Sustainable Mountain Development - Mountain Geography: Physical and Human Dimensions - page 623

2008). Mountain and lowland people have long recognized the importance of mountains
as sources of water by worshipping them as the home of deities and the source of clouds
and rain that feed springs and rivers (Liniger and Weingartner 1998).
While mountain areas occupy only relatively small proportions of most river basins, a
relatively large proportion of the precipitation falls in them due to the orographic effect:
As air rises over the mountains, it cools, releasing the moisture it holds, as discussed in
Chapter 3. The greater height of the mountains is important not only for triggering pre-
cipitation, but also because temperature decreases with altitude. Consequently, there
is less evaporation once the precipitation has fallen, and it is also more likely to fall as
snow than as water—one reason that many of the world's mountains have names that
mean, or include, the local word for “white.” For people living in the lowlands below,
the storage of winter precipitation as snow or ice is crucial, because when temperatures
rise in the spring and summer, the snow and ice melt. The water that is released enters
the rivers, flowing downstream at exactly the time when it is most needed in the low-
lands, sometimes thousands of kilometers away, for irrigation and other uses. A total of
65 countries use over 75 percent of available freshwater for food production, including
China, Egypt, and India, all of which rely heavily on mountain water, much of it from dis-
tant mountain regions. The river basins of these 65 countries cover over 40 percent of
the global land surface and are home to over 50 percent of the global population (Vivir-
oli et al. 2003).
At a global scale, the mountain regions that are most critically important for provid-
ing water for human needs are in the Middle East, South Africa, the western and east-
ern Himalayas, the mountains of Central Asia, and parts of the Rocky Mountains and
the Andes (Viviroli et al. 2007; Viviroli and Weingartner 2008). The greatest importance
of mountain rivers is thus in arid and semiarid regions, where mountains are “wet is-
lands”—often the only areas that receive enough precipitation to generate runoff and
groundwater recharge (Fig. 12.13). Their importance increases in proportion to the dur-
ability and volume of their snowcover and the size of their glaciers (Viviroli et al. 2003).
Mountains in semiarid and arid regions typically provide 70-95 percent of the flow to
nearby lowlands (Fig. 12.13). For example, the watersheds of the Blue Nile and Atbarah
rivers, which rise in the Ethiopian highlands, occupy only 10 percent of the Nile River
basin, but contribute 53 percent of the annual inflow to Lake Nasser—and 90 percent
of the sediment input. The remainder of the inflow comes mainly from the White Nile,
which flows from the mountains of East Africa (Hurni 1998). The semiarid Indus Basin
in Pakistan, one of the largest irrigation areas in the world, which ensures the country's
food supply and generates 23 percent if its GDP, derives 80 percent of its waters from
the Hindu Kush-Himalaya (Immerzeel et al. 2010). In Central Asia, the Tien Shan and
Pamir Mountains occupy only 38 and 69 percent, respectively, of the area of the basins
of the Syr Darya and Amu Darya Rivers, which used to provide 95 percent of the water
flowing into the Aral Sea. The sea has been rapidly drying since the 1960s, and split
into 4 lakes in 2009, having lost 88 percent of its surface and 92 percent of its volume.
The main cause of this decline was excessive irrigation development. In a bid to restore
some of the water body, a dam was completed in 2005 and has been successful in rees-
tablishing the smaller northern part of the Sea (Micklin 2010).