Chapter 3: Snow cover area variation study of the Brahmaputra River Basin and
3.3 Results and Discussions
3.3.2 Effect of change in snow cover area on discharge of the River
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.
Table 3.3: Snow Cover Area and discharge for different months
Year
January April July October
Snow cover area (sq.km) Discharge (MCM) Snow cover area (sq.km) Discharge MCM) Snow cover area (sq.km) Discharge MCM) Snow cover area (sq.km) Discharge MCM)
2002 30474 20265 114588 24218 14536 32081 84551 33407 2003 98414 17855 99972 23967 25851 81635 21069 58528 2004 64472 17591 56236 24744 23695 82136 63894 41276 2005 74004 17197 83658 23364 11262 67365 28355 47279 2006 37285 16450 80162 22878 12678 52740 16157 49474 2007 49892 16260 87833 24571 7881 66558 12347 57555 2008 29822 16189 92263 17822 15540 62268 32572 41136 2009 112736 16262 89539 20595 14328 62966 25427 33734 2010 66618 16207 68848 39502 12401 68380 30162 57233 2011 62421 17059 74142 24578 13038 68352 13129 51521 2012 138555 16064 89930 21631 12236 79806 20526 63098 2013 126798 16713 52121 23457 9088 69891 16915 40634 2014 112355 17644 50246 25651 4503 64247 9180 49275 2015 102833 17500 51138 24657 6584 59506 9983 38262
Fig. 3.13: Trend showing variation of discharge w.r.t. snow cover area for the month (a) January, (b) April, (c) July and (d) October.
A relationship has been established between the change in snow cover area and discharge.The output shows that for the month of January, the discharge shows a decreasing trend with respect to the increasing trend of snow cover area. The minimum discharge was observed in the year 2012 (16064 MCM) and maximum was observed in the year 2002 (20265 MCM).
For the other three months i.e. April, July and October, increasing trends of discharge have been observed with respect to the decreasing trends of snow cover area. This may be attributed to the fact that due to rise in temperature the melting of snow is occurring at the upper catchment of the Brahmaputra River basin and the snowmelt water is contributing to the increase in discharge of the River at the downstream. This in turn is affecting the
downstream hydrology of the basin, causing severe flood in the monsoon season as well as riverbank erosion at some vulnerable points which is definitely a threat to the development of the entire region. For the month of April, the minimum discharge has been observed in the year 2008(17822 MCM) and maximum has been observed in the year 2010 (39502 MCM).
For July, minimum has been observed in the year 2002 (32081 MCM) and maximum has been in the year 2004 (82136 MCM). On the other hand, for the month of October, the minimum discharge has been observed in the year 2002(33407 MCM) and maximum has been observed in the year 2012 (63098 MCM). Melting of snow/glacier with respect to the rise in temperature has a major impact on the hydrology of the rivers originating from glaciers. The Brahmaputrais responsible for many natural calamities among which flood and erosion are the severe ones. Along with the heavy rainfall of this region, the melting of snow/glacier at the upper catchment of the basin also has a major effect on change in its discharge. In mountain regions particularly, global mean temperature has not been spatially homogeneous. A minor variation in temperature under low magnitude warming is more effective to changes in volume and area of glacier than high magnitude warming.An increased seasonal melt coupled with rains will bring more intense floods to the downstream areas. Impacts of the shrinkage and disappearance of mountain glaciers include changes in the flow characteristics of glacier fed rivers; glacier lake outburst floods (GLOFs) as well as the change in flood severity and frequency. The GLOFs are capable of releasing billions of cubic meter of glacier water, stored for decades without warning to those living at the downstream.
Melting of snow with respect to the rise in temperature has a major impact on the hydrology of the rivers. The flood and river bank erosion caused by the river Brahmaputra at its downstream are the main calamities that affect the livelihood of the people. Though precipitation is considered as the main cause of flood in this region, attention should also be paid to the impact of the contribution of snowmelt water to the discharge of the river. From this study, it has been found that the melting of snow is proportional to rise in temperature.
Here a quantitative analysis has been performed to show how much area covered by snow has been reduced due to the rise in temperature from 2002 to 2015 for four different months viz., January, April, July and October. On the basis of these findings, in future, further analysis could be carried out to determine the volume of snowmelt water that contributes to the discharge of the river Brahmaputra. Increase in glacier lakes due to rise in temperature should also be a serious matter of concern. Glacier lakes are formed when the glacier erodes the
land, and then melts, filling the hole or space that it has created. A retreating glacier often left behind large deposits of ice in hollows between hills. As the ice age ended, these melted to create lakes.
The accelerated melting and the accelerated retreat of snow cover area under warmer climate have direct implications on water resources and their distribution in time as well as on the regional climate. The reduction in snow cover area under warmer climate will reduce the volume of snowmelt runoff. Because of major differences in the physical properties of snow and other natural surfaces, the energy balance of the basin will be significantly changed.
Evaporation would significantly increase because of higher temperatures and fast conversion of snow cover areas to snow free areas. Moreover, since evaporation from the snow cover area is far lower than from snowfree areas (Bengtsson, 1980), snow-free areas would be exposed to evaporation for longer durations. Under warmer climate, glaciers located in the upper part of the basin will be exposed earlier and this will result in higher melting from the high altitude area (Braithwaite & Olesen, 1989; Braun et al., 1994), which will affect the mass balance of the glaciers. Thus, faster depletion of snow in a basin may influence the total runoff of that particular area and its distribution.