The impact ofclimate change onfresh water resources 159summer runoff still remains well below normal. Watersheds in arid or
semi-arid regions are especially sensitive because the annual runoff is in
any case highly variable.
Some watersheds in mid latitudes in the Northern Hemisphere, where
snowmelt is an important source of runoff, can also be severely affected.
For these places, as temperatures rise, winter runoff will increase sub-
stantially and spring high water will be much reduced. Further, as we
saw earlier in the chapter, up to one-half of the mass of mountain glaciers
and small ice caps may melt away over the next hundred years which
could substantially change the seasonal distribution of river flow and
water supply for hydroelectric generation and agriculture.
A detailed hydrological study of Asia under climate change illus-
trates the very different impacts on different parts of the continent.^24 Pro-
jections from a climate model for 2050 under a scenario similar to A1B
were introduced into a hydrological model of different river catchments
and the changes in runoff in the river basins estimated. In arid or semi-
arid regions of Asia, surface runoff is expected to decrease drastically
so affecting the volume of water available for irrigation and other pur-
poses. Average annual runoff in the basins of the Tigris, Euphrates, Indus
and Brahmaputra rivers was estimated to decline by twenty-two, twenty-
five, twenty-seven and fourteen per cent respectively. Other areas will
experience substantial increases in runoff, for instance by thirty-seven
and twenty-six per cent, respectively, in the Yangtze (Changjiang) and
Huang He rivers. Substantial increases were also projected for Siberian
rivers.
Watersheds that are particularly vulnerable to climate change can be
identified by asking certain questions about them.^25
How much water storage is there in the watershed relative to the annual
flow? In Colorado in the United States, for instance, the storage is four
times the annual flow, whereas in the Atlantic States it is only one-tenth
of the annual flow.
How large is the demand as a percentage of the potential supply? This
varies a great deal. For instance, in North America, for the Rio Grande
and for the lower Colorado demand approximately equals supply and
very little of either of these rivers reaches the sea. Therefore, though
the Colorado has substantial storage and is therefore not very sensitive
to annual variations, the amount of use in its lower reaches means that,
over a number of years, any reduction of its flow is bound to imply
lower water availability.
How much groundwater is being used? There are many places in the
world where groundwater is being used faster than it is being replen-
ished. To give two examples, for more than half the land area of the