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The below mentioned article provides a case study on the Gangotri Glacier in India.
Study Area:
The Gangotri glacier, one of the largest ice bodies in the Garhwal Himalayas, is located in the Uttarkashi district of the state of Uttarakhand in India. It is one of the most sacred shrines in India, with immense religious significance. Being the main source of the river Ganga, it attracts thousands of pilgrims every year. The Gangotri glacier is a vital source of freshwater storage and water supply, especially during the summer season for a large human population living downstream.
The discharge from the glacier flows as the river Bhagirathi initially before meeting the Alaknanda river at Devprayag to form the river Ganga. Snow and glaciers contribute about 29 per cent to the annual flows of the Ganga (up to Devprayag) and hence any impacts on these glaciers are likely to affect this large river system.
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The north-west facing Gangotri glacier is a valley type glacier originating in the Chaukhamba group of peaks. Numerous smaller glaciers join the main stem of the main glacier to form the Gangotri group of glaciers. The complete Gangotri glacier system along with its tributaries covers an area of 210.60 sq km (ETM + 1999). The area and length of the main trunk of the glacier is 56.59 sq km and 29.13 km respectively.
The average width of the glacier is 1.85 km. The glacier, lies between 79°4’46.13″E – 79°16’ 9.45″E and 30°43’47.00″N – 30°55’51.05″N (ETM+1999). It has varying elevation of 4,015 -6,145 m above sea level (SRTM data analysis). The snout of the glacier occurs at an altitude of about 3,949 m above sea level, and this is the place from where the Bhagirathi originates. Its snout position is at 73°4’47.26″E and 30°55’36.45″N.
Gangotri Glacier Retreat – A Historical Perspective:
The Gangotri glacier has been receding since the last ‘Little Ice Age’, which ended in the 19th century. The tributary glaciers have also shrunk and some of them have even got separated from the main trunk of the glacier. This fact is evident by systematic studies going on since 1935 on the movement of the glacier snout, and by the presence of recessional features such as terminal and lateral moraines.
In the past century, the retreat rate of the glacier has shown a rising trend. It has been observed by glaciologists that the snout of the Gangotri glacier has retreated by about 2 km in the last 100 years. Both the snout retreat and area vacated by the Gangotri glacier have been documented by several scientists.
Meteorological Conditions around the Gangotri Glacier:
Meteorological conditions play a pivotal role in governing the state of glaciers and their associated hydrological features such as water storage in downstream areas. Meteorological analysis of long-term data becomes a fundamental aspect in determining the structural changes taking place in glaciers.
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Rainfall:
Precipitation data collected from the region indicates that the area around Gangotri usually receives less than 15 mm of daily rainfall during the summer season. There are only few unusual days when the rainfall patterns vary due to a storm or some unusual heavy rainfall event. The study also shows that August and September usually receive higher rainfall as compared to the other months. It has also been found that early morning and late evening are the most probable times for the occurrence of rainfall.
Temperature:
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Temperature in the ablation season increases for a few months after which it starts decreasing. July has been found to be the warmest month on the basis of the mean-maximum and minimum temperatures. Diurnal variations in temperature show that the maximum temperature is observed around 1400 hours, while the minimum is observed in the early morning hours.
Changes in minimum temperature have been seen to be more significant than those in the maximum temperature. It has also been found that the ‘maximum diurnal temperature range’ occurs in May and October while August shows the least variation in temperature range. This is probably due to the presence of a cloud cover during the rainy season.
Hydrological Characteristics of the Gangotri Glacier:
For the Gangotri glacier, the major sources of runoff are melting snow and ice. Since this area receives less rainfall, it does not contribute much to the runoff. Stream flow at the Gangotri glacier shows a wide variation depending on various factors. In the beginning and at the end of the ablation season, there is not much difference in the day and night time flow volume; however, as the peak melting time approaches there is a comparative reduction in the night time flow.
Still, a significant flow is observed at night in spite of very little or no melting taking place at that time. This, reflects the fact that the Gangotri glacier has strong melt water storage characteristics. Monthly variation of flow shows that the discharge starts rising from May and maintains a high flow during June to August with maximum average discharge in August. The discharge starts reducing from September onwards. Singh et al. (2006) have also reported similar results in the discharge flows from the glacier.
Methodology:
The methodology for field studies at Gangotri involved a combination of primary field data through the use of Differential Global Positioning System (DGPS) and Remote Sensing Applications along with the existing secondary data. DGPS measurements have been extensively used for locating the snout position of the glacier during various field visits starting from 2006. Long-term observations are an important factor for accuracy in any glacial research as the snout position is quite dynamic and inter-seasonal changes are seen in the exact position of the snout.
Snout position was recorded in the months of September and October of each year since 2006. For the collection of meteorological data, an automatic weather station (AWS) has been installed at Bhojwasa near the snout of the glacier. Apart from the glacial and meteorological monitoring, data has also been collected about the discharge patterns of the glacial melt water. The glacial area was calculated using satellite imageries of 1976, 1990, 1999 and 2006 by delineating the glacier boundary using ERDAS 9.0 and auto calculating the area using GIS methodology.
Results and Analysis:
A comparative analysis of the glacier’s snout position was carried out using data from secondary sources and interpretations from various satellite imageries over the past three decades. Satellite imageries available since 1976 formed the baseline for the analysis of the fluctuations in snout position together with DGPS observations since 2006.
A study of data from all available sources illustrates that the main trunk of the Gangotri glacier has been in a continuous state of recession and fragmentation during the past century. The length of the glacier has been computed for different years based on available data. The trend shows that the length of the glacier has reduced by about 0.59 km in 33 years, from 1976-2009, with an average retreat rate of 17.59 m/year (Figure 5.1).
Based on the comparison of satellite imageries of the Gangotri glacier for the years 1976, 1990, 1999 and 2006, our analysis shows that the glacier is not only receding in length but also in terms of glaciated area from all the sides.
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The possible reasons behind this retreat may be linked with two main factors:
(a) Reduction in snowfall and
(b) An increase in the temperature of the region.
Analysis shows that between 1976 and 2006, the glacier area has reduced by 15.5 km2, with an average loss of 0.51 km2 per year. This reduction in glacier area is 22.9 per cent over 1976 (Figure 5.2). The glacier area reduced by 7.2 Km2 between 1976 and 1990 in fourteen years, with a 10.6 per cent reduction in the glacial area. In nine years, between 1990 and 1999, the glacial area reduced by 4.3 km2 and resulting in 7 per cent reduction in area when compared to 1990. However, the rate has increased between 1999 and 2006, with glacial area reducing by 4.1 km2 and a 7.25 per cent reduction in area over 1999.
With a reduction in the area and length of the Gangotri glacier, there has also been a retreat in the snout position. Data collected from various sources shows that this has been a continuous process; however, there have been fluctuations in the rate of retreat for different time intervals.
The calculation of the snout retreat by DGPS observation is made from point to point that gives the straight line distance of the two points that may include the shift of the water outlet point (left/right) in the curvilinear shape of the larger snout. The other way of measuring shift of the snout position in the glacier flow direction is to measure the perpendicular distance on the curvilinear shape of the snout.
This measurement normally gives a lesser value if the snout point has moved either left or right rather than in the glacier flow direction as compared to the previous position. Based on this measurement we have observed that the snout retreat during 2006-07 was 17.85 m when compared to snout position of 2006 (Figure 5.3).
In the subsequent years, the retreat rate has reduced-13.06 m in 2007-08 and 10.57 m in 2008-09. It can be said that retreat has taken place between 2007 and 2009, however, it is lower than the long-term average. Our observation from the glacier indicates that while there is a retreat, it is the fragmentation of the Gangotri glacier is of larger concern.
Hydro-Meteorological Study:
Trends emerging from the analysis of summer season discharge data for Gangotri show that there are variations on a diurnal and monthly basis. Flows recorded at different times of the day were found to be highest during the evening horn’s (1700 hours). The discharge is lowest during the morning hours (Figure 5.4). These results are also in agreement with the findings of an earlier study by Singh et al. (2006), in which they observed a high discharge rate during daytime and low rates during the night time for the Bhagirathi River at Bhojwasa.
On a monthly scale, flows increase with the commencement of the summer season reaching a peak during July and August due to the combined impact of more melting of ice, which is driven by higher average temperatures and rainfall in the catchments.
The discharge starts reducing in September and reaches a very low level throughout the accumulation season. During winter, the flow is reduced and keeps almost at a constant value. The combined impact of low winter temperature (sub-zero) and higher albedo (90-95 per cent for fresh snow) reduces the winter discharge rate to quite a low level.
The daily average temperature and average discharge for the ablation and accumulation season at Bhojwasa are shown through Figures 5.5 and 5.6 respectively. A good correlation of 0.73 has been found to exist between the average temperature and average discharge in the ablation season. However, these parameters are not well correlated (0.46) in the accumulation season, which shows a lower influence of temperature on melting because most of the solar energy is reflected back to the atmosphere due to the snow-covered valley that has a high albedo.
The preliminary trends observed during the study in 2008 seem to suggest that the volume of discharge and flow patterns is also being influenced by the temperature variations in the valley. This is in consonance with results obtained by other studies over the past decade. While these are initial results, comparisons with similar studies indicate a consistent pattern in the rate of discharge and temperature increase for the region.
Where the general trend of the impact of mid-day temperature on evening discharge in the glaciated basin is concerned, we have found a correlation of 0.63 between the 1200 hours temperature and the 1700 hours discharge at Bhojwasa on the Bhagirathi River (Figure 5.7). In the month of October the discharge rate falls as the average temperature starts reducing.
Trends in the Gangotri region (Figure 5.8) reflect that temperature start falling with the arrival of September. The maximum temperature reaches up to 18°C in June and July and remains mostly around 14°C during May to August. The morning temperature (at 0800 hours) remains at around 9°C till August and falls to zero in the mid of September.
The winter temperature is quite low especially in December when the morning temperature is always sub-zero and can dip down to -15°C. The noon temperature remains at around 10°C. During January and February, the morning and evening temperature is always below zero while in the noon it just crosses the zero degree mark. During March, the morning temperature remains at below zero, while the noon and evening temperature varies around 5°C.
