FRIEND 2002—Regional Hydrolog}': Bridging the Gap between Research and Practice (Proceedings ofthe
Fourth International FRIEND Conference held at Cape Town. South Africa. March 2002). IAI IS I'ubl. no. 274. 2002.
417
Himalayan glaciers meltdown: impact on
south Asian rivers
SYED I Q B A L HASNAIN
School of Environmental
Sciences, Jawaharlal Nehru University, New Delhi 110067, India
e-mail: [email protected]
Abstract The Himalaya mountains contain the largest volume of ice outside
the polar regions. About 17% of the Himalaya and 37% of Karakoram
mountains are covered by glacier ice. The meltwaters from its glaciers form
the headwaters of major rivers as the Indus, the Ganges and the Brahmaputra.
The discharge of headwater rivers comprises about 70-80% of snow and ice
melt. In recent years the summers have been particularly warm as average air
temperatures have risen rapidly by more than 1.5°C, increasing the year to
year variability of river flow. The majority of Himalayan glaciers are
nourished by the precipitation during the summer-monsoon and therefore they
are very sensitive to summer air temperature fluctuations. This paper will
examine how climate change influences glacier behaviour and the quantity of
discharge in rivers draining from glaciated Himalayan basins. A case study of
the Dokriani Glacier (Ganga headwaters, India) is presented.
Key words glacier mellwater; global climate; south Asian rivers; summer-accumulation glaciers;
Dokriani Glacier; Ganga; India
INTRODUCTION
With a length of 2400 km and a width of 150-400 km, the Himalayan ranges are the
biggest and tallest mountainous structure on Earth. These high mountains are not only
the source for several perennial rivers but also influence the climate and water cycle of
the region (Reilly, 1996). Such impacts have the potential to directly affect the lives of
10% ofthe world's population living in India, Pakistan, Bhutan, Nepal and Bangladesh.
More than 7 0 - 8 0 % of the global freshwater supply comes from high mountains.
One such important region is the Karakoram-Himalayan mountain range in south
Asia. The Himalaya contains the largest volume of ice outside the polar regions,
meltwaters from its glaciers form the headwaters of such important rivers as the Indus,
the Ganges and the Brahmaputra (Thompson et al., 2000). The water reserves stored in
the Himalayan glaciers is estimated to be about 1 0 m (Puri, 1994).
As global climate warms, recession and wasting of glaciers are perhaps more
significant in the Karakoram-Himalaya than elsewhere and are expected to continue
this century (Hasnain, 1999; Fujita et ai, 1997; Naito et al, 2000). Retreat of
Himalayan glaciers linked to a decrease in precipitation since the 1940s (Dahe, 2000)
and a rise in summer air temperature on these summer-accumulation type glaciers have
an especially strong negative impact on the mass balance (Ageta & Kadota, 1992). The
proportion of rain is increased by the temperature rise and snow accumulation
decreases; surface albedo has decreased due to snowfall decrease and ablation
increases (Ageta et al, 2000).
12
J
Syed Iqbal Hasnain
418
Meltwater runoff from countries with glacier-covered drainage basins is a valuable
water resource for south Asian countries sharing the Himalayan mountain system.
Hence, it is important to understand the effect of climate change on the future
variability of river runoff from these basins. The aim of the present research is to
enhance our understanding of the hydrological sensitivity of monsoonal swept
glaciated Himalayan basins to climate change.
H Y D R O L O G Y A N D C L I M A T E C H A N G E IN T H E H I M A L A Y A N R E G I O N
Hydrology of the Bhagirathi-Ganga basin
The Bhagirathi River rises at the Gangotri Glacier known as Goumukh, at an elevation
of 4000 m (Fig. 1). It is one of the largest and most important source streams of the
Ganga fluvial system. Along with Alaknanda (originating from Satopanth and
Bhagirath Khark glaciers) it forms the mountainous catchment of the River Ganga and
flows through the deep gorges of the Garhwal Himalaya. The River Bhagirathi
traverses a distance of around 225 km across the Himalaya before it joins the
Alaknanda at Devparyag to form the River Ganga. Kaul et al. (1999) have identified
238 glaciers in the basin covering a glaciated area of 755.43 km" with a total ice
volume of 67.02 k m . The average monsoonal precipitation in the Ganga headwaters is
between 1000 and 2500 mm, of which 80% falls during the period July-August.
In 1992 the Dokriani Glacier (30°50'N and 78°49'E) was identified by the Depart­
ment of Science and Technology, Government of India, for the compilation of long-term
data sets in the Bhagirathi-Ganga headwaters. Since then the Glacier Research Group,
Jawaharlal Nehru University, N e w Delhi, has been studying the hydrology of the glacier.
J
Discharge, precipitation and temperature measurements
In 1994 a gauging station was established about 600 m downstream from the glacier
snout which has a 10-m straight reach. Frequent cross-section measurements were
carried out during the ablation period each year using a dip-stick.
Flow velocity was measured with a standardized wooden float, and water level was
monitored using a wooden staff gauge. Discharge was calculated from the rating curve
(r = 94%>). A self-recording raingauge was installed for continuous recording at 3550
and 4500 m elevation. Daily maximum and minimum air temperature were observed
between 1994 and 2000 at an elevation of 3500 m near the base camp. Daily mean air
temperatures were calculated by taking the average of daily maximum and minimum
temperatures.
2
R E S U L T S A N D DISCUSSION
Mean monthly values of discharge, daily air temperature and daily precipitation for the
summer monsoonal months (July and August) in the Bhagirathi-Ganga headwaters,
India, are listed in Table 1. The average air temperature was around 10°C between 1994
420
Syed Iqbal Hasnain
Table 1 Mean monthly values of the discharge (Q in nr' s"'), daily air temperatures (T in °C) and daily
precipitation (P in mm) for the summer-monsoonal months in the Bhagirathi-Ganga headwaters, India.
Months
July
August
1995:
1996:
1994:
T P 0
T
Q T P 0
11 10 2
9
6
11 17 5
5
8 15 5
10 9 6
9
P
1997:
Q T
6 2
13 6
11
9
P
1998:
0
T
8 9
10 9
11
10
P
1999:
Q T
7 13
12 31
P
12 9
10 7
2000:
Q T
P
35
36
18
11
11
11
and 1 9 9 8 . From 1 9 9 8 onwards an increase in air temperature of 0 . 5 ° C in the Dokriani
Glacier valley may have resulted in enhanced glacier ice melt. It is evident from Fig. 2
that anomalously high rates of glacier shrinkage due to climate warming have resulted
in high runoff. The increasing trend in the discharge is calculated by linear regression.
The separated components of the bulk hydrograph of the Dokriani Glacier between 1 9 9 4
and 2 0 0 0 are shown in Table 2 . The snow and glacier melt component during the obser­
vation period is above 8 0 % , which indicates a negative mass balance of the glacier.
Figure 3 shows temperature, precipitation and discharge trends that are likely to take
place until 2 0 1 0 . The trends that are calculated by linear regression indicate that the mon­
soonal runoff during July and August will increase by more than 1 0 0 % with an increase
of 60%o in monsoonal precipitation and 1 . 5 ° C in air temperature. If air temperature is
higher than it used to be, there are three negative effects on the glacier mass balance:
(a) the increased proportion of rain in the total precipitation reduces the accumulation
of snowfall,
(b) higher temperature increases ablation by sensible heat, and
1994
1995
1996
1997
1998
1999
2000
Fig. 2 Time series of the air temperature (top) and discharge (bottom) ofthe Dokriani
Glacier recorded between 1994 and 2000.
Himalayan glaciers meltdown: impact on south Asian rivers
421
Table 2 Separated components of bulk hydrograph of Dokriani Glacier meltwater.
Year
Total discharge
(10 m )
Rainfall
(10 m )
Snow and ice
(10 m )
243
266
468
720
932
1915
63
34
57
63
75
345
424
233
411
657
857
1570
5
1994
1995
1996
1997
1999
2000
3
5
3
5
3
Rainfall
(%)
13
13
12
9
8
18
Snow and ice
(%)
87
87
88
91
92
82
(c) decreased albedo, due to a decrease in snowfall. This increases ablation by
insolation.
Thus, variations in the glacier mass of summer-accumulation glaciers such as the
Dokriani Glacier depend strongly on the summer air temperature. Therefore, in the case
of constant precipitation, rising air temperature will reduce accumulation, and this inten­
sifies the ablation increase. Then the mass balance declines further due to cumulative
effects of the negative change of both accumulation and ablation (Ageta, 1998).
Similarly, glaciers in the Nepalese Himalayas are of the summer accumulation
type, i.e. about 8 0 % of nourishment comes from summer-monsoonal precipitation
20
15
10 -I
t »*'»^»
r
5
0
-i
1
1
1
1
r-
2000 - i
Fig. 3 Trends in precipitation, air temperature and runoff between 1994 and 2010 in
the Bhagirathi-Ganga basin, India.
422
Syed Iqbal Hasnain
(Ageta, 1998). In such glaciers, accumulation conditions strongly affect ablation since
both occur simultaneously in summer. In the glaciated basin of northwest China,
continuous high flow occurs throughout the summer, when the hot and rainy season
prevails in the mountains. Precipitation is also seasonally variable there; 80% falls in
the warm seasons (May-September), with a maximum in June. The annual maximum
floods are closely related to extreme storms and high temperature in summer (Liu
Jingshi et al., 1999). The glaciers of the central Asian republics have also experienced
the effect of the global warming which caused changes in the regional temperature and
precipitation (Li Dongliang, 1995; Aizen & Aizen, 1997). Analysis of a 56-m-long ice
core recovered from the Dasuopu Glacier (7200 m a.m.s.l.) in Tibet (Thompson et ai,
2000) shows other evidence of a significant rate of warming of about 0.35°C per
decade across the Tibetan Plateau. This isotopic warming is the most regionally
consistent climate signal ofthe last 1000 years.
CONCLUSIONS
Increases in monsoonal precipitation and rise in air temperature since the 1980s have
increased flow in the rivers along the Himalayan arc by enhanced glacier ice melting.
In the Ganga headwaters, where drainage basin studies are being conducted, an
increasing trend both in air temperature and discharge has been found. Small glaciated
basins with low ice coverage are highly sensitive to air temperature change.
Recent observations from India, China, Pakistan, Nepal, and Bhutan indicate rapid
shrinkage of glaciers. Such tendency has been observed both in large debris-covered
and small debris-free glaciers, with varying response times and sensitivity to climate.
There is a growing consensus that regional cooperation on the conjunctive use of great
rivers of south Asia, particularly the Ganges, the Indus and the Brahmaputra, will be
essential to solve regional water problems.
A c k n o w l e d g e m e n t The author has greatly appreciated the financial support given by
the Department of Science and Technology, Government of India.
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