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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,