In the Bhagirathi Basin, selected glaciers have been conserved and glacier length, glacier area, topographic parameters etc., were computed using Landsat satellite images and ASTER DEM to evaluate the status of glaciers and to assess the glacier changes in the Bhagirathi River Basin. Here, the results of all the glaciated basin and sub-basins have been discussed below to get the microscopic overview of the Bhagirathi Basin.
3.1 Spatio-temporal variability of glaciers in Bhagirathi basin during 2000-15
Bhagirathi basin consists of five sub-basins, mainly Bhagirathi, Bhilangana, Jahnvi, Jalandhari, Pilang, and Kaldi. Kaldi sub-basin has no glaciered region, whereas the remaining sub-basin has 288 glaciers. However, in the present study, 29 glaciers with an area of more than 5 km2 have been considered and given in Table 2. It is observed from the analysis and shown in Fig. 2 that Dokarni and Dehigad glacier lies in Pilang and Jahnvi sub-basin respectively, shows maximum retreat (~ 0.204 km2) and advancing (~ 0.016 km2) in terms of area during 2000–2015. Snout elevation is observed 3923 m a.s.l. and 4028 m a.s.l. in Dokarni glacier, and 4099 m a.s.l. and 4039 m a.s.l. in Dehigad glacier during 2000 and 2015, respectively. Study also reveals that changes in snout elevation and length as -2.7 m a.s.l. and − 291 m in Dokarni glacier, and 1.5 m a.s.l. and 152 m in Dehigad glacier, during the data period. Further, all the glaciers show retreating activities, except four glaciers (Dehigad, Maitri, 52Bhagirathi, and Rudugaira) during the study period.
3.2 Spatio-temporal variability of glaciers in Bhagirathi sub-basin during 2000-15
It is observed from that Table 3, that in the Bhagirathi sub-basin, Bhrigupanth, 54 Bhagirathi, Kedar, Chaturangi, Raktavarn, Swetvaran, Gangotri and 13 Bhagirathi glaciers have shown a decrease in glacier length by ~ 62 m, ~ 92 m, ~ 150 m, ~ 151m, ~ 152 m, ~ 158 m, ~ 160 m and ~ 197 m, respectively. Whereas, Maitri, 52 Bhagirathi and Rudugaira glaciers have shown increase in the length by ~ 85 m, ~ 34 m and ~ 3 m, respectively during the study period.
Interpretation of satellite imagery for the year 2000 and 2015 indicated a decrease of the area in 8 glaciers. A marked decrease of roughly ~ 1 km2 is observed in Gangotri and Raktvaran glaciers. Other glaciers namely, Bhrigupanth, 54 Bhagirathi, Kedar, Chaturangi, Swetvarn and 13 Bhagirathi have shown a decrease in the area by 0.011 km2, 0.016 km2, 0.017 km2, 0.118 km2, 0.002 km2 and 0.087 km2, respectively. Whereas, Maitri, 52 Bhagirathi and Rudugaira have shown increase in glacier area by ~ 0.025 km2, 0.020 km2, and 0.016 km2, respectively (Table 3).
Change in snout elevation between the data period shows that maximum positive change is observed in 54 Bhagirathi glacier (~ 212 m a.s.l.) and maximum negative change in Maitri glacier (~ 25 m a.s.l.). 54 Bhagirathi, Kedar, Chaturangi, Raktavarn, Swetvaran, Gangotri and 13 Bhagirathi glaciers have shown an increase in snout elevation, whereas, Maitri, 52 Bhagirathi and Rudugaira glaciers have shown decrease in snout elevation during 2000 to 2015 in study area. Detailed observations are given in Table 3.
Further, glacier terrain parameters (i.e., aspect and slope) have been derived from ASTER DEM and observed that advancing glaciers are more than 50% in north-oriented, while less than 20% are oriented towards the south. It has been widely reported that north-facing glaciers persist, whereas south-facing glaciers are disappearing (Sakai et al. 2002; Dobhal et al. 2010; Garg et al. 2016). The amount of depletion is not significant but this shows a general trend of depleting glaciers in the Bhagirathi basin. Also, it was found that the maximum mean slope of ~ 26.50⁰ in the Maitri glacier, which has shown the highest advancing trend. Similar results have been reported in various studies (Bhambri et al. 2010, Dobhal et al. 2012, Bolch et al. 2013).
