The Changing Himalayas
Table 1: Principal rivers of the Himalayan region – basin statistics
River River basin
River Annual mean % of glacier melt Basin area Population Population Water availability
discharge m
3
/sec
a
in river flow
b
(km
2
) density x1000 (m
3
/person/year)
(pers/km
2
)
Amu Darya 1,376
a
not available 534,739 39 20,855 2,081
Brahmaputra 21,261
a
~ 12 651,335 182 118,543 5,656
Ganges 12,037
a
~ 9 1,016,124 401 407,466 932
Indus 5,533
a
up to 50 1081,718 165 178,483 978
Irrawaddy 8,024
a
not available 413,710 79 32,683 7,742
Mekong 9,001
a
~ 7 805,604 71 57,198 4,963
Salween 1,494
a
~ 9 271,914 22 5,982 7,876
Tarim 1,262
a
up to 50 1,152,448 7 8,067 4,933
Yangtze 28,811
a
~ 18 1,722,193 214 368,549 2,465
Yellow 1,438
a
~ 2 944,970 156 147,415 308
Total 1,345,241
a
The data were collected by the Global Runoff Data Centre (GRDC) from the following most downstream stations of the river basins:
Chatly (Amu Darya), Bahadurabad (Brahmaputra), Farakka (Ganges), Pakse (Mekong), Datong (Yangtze), Huayuankou (Yellow)
b
Estimation of the meltwater contribution is difficult and varies in an upstream and downstream situation; approximates are given here.
Source: IUCN et al. 2003; Mi and Xie 2002; Chalise and Khanal 2001; Merz 2004; Tarar 1982; Kumar et al. 2007; Chen et al. 2007
Note: The hydrological data may differ depending on the location of the gauging stations. The contribution of glacial melt is based on limited data
and should be taken as indicative only.

Himalayan Climate and Water
A substantial portion of the annual precipitation falls as
snow, particularly at high altitudes (above 3000 masl)
The Himalayas display great climatic variability. The
feeding the Himalayan glaciers. The high Himalayan and
mountains act as a barrier to atmospheric circulation for
inner Asian ranges have the most highly glaciated areas
both the summer monsoon and the winter westerlies. The
outside the polar regions (Owen et al. 2002; Dyurgerov
summer monsoon dominates the climate, lasting eight
and Meier 2005). Glaciated areas in the greater
months (March-October) in the eastern Himalayas, four
Himalayan region cover an area of more than 112,000
months (June-September) in the central Himalayas, and two
km
2
(Table 2). The Himalayan range alone (a sub-region)
months (July-August) in the western Himalayas (Chalise and
has a total area of approximately 33,000 km
2
of glaciers
Khanal 2001). The east-west variation is based on the
or 17% of the mountain area (as compared to 2.2% in the
dominance of different weather systems, which in turn cause
Swiss Alps) with a total ice volume of ca 3,420 km
3

the monsoon to weaken from east to west. The monsoon
(Table 3) which provides important short and long-term
penetrates northwards along the Brahmaputra River into
water storage facilities. These figures are very tentative,
the southeast Tibetan Plateau, but rarely as far as the
however, and need to be followed up with more research.
Karakoram (Hofer and Messerli, 2006; Rees and Collins,
2006). The highest annual rainfall in the region occurs in
Glaciers undergo winter accumulation and summer ablation in
Cherapunjee in India, amounting to more than 12,000 mm.
the west, but predominantly synchronous summer accumulation
and summer melt in the east. The main melting occurs in high
The monsoon rainfall is mainly of an orographic nature,
summer; however, when this coincides with the monsoon, it
resulting in distinct variations in rainfall with elevation
may not be as critical for water supply as when the melting
between the southern slopes of the Himalayas and the rain
occurs in the shoulder seasons: spring and autumn. When the
shadow areas on the Tibetan Plateau (Mei’e 1985). On the
monsoon is weak, delayed, or fails, meltwater from snow and
meso-scale, the impacts of climate are mainly due to local
ice may limit or avert catastrophic drought.
topographic characteristics (Chalise and Khanal, 2001),
with dry inner valleys receiving much less rainfall than the
Water from both permanent snow and ice and seasonal
adjacent mountain slopes as a result of the lee effect. This
snow is released by melting, some to be temporarily stored
suggests that the currently measured rainfall, which is mainly
in high altitude wetlands and lakes, but most flowing directly
based on measurements of rainfall in the valley bottoms, is
downstream in the large river systems, giving a distinct
not representative for the area, and the use of these data
seasonal rhythm to annual stream flow regimes in these
results in significant underestimates.
rivers. The contribution of snow and glacial melt to the
2
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