uncertain since small pelagic biomass declines steadily to­ward the end of the modeled time period and the biomass of the large bentho-pelagic fish also declines.

5.3.3.2 Global trends in water availability and emerging challenges to water supply

Changes in human use of freshwater are driven by population growth, economic development and changes in water use ef­ficiency. Historically, global freshwater use had increased at a rate of about 20% per decade between 1960 and 2000, with considerable regional variations due to different devel­opment pressures and efficiency changes. Because of uneven distribution of fresh water in space and time, however, to­day only 15% of the world population lives with relative water abundance, and the majority is left with moderate to severe water stress (Vorosmarty et al., 2005).

     This global water picture may be worsened in the future under climate change and population growth. For the refer­ence run, by 2050 internal renewable water (IRW) is esti­mated to increase in developed countries but is expected to decrease in the group of developing countries (Table 5-12). The disparity of changes of IRW and population by 