The Fall of the Water

Introduction At the roof of the world, the Tibetan plateau supplies, together with the Himalayas, Hindu Kush and the Tian Shan mountain ranges, water to people in Central, Southern, Western and South-east Asia, the largest river run-off from any single location in the World (UNEP, 2002a) (Fig. 1). Major rivers originating from these mountain regions include the Syr Darya, the Amu Darya; the Ganges, the Indus, the Arun, The Sankosh, the Manas, the Yarlung/Brahmaputra, the Chindwin, the Salween/Nu Jiang, the Lancang Jiang/Mekong, the Jinsha Jiang, the Huang He and the Yangtze, in addi- tion to numerous other rivers. While the mountains are homes to some 170 million people, the water resources influence the lives of close to half of the world’s popula- tion downstream. The region comprises unique biodi- versity, ranging from desert, steppe and high-altitude fauna to tropical rainforests with global biodiversity hotspots, such as in South-Western China.

sity, particularly between north-western and south-east- ern regions in the hydrological significance of moun- tains for water supply further downstream (Viviroli et al., 2003). For several of the rivers from the Tian Shan and Hindu Kush into Central Asia and Pakistan, such as the Amu Darya and the Indus, the mountain sections are responsible for >90% of the estimated discharge, while rivers like the Mekong also receive substantial water from lowland catchments and the monsoons. There is also extreme variability within this region with regard to vulnerability to land use patterns, land slides, floods, drought or glacial outbursts (Semwal et al., 2004; Gau- tam et al., 2003; Blyth et al., 2002; Gurung and Gurung, 2002; Chettri et al., 2002,; Dongol et al., 2002). In spite of the vast water supply, water scarcity is a major prob- lem in the region, both up- and downstream, including both drinking water and irrigation (Merz et al., 2003). Desertification is a major problem in north-western parts of China, such as in parts of Xinjiang, particularly as a result of intensified land use along road corridors. The rivers form basic lifelines to people including access to

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While mountains traditionally have been considered the major water sources of the region, there is great diver-

Figure 1: While Asia has the highest share in the run-off of all the World’s rivers, it holds an estimated 60% of the World’s population (~3, 675,000,000 people in 2000), but only 36% of its river run-off (UNEP, 2002a).

River Runoff through the 20th Century Average Annual Volumes by Continent, 1921-1985

km 3 per year

Asia

14 000

12 000

South America

12 000

10 000

10 000

8 000

8 000

North America

6 000

8 000

6 000

4 000

6 000

4 000

Africa

2 000

4 000

2 000

0

Europe

4 000

0

2 000

1980

0

2 000

1970

Australia and Oceania

1960

0

2 000

1950

1940

0

2 000

1930

0

1920

FEBRUARY2002 PHILIPPEREKACEWICZ

UNEP

Source: Igor A. Shiklomanov, State Hydrological Institute (SHI, St. Petersburg) and United Nations Educational, Scientific and Cultural Organisation (UNESCO, Paris), 1999.