Water towers for humanity

Waterfalls, Solu Khumbu, Nepal
© John Burbank/The Hutchison Library

Mountains intercept air masses, forcing the air to rise and cool, triggering precipitation as rain or snow. The water may be stored as glacier ice, in snow, or in lakes and man-made reservoirs. The summer melting of glaciers and snow is often essential is providing water during the season when precipitation and runoff are often least in the lowlands, and demands are highest. All of the world's major rivers rise in mountain regions.

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• In humid regions, mountains generate 30-60 per cent of the water flowing to the lowlands. In Europe, although the Alps cover only 23 per cent of the area of the Rhine river basin, they provide half the total flow. Other parts of the Alps form a third of the watershed (or catchment area) of both the Rhone and Po rivers and contribute 47and 56 per cent, respectively, to the lowland flow.
  

• In semi-arid and arid regions, mountains are often the only areas which receive enough precipitation to generate runoff and groundwater recharge. They typically provide 70-95 per cent of the flow to nearby lowlands.
  

• About 90 per cent of the annual flow in the Indus river basin comes from the mountains of the Hindu Kush, Karakorum and western Himalaya: 37 per cent from the Indus river, 15 per cent from the Chenab, 14 per cent from the Jhelum, 13 per cent from the Kabul, 10 per cent from the Sutlej, 7 per cent from the Beas, and 4 per cent from the Ravi. Three-quarters of this water is used in the world's largest irrigation network, on which most of Pakistan's 130 million inhabitants depend.
  

  Rogoun Dam and the Vakhsh River, Tajikistan. ©UN University Mountains Programme

  
  

• Worldwide, 214 river basins, home to 40 per cent of the world's population and covering more than half the land area, are shared by two or more countries. Transboundary water flows can create political tension, for instance in the Euphrates, Ganges, Jordan, and Nile.
  

• Such conflicts are likely to increase in number as populations grow, demands for water increase, and climate change affects precipitation and evaporation. Improved methods of equitable water allocation and improved regulation of water abstractions are essential to minimize the risk of conflict, especially in semi-arid and arid regions.
  

• Because of their steep slopes and often extreme climates, mountains are dynamic ecosystems. Storms can lead to soil erosion and landslides; the soil reaching the rivers is often carried far downstream. Deforestation, agriculture, mining, the development of transport infrastructure, and tourism in mountain watersheds can all significantly affect the quality and quantity of water, sometimes leading to floods and increased soil erosion.
  

• While many statistics are quoted above, our knowledge of the quantity and quality of global mountain water resources is very uneven. While we can say with some certainty that at least 50 per cent of mountain areas play an essential or supportive role in supplying water for downstream regions, we cannot say what proportion of the world�s water comes from the mountains. This is due to many factors, including the high variability of precipitation across mountain regions, and the physical difficulties and costs of collecting information in harsh mountain environments. Even where data are collected, they are often not made available by governments for 'security reasons'.
  

Viviroli, D., H.H. D�rr, B. Messerli, M. Meybeck, R. Weingartner (2007) Mountains of the world, water towers for humanity: Typology, mapping and global significance. Water Resources Research 43, W07447.

Mountain Agenda (1998) Mountains of the World: Water Towers for the 21st Century. Mountain Agenda, Bern.

Compiled by Professor Martin Price, Director of the Centre for Mountain Studies at Perth College, within Scotland's UHI Millennium Institute, 'creating the University of the Highlands and Islands'.

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