162 The impactsof climate changeGCM–3 –1 –0.2 0 0.2 1 3RCMPrecipitation (mm day−^1 )Figure 7.8Predicted
changes in monsoon rainfall
(mm/day) over India between
the present day and the
middle of the twenty-first
century from a 300-km
resolution GCM and from a
50-km resolution RCM. The
RCM pattern is very different
in some respects from the
coarser resolution pattern of
the GCM.
Bangladesh where the projected increase is about twenty per cent. What
is urgently required for this part of the world and elsewhere is much
better information linking changes in average parameters with likely
changes in frequency, intensity and location of extreme events.
There is another reason, not unconnected with global warming, for
the vulnerabilityof water supplies: the link between rainfall and changes
in land use. Extensive deforestation can lead to large changes in rainfall
(see box on page 173). A similar tendency to reduced rainfall can be
expected if there is a reduction in vegetation over large areas of semi-
arid regions. Such changes can have a devastating and widespread effect
and assist in the process of desertification. This is a potential threat to
the drylands covering about one-quarter of the land area of the world
(see box on page 163).
What sort of action can be taken to lessen the vulnerability of hu-
man communities to changes in water availability or supply? Irrigation
accounts for about two-thirds of world water use, and is of great im-
portance to world agriculture. Irrigation is applied to about one-sixth
of the world’s farmland, which produces about one-third of the world’s
crops. In some areas the ratio is much higher; for instance, over eighty
per cent of the agricultural land in California is irrigated. Most irriga-
tion is through open ditches, which is very wasteful of water; over sixty
per cent is lost through evaporation and seepage. Microirrigation tech-
niques, in which perforated pipes deliver water directly to the plants,
provide large opportunities for water conservation, making it possible
to expand irrigated fields without building new dams.^30 Management of
the existing infrastructure can be improved, for instance by arranging for
the integration of different supplies, and conservation in the domestic
and industrial sectors can be encouraged. Most of these actions will cost