Table 3
Change in length, area, snout elevation during data period and terrain parameters in the Bhagirathi Sub-basin
Glaciers | Length (m) | Area (sq. km) | Snout Elevation (m) a.s.l. | North Aspect | Mean Slope (Degree) |
Maitri | 85 | 0.025 | -25 | 54 | 26.55 |
52 Bhagirathi | 34 | 0.020 | -15 | 53 | 23.42 |
Rudugaira | 3 | 0.016 | -1 | 63 | 24.74 |
Bhrigupanth | -62 | -0.011 | 25 | 70 | 25.35 |
54 Bhagirathi | -92 | -0.016 | 212 | 72 | 25.08 |
Kedar | -150 | -0.017 | 146 | 57 | 23.13 |
Chaturangi | -151 | -0.118 | 93 | 53 | 21.67 |
Raktvarn | -152 | -0.100 | 135 | 31 | 25.14 |
Swetvarn | -158 | -0.022 | 14 | 31 | 19.08 |
Gangotri | -160 | -0.862 | 32 | 56 | 18.06 |
13 Bhagirathi | -197 | -0.087 | 76 | 10 | 18.73 |
Additionally, the analysis shows a positive correlation ~ 0.3 between variation in the glacier length and area, while a negative correlation value of ~ 0.62 between glacier length and snout elevation for retreating glaciers. A positive correlation between mean glacier slope and area variation has been observed for the advancing glaciers (~ 0.68) and retreating glaciers (~ 0.5).
3.3 Spatio-temporal variability of glaciers in Jalandhari sub-basin during 2000-15
It is observed from that Table 4, that in the Jalandhri sub-basin all the glaciers length is decreasing during the study period. Rohilla glacier has shown a minimum decrease in length (86 m) followed by 66 Chhaling (100 m), Sian (119 m), 61 Jalandhari (123 m), 5 Bartikhunt (180 m), and 2 Chhalan (251 m). A decrease in glacier length up to ~ 251 m in the Jalandhari sub-basin is indicative of overall depletion of glacial cover.
Interpretation of satellite imagery for the year 2000 and 2015 indicated a decrease the area in all the selected glaciers. A marked decrease of approx. ~0.1 km2 is observed in 61 Jalandhari, Bartiakhunt, and Chhalan glaciers. Other glaciers namely, Rohilla, 66 Chaling, and 13 Sian have shown a decrease in the glacier area by ~ 0.02 km2, ~ 0.02 km2 and ~ 0.05 km2, respectively (Table 4).
Table 4
Change in length, area, snout elevation during data period and terrain parameters in the Jalandhari Sub-basin
Glaciers | Length (m) | Area (sq. km) | Snout Elevation (m) a.s.l. | North Aspect | Mean Slope (Degree) |
Rohilla | -86 | -0.021 | 31.12 | 19 (N) | 19.91 |
66 Chaling | -100 | -0.020 | 31.14 | 56 (NE) | 20.95 |
Sian | -119 | -0.048 | 31.11 | 45 (NE) | 17.82 |
61 Jalandhari | -123 | -0.100 | 31.14 | 31(NE) | 13.18 |
5 Bartiakhunt | -180 | -0.100 | 31.04 | 68(E) | 20.74 |
2 Chhalan | -251 | -0.110 | 31.00 | 31(NE) | 34.39 |
Change in snout elevation between the data period shows that maximum positive change is observed in 54 Bhagirathi glacier (~ 212 m a.s.l.) and maximum negative change in Maitri glacier (~ 25 m a.s.l.). 54 Bhagirathi, Kedar, Chaturangi, Raktavarn, Swetvaran, Gangotri and 13 Bhagirathi glaciers have shown an increase in snout elevation, whereas, Maitri, 52 Bhagirathi and Rudugaira glaciers have shown decrease in snout elevation during 2000 to 2015 in study area. Detailed observations are given in Table 3
Further, glacier terrain parameters (i.e., aspect and slope) have been derived from ASTER DEM and observed that north and north-east aspect glaciers are retreating fast in this sub-basins. The maximum mean slope of 34.40º has been found in Chhalan glacier, which has shown the highest retreating trend and is given in Table 4.
Additionally, the analysis shows a positive correlation ~ 0.8 (in Sian glacier) between glacier length and area, however, a high negative correlation ~ 0.9 between glacier length and snout elevation for retreating glaciers has been observed. A positive correlation between length and area advises that glacier area decreases with a decrease in length, whereas a negative correlation between length and snout elevation demonstrates that glacier length decrease with the increase in snout elevation. A low negative correlation value of 0.119 between length and the North aspect suggests that glaciers facing north have no significant length variation. This result conforms with the universally accepted thesis that topographic shading is an important factor affecting glacier mass balance and that glaciers facing north are more persisting in nature.
In the Northern Hemisphere, north-facing slopes in latitudes from about 30 to 55 degrees receive less direct sunlight than south-facing slopes and remain therefore cooler. This causes snow on north-facing slopes to melt slower than on south-facing ones. The scenario is just the opposite for slopes in the Southern Hemisphere, where north-facing slopes receive more sunlight and are consequently warmer.
Also, a negative correlation (~ 0.68) between the mean glacier slope and area has been estimated for the glaciers of the Jalandhari sub-basin.
3.4 Spatio-temporal variability of glaciers in Jahnvi sub-basin during 2000-15
It is observed from that Table 5, that in the Jahnvi sub basin, out of 6 glaciers of this sub-basin, 5 glaciers have shown a marked decline in length whereas one glacier has shown marked positive length. The maximum positive length change of ~ 152 m was found in the Dehigad glacier during the data period. Whereas, Surali glacier has shown a maximum negative decrease in length (172 m), followed by Kailash (133 m), Guligad (119 m), 60 Bombigad (106 m), and Mana (90 m), respectively.
Interpretation of satellite imagery for 2000 and 2015 indicated a decrease in the area in all the selected glaciers, except Dehigad glacier. A marked decrease of glacier area approx. ~0.33 km2 is observed in Surali glacier. Other glaciers namely, Mana, 60 Bambigad, Guligad, and Kailash shown a decrease in the glacier area by ~ 0.013 km2, ~ 0.05 km2, ~ 0.06 km2, and ~ 0.097 km2, respectively, while Dehigad glacier is increasing by ~ 0.016 km2, during data period (Table 5).
Change in snout elevation between the data period shows that maximum positive change is observed in 54 Bhagirathi glacier (~ 212 m a.s.l.) and maximum negative change in Maitri glacier (~ 25 m a.s.l.). 54 Bhagirathi, Kedar, Chaturangi, Raktavarn, Swetvaran, Gangotri and 13 Bhagirathi glaciers have shown an increase in snout elevation, whereas, Maitri, 52 Bhagirathi and Rudugaira glaciers have shown decrease in snout elevation during 2000 to 2015 in study area. Detailed observations are given in Table 5.
Table 5
Change in length, area, snout elevation during data period and terrain parameters in the Jahnvi sub-basin
Glaciers | Length (m) | Area (sq. km) | Snout Elevation (m) a.s.l. | North Aspect | Mean Slope (Degree) |
Dehigad | 152 | 0.016 | 31.09 | 92 (E) | 30.20 |
Mana | -91 | -0.013 | 31.03 | 65 (ne) | 15.56 |
60 Bambigad | -106 | -0.050 | 31.05 | 72 (E) | 25.04 |
Guligad | -119 | -0.060 | 31.04 | 85 E | 24.72 |
Kailash | -133 | -0.097 | 31.04 | 58 NE | 18.38 |
Surali | -172 | -0.333 | 31.08 | 68E | 17.66 |
Further, glacier terrain parameters (i.e., aspect and slope) have been derived from ASTER DEM and observed that north-east and east aspect glaciers are retreating in this sub-basins. The maximum mean slope of 30.50º has been found in the Dahigad glacier, which has shown some advancing trend. Various authors have reported that persisting glaciers are found on steeper slopes.
Additionally, the analysis shows a low positive correlation value of ~ 0.13 between glacier length and the north aspect slope. It suggests that glaciers facing north are having no significant length variation. This result conforms with the universally accepted thesis that topographic shading is an important factor affecting glacier mass balance and that glaciers facing north are more persisting in nature.. A low negative correlation value ~ 0.15 was observed between glacier length and south aspect. South-facing glaciers have shown length variations owing to direct solar radiation. But here the result deviates from the common understanding. A positive correlation (~ 0.47) between glacier mean slope and area has been observed for the glaciers of Jahnvi sub-basin. A moderate negative correlation (~ 0.35) has been observed between glacier snout elevation and area. This implies that with the increase in snout elevation, the glacial area decrease. Further, the calculated results of glacier area change clearly show that all glaciers except one have lost glacier area during the study period. The total glacier area changed from 61.77 km2 to 61.23 km2 (0.88%).
3.5 Spatio-temporal variability of glaciers in Bhilangana sub-basin during 2000-15
It is observed from that Table 6 that in the Bhilangana sub-basin, all the glaciers length is decreasing during the study period. Khatling glacier has shown a maximum decrease in length (212 m) followed by 15 Bhilangana (181m) and 19 Bhilangana (47 m), respectively.
Interpretation of satellite imagery for the year 2000 and 2015 indicated a decrease in area in all the selected glaciers of the Bhilangana sub-basin. 19 Bhilangana, 15 Bhilangana and Khatling have shown a decrease in the area by ~ 0.01 km2, ~ 0.03 km2, ~ 0.06 km2 and 0.07 km2, respectively. Although the magnitude of the area variation is very low, these results still reflect the general trend of glacial area retreat in the Bhilangana sub-basin (Table 6).
Change in snout elevation between the data period shows that maximum positive change is observed in 54 Bhagirathi glacier (~ 212 m a.s.l.) and maximum negative change in Maitri glacier (~ 25 m a.s.l.). 54 Bhagirathi, Kedar, Chaturangi, Raktavarn, Swetvaran, Gangotri and 13 Bhagirathi glaciers have shown an increase in snout elevation, whereas, Maitri, 52 Bhagirathi and Rudugaira glaciers have shown decrease in snout elevation during 2000 to 2015 in study area. Detailed observations are given in Table 5. Snout elevation variation in the order of 85 m, 37.7m and 34m was estimated for Khatling, 15 Bhilangana and 19 Bhilangana glaciers respectively.
Further, glacier terrain parameters (i.e., aspect and slope) have been derived from ASTER DEM and observed that north aspect glaciers are retreating at very slow rate in this sub-basins. The maximum mean slope of ~ 27.88º has been observed in 15 Bhilangana glaciers and results into a minor decrease in area. Various authors have reported that persisting glaciers on steeper slopes.
Table 6
Change in length, area, snout elevation during data period and terrain parameters in the Bhilangana sub-basin
Glaciers | Length (m) | Area (sq. km) | Snout Elevation (m) a.s.l. | North Aspect | Mean Slope (Degree) |
19 Bhilangana | -47 | -0.009 | 30.76 | 27 | 13.38 |
15 Bhilangana | -181 | -0.032 | 30.85 | 25 | 27.88 |
Khatling | -212 | -0.069 | 30.82 | 35 | 16.44 |
Additionally, the analysis shows a high positive correlation value ~ 0.89 between glacier length and area, whereas a moderately high negative correlation value ~ 0.69 between glacier length and snout elevation in this sub-basin. A positive correlation between length and area suggests that glacier area decreases with a decrease in length, whereas a negative correlation between length and snout elevation shows that glacier length decreases with the increase in snout elevation and this conforms with the existing hypothesis.
A high negative correlation value (~ 0.80) has been estimated between the glacier north aspect slope and area, this may be due to glaciers facing north direction slope are showing very low variation in area change. A moderate negative correlation value ~ 0.45 has been observed between glacier length and the north aspect slope. The analysis shows that glaciers facing north aspect having no significant length variation. This result conforms with the universally accepted thesis that topographic shading is an important factor affecting glacier mass balance and that glaciers facing north are more persisting in nature. Similar, findings have also been reported by several authors working on Himalayan glaciers.
High negative correlation (~ 0.94) was estimated between glacier snout elevation and area. This implies that with an increase in snout elevation, glacial area covers decrease. Generally, at higher elevations, persisting glaciers are found owing to prevailing low-temperature conditions. The overall results of area change estimation show that all glaciers of the Bhilangana sub-basin have depleted during the period between 2000 and 2015 and results into total glacier area changed from 54.72 km2 to 54.61 km2 (~ 0.20%).
3.6 Spatio-temporal variability of glaciers in Pilang sub-basin during 2000-15
In the Pilang sub-basin, a significant negative change in length ~ 291 m was found in the Dokriani glacier followed by Jaonli (109 m) and 27 Ganigad (1 m), respectively. All the three glaciers of this sub-basin has shown a decrees in length (Table 7).
Analysis of Landsat temporal imageries (2000 and 2015) shows a decrease in area for all the glaciers of the Pilang sub-basin. 27 Ganigad, Jaonli and Dokrani glaciers have shown a decrease in the area by ~ 0.1 km2, ~ 0.01 km2 and ~ 0.2 km2, respectively. A marked decrease in the glacial cover area has been observed in the Dokriani glacier and this has also been reported earlier Dobhal et al. (2008). Although the magnitude of the area variation is low in the other two glaciers of this sub-basin, these results broadly reflect the general trend of glacial area retreat in the Pilang sub-basin. Apart from glacier change in length and area, snout elevation change has also been estimated for this sub-basin. It is observed that snout elevation changes by ~ 151.3 m a.s.l., 22 m a.s.l., and 1.1 m a.s.l., for Dokrani, Jaonli and 27 Ganigad glaciers, respectively (Table 7). The maximum mean slope of 27.88º has been found in 15 Bhilangana glaciers which have shown a minor decrease in area.
Table 7
Change in length, area, snout elevation during data period and terrain parameters in the Pilang sub-basin
Glaciers | Length (m) | Area (sq. km) | Snout Elevation (m) a.s.l. | North Aspect | Mean Slope (Degree) |
27 Gani gad | -1 | -0.100 | 30.83 | 28 | 18.80 |
Jaonli | -109 | -0.014 | 30.92 | 56 | 21.25 |
Dokrani | -291 | -0.204 | 30.86 | 60 | 25.95 |
Additionally, the analysis shows a positive correlation (~ 0.66) has been observed between glacier length variation and area, whereas a very high negative correlation (~ 0.97) between glacier length and snout elevation in this sub-basin. A positive correlation between length variation and area variation suggests that glacier area decreases with a decrease in length and vice versa whereas a high negative correlation between length variation and snout elevation shows that glacier length significantly decreases with the increase in snout elevation
A high negative correlation (~ 0.85) has been estimated between glacier length and the north aspect slope. It suggests that glaciers facing north are having no significant length variation. A very high negative correlation (~ 0.99) was estimated between glacier length and mean slope. This implies as the mean slope increase the glacier length decreases significantly. Different rates of retreat/advance of glaciers within a region, over which the climatic conditions do not change significantly, is due to the important role played by the dynamics of ice movement, which in turn is controlled by the mean slope and length of the glacier.
A high negative correlation (~ 0.83) was estimated between glacier snout elevation and area in this sub-basin. Several authors (Kulkarni et al. 2005; Deota et al 2011; Venkatesh et al. 2012) have found the same trend in the Himalayan glaciers. Generally, at higher elevations, persisting glaciers are found owing to prevailing low-temperature conditions. A negative correlation value (~ 0.16) has been estimated between the glacier north aspect slope and area. This result indicates that north-facing glaciers can be more or less persisting. The overall results of area change estimation show that all glaciers of the Pilang sub-basin have depleted during the period between 2000 and 2015 and the total glacier area changed from 25.36 km2 to 25.04 km2 (1.26%).