area with respect to the change in temperature for all the four months from 2002 to 2015 are shown in Fig.3.10 [(a), (b), (c) and (d)]. The results clearly displayedthat except for the month of January,the values of temperature taken from the GCM (HadCM3 A2 scenario) for the aforesaid months are increasing gradually from 2002 to 2015 which can be attributed to the fact of global warming. While establishing a relationship between the snow cover area and temperature data, it has been observed that for the month of January, the snow cover area shows an increasing trend with respect to the decreasing trend of temperature. The minimum snow cover during this period was in the year 2008 with a value of 29822 sq. km. and the maximum was in the year 2012 with a value of 138555sq.km. While the minimum temperature was in the year 2013 (-15.70˚C) and the maximum was in the year 2008 (-10.20˚C). For the entire study period from 2002 to 2015, the effect of temperature on snow melt was found to be the maximum in 2008. For the other three months i.e. April, July and October, the results are plotted in Fig. 3.10(b), (c) and (d).
2008 2009 2010
2011
2002
2012 2013
2014 2015
The graphs show that snow cover area for these three months decreases with increase in temperature. Maximum changes have been observed in the month of October. For the month of April, the minimum snow cover was observed in the year 2014 (50246 sq. km.) and the maximum was observed in the year 2002 (114588sq. km.). For July, the minimum was observed in the year 2014 (4503sq. km.) and the maximum was observed in the year 2003 (25851sq. km.). Again, for the month of October, the minimum snow cover area was found to be 9180 sq. km. in the year 2014 while the maximum was found to be 84551sq. km. in the year 2002. In case of temperature, the minimum in the month of April was observed in the year 2006(-3.81 ˚C) and the maximum was observed in the year 2007 (0.81 ˚C). For July, the minimum and maximum temperatures were observed in 2004 (7.20˚C) and 2015 (8.97˚C) respectively. On the other hand, for the month of October, the minimum temperature was observed in the year 2010 (-3.25˚C) and the maximum was observed in the year 2011 (1.43˚C).
Fig.3.10: Trend showing Variation of Snow Cover Area w.r.t temperature for the month of (a)January, (b) April (c) July and (d) October from 2002 to 2015
Fig. 3.11 shows the percent change in snow cover area from 2003 to 2015 with respect to the snow cover area in 2002 for the months January, April, July and October.
Fig.3.11: Percent change in snow cover area from 2003 to 2015 w.r.t. snow cover area in 2002 for (a) January, (b) April, (c) July and (d) October for the Brahmaputra
River basin
It is observed from the above figure that for the month of January, except for 2008 where the snow cover area was less than the snow cover area in 2002, for all other years, the snow cover area increases with respect to the snow cover area in 2002. The maximum increase was observed in 2012 (354.68%) while the minimum increase was observed in 2006 (22.35%). In 2008, the snow cover area decreased by 2.14% with respect to that in 2002.
For the month of April, for each year from 2003 to 2015, the snow cover area was observed to be less than the snow cover area in 2002. The maximum decease has been observed in
2014 with percent decrease value of 56.15% compared to the snow cover area in 2002. The minimum decrease has been observed in 2003 (12.75%). The snow cover area also decreased significantly in the years 2004, 2013 and 2015 by 50.92%, 54.14% and 55.37% respectively.
For July, the snow cover area were more in 2003, 2004 and 2008 compared to that in 2002. In 2003 and 2004 it increased by 77.84% and 68% respectively. For other years, the snow cover areas were less compared to the snow cover area in 2002. The maximum percent decrease in snow cover area was in 2014, where it decreased by 69.02% with respect to 2002 and the minimum percent decrease was in 2009 where snow cover area decreased by 1.43%
compared to that in 2002.
Compared to the snow cover area in 2002, the snow cover areas were less for other years from 2003 to 2015, for the month of October. The maximum reduction in snow cover area was observed in 2014 (89.14%) and the minimum reduction was observed in 2004 (24.43%).
Except for the year 2004, the percent decrease in snow cover areas are more than 50% for the rest of the years compared to that in 2002.
Fig.3.12 shows the scatter plot for percentage reduction in average snow cover area of the Brahmaputra River basin from 2003 to 2015compared to the snow cover area of 2002.
Fig.3.12: Percentage change in average snow cover areas from 2003 to 2015 for the Brahmaputra River basin
It is observed that average snow cover area for 2003 and 2012 increases compared to the snow cover area in 2002 by 0.47% and 7% respectively. For the rest of the years, the average snow cover area of the basin reduces compared to the snow cover area in 2002. The maximum percentage reduction in snow cover area as compared to that of 2002 has been observed for the year 2006 (40.08%) and the minimum percentage reduction was observed in the year 2009 (0.87%). The increase in percentage reduction in snow cover area implies that the river is carrying a significant amount of melting water and is affecting the downstream hydrology of the river Brahmaputra.
Temperature being a dominant factor, this study evaluated the impact ofincrease in temperature on the snow cover area. Apart from the role of temperature, other factors should also be taken into account to explain the reason behind the difference in snow melt trend of the other three months from that of January. For example, if the snow is older, its albedo will be lower and in such situation it absorbs more solar radiation. Again, if the temperature variation is not significant, snow melts faster in the windy condition than in the calm conditions. According to IPCC (2001a), it is likely that changes in precipitation may occur during winter in Tibet and central Asia. Therefore, such variations should also be considered along with temperature and there is a need to study the combined effect of changes in temperature and precipitation. Once the effect of increase in temperature (no precipitation change) is known, then it is possible to assess the trend of snow cover depletion along with any change in precipitation (Singh and Bengtsson, 2003). For example, under a particular higher temperature scenario, an increase in snowfall will slow down the depletion of snow cover area and, consequently, would prolong the melt season. On the other hand, a reduction in snowfall would accelerate the depletion of snow and the melt season would be reduced.