49 / 104
49
This also includes supply to regions of Central Asia, China and
Pakistan, which are under direct water stress today.
A decline of 10–30% in irrigated yields in the basins originat-
ing from the mountains of the Himalayas and Central Asia
corresponds to 1.7–5.0% of the world cereal production (see
box). A 10–30% yield loss due to lower availability of water for
irrigation (without increased water efficiency) on the world’s
irrigated croplands would equate to losses in the range of
4–12% of world cereal production. In many regions, greater
losses have already been observed due to over-extraction of
water resources from aquifers and rivers. Studies suggest that
almost half of the irrigation water comes from non-renewable
and non-local sources (Rost
et al
., 2008). Indeed, river dis-
charge is decreasing in many areas mainly as a result of an-
thropogenic use, particularly irrigation (Gerten
et al
., 2008).
Currently, an estimated 24% of the world river basin area has
a withdrawal/availability ratio greater than 0.4, which some
experts consider to be a rough indication of “severe water
stress”. Under a “business-as-usual” scenario of continuing
demographic, economic and technological trends up to 2025,
water withdrawals are expected to increase in 59% of world
river basin area, outweighing the assumed improvements in
water-use efficiency, although with great geographic variation
(Alcamo
et al
., 2003). On the assumption that the melting gla-
ciers would cause reduced production by 2050, as indicated,