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 originating 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 discharge is decreasing in many areas mainly as a result of anthropogenic 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 glaciers would cause reduced production by 2050, as indicated,
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