gangotri glacier retreat – From a Glaciers Perspective

[ngg_images source=”galleries” container_ids=”22″ display_type=”photocrati-nextgen_basic_imagebrowser” ajax_pagination=”1″ template=”/nas/wp/www/sites/blogsorg/wp-content/plugins/nextgen-gallery/products/photocrati_nextgen/modules/ngglegacy/view/imagebrowser-caption.php” order_by=”sortorder” order_direction=”ASC” returns=”included” maximum_entity_count=”500″]

Purple dots indicate the transient snowline on Gangotri Glacier in the fall and early winter of 2016. Red arrow indicates the terminus of the glacier.

The Gangotri Glacier is the largest glacier in the Bhagirathi River watershed, situated in the Uttarkashi District, India. It is one of the larger glaciers in the Himalaya, and like all of the nearby Himalayan glaciers is retreating significantly. Gangotri Glacier provides hydropower as its meltwater passes through three hydropower plants generating 1430 MW, including the 1000 MW Tehri Dam and reservoir and Maneri Bhali I and II, see map below. From 1968-2006 the glacier retreated 800 meters, close to 20 meters per year (Bhambri et al, 2012). The glacier continues to thin and tributary inflow decline, while terminus retreat is slowe due to the thick debris cover that heavily insulates the ice. Bhambri et al (2011) inventoried glaciers in the upper Bhagirathi basin and found they lost 9 square kilometers in area, 3.3% to the total, from 1968 to 2006. They further noted that recession rates have increased since 1990 and that the number of glaciers increased from 82 in 1968 to 88 in 2006 due to fragmentation of glaciers. From 1968 to 2006, the debris-covered glacier area increased by ~12% in the upper Bhagirathi basin. Bhattachaya et al (2016) expanded on this work noting that the velocity of Gangotri Glacier declined during 2006-2014 by 6.7% from 1993-2006, this suggests reduced accumulation being funneled downglacier. They also noted an increase in the rate of debris-covered area expansion on the main trunk of Gangotri Glacier from 2006-2015, which is indicative of an expanding ablation zone. Bhattachaya et al (2016) report a retreat rate of 9 m/year 2006-2015, which is less than before, but the down-wasting in the same period 2006-2015 was higher than during 1968-2006. The study reinforced that glacier retreat is a delayed response to climate change, whereas glacier mass balance is a more direct and immediate response. This underlines the importance of mass balance studies for assessing climate change impact on glaciers,that the World Glacier Monitoring Service has emphasized. Gangotri Glacier is a summer accumulation glacier with the peak ablation period low on the glacier coinciding with peak snowfall high on the glacier during the summer monsoon. In the post monsoon period of October and November precipitation is low and melt rates decline, Haritashya et al (2006) note a sharp decline in discharge and suspended sediment load beginning in October. . Kundu et al (2015) from Sept. 2012 to January 2013 noted that the snowline elevation varied little, with the highest elevation being 5174 m and the lowest 5080 m.

The increase in temperature has led to a tendency for snowlines to rise in the post monsoon period and remain high into the winter season on many Himalayan glaciers. In 2016 this has been the case. On October 9, 2016 a Sentinel image indicates the snowline is at 4850 m on the main trunk and on the tributary Ghanohim Glacier the snowline, while it is 4750 m on the tributaryKirti Glacier. A Landsat image from October 13th indicates the snowline on Kirti has risen to 4800 m, and remains at 4850 on the main trunk and Ghanohim Glacier. By November 30th a Landsat image indicates the snowline has risen to 5400 m on the main trunk and Ghanohim, the snowline is at 5800-5900 m on the glaciers in the Swachhand tributary valley, at 5600 m on Maiandi Glacier and 5700 m on the last tributary entering from the north. Note the impact of radiational shading is apparent on the main trunk with the snowline descending down the middle of the main trunk from 5400 m to 5100 m and on Kirti Glacier which is too dark to confidently discern the snowline. Temperatures are typically cool in December, but sunshine is common. A Sentinel image from December 8th and Landsat from Dec. 9th indicate that the snowline remains approximately the same as on Nov. 30th. The accumulation area ratio is the percentage of a glacier in the accumulation zone and is typically above 50%. On Gangotri Glacier in December 2016 the accumulation area ratio is only 20%, indicating a large mass balance deficit. High winter snowlines on Chutenjima Glacier, Tibet, from October, 2015 to February 2016. This tendency is also noted at Nup La-West Rongbuk Glacier, on the Nepal-China border, West Hongu Glacier, Nepal and Lhonak Glacier, Sikkim.

Gangotri Glacier and its key tributaries, with the red line being the outline of the glacier from GLIMS.

Hydropower in the Bhagirathi River watershed

In India the Gangotri Glacier is the largest glacier at the headwaters of the Bhagirathi River. The false-color image below provided by NASA shows the retreat of Gangotri Glacier, situated in the Uttarkashi District of Garhwal Himalaya. It is one of the larger glaciers in the Himalaya, and like all of the nearby Himalayan glaciers is retreating significantly. The Bharigrathi River has the Tehri Dam, a 2400 mw hydropower facility. With an area of 286 square kilometers Gangotri Glacier (Singh and others, 2006) provides up to 190 cubic meters per second of runoff for this river. Gangotri Glacier provides hydropower as it passes three hydropower plants generating 1430 MW, including the 1000 MW Tehri Dam and reservoir and maneri Bhali I and II, see map below. The Tehri also provides flood control, such as this past week of June 17, 2013. The Tehri Reservoir level rose 25 m within 48 hours which is a storage of approximately 1.3 billion cubic meters. Below is a view of the Tehri Reservoir, images of the dam and its operations are here.
Map from the Southeast Asian Network on Dams, Rivers and People
Gangotri Glacier retreated 26.5 meters per year form 1935-1971. From 1968-2006 the glacier retreated 800 meters, close to 20 meters per year (Bhambri et al, 2012). Srivastava et al (2013) indicate the retreat rate of 21 m/ year from 2004-2010. The glacier continues to thin and tributary inflow decline, while the thick heavily insulated by debris terminus retreat is slow. Srivastava (2012) published a report with numerous terminus pictures though they do not have a common reference point beginning on page 90. Where the river exits the glacier is referred to as Gomukh.
Here we compare both Landsat and Google Earth images during the 2000-2013 period. First the 2000 and 2013 Landsat images. A 2000 and 2013 landsat image pinpoint the terminus change, the yellow and red arrows converge on the 2000 location of Gomukh. The blue arrow indicates the mouth of a side valley from the east that is at the terminus in 2013 and actively cutting the face, which is not the case in 2000. The orange dots indicate the course of this stream. A 2006 Cartosat image from Bhambri et al (2012) can be compared to the 2010 and 2013 Google Earth images. In Google Earth the 2010 image gives a clear view of Gomukh which can be compared to the 2006 Cartosat image from Bhambri et al (2012). In 2000 and even 2006 this was not the case. A 2013 Google earth also indicates this point,with the glacier having retreated to the side valley from the east. The retreat from the location of Gomukh in 2000 to 2013 is 240-270 m, approximately 20 m per year as noted by Srivastava et al (2013) for a shorter interval.

2000 Landsat image

2013 Landsat image

.
2006 Cartosat image

2010 Google Earth image

2013 Google Earth image

2013 Google Earth image

This glaciers remains over 30 km long, and is not in danger of disappearing anytime soon. The lower section of the glacier is heavily debris covered, which slows melting. The debris cover prevents black carbon-soot from enhancing melt over most of the ablation zone. The upper reaches of the glacier extends above 6000 meters and remains snow covered even during the summer melt season June-August, as this is also a main accumulation season due to the summer monsoon. This is different from other alpine regions, where the melt season is also the dry season, here it coincides with the wet season and the accumulation season on the upper glacier. Compare the differences in hydrographs from Thayyen and Gergen (2009) Figure 3 and 4. The new snowcover on the upper glacier also limits the impact of black carbon or soot on ablation. The glacier is fed from avalanches off of the even larger area of mountains above 6000 meters adjacent to it. This is one of many glacier in the Himalaya that is being tapped for hydropower. The retreat is slower than that of nearby Malana Glacier and Samudra Tupa Glacier but similar to Durung Drung Glacier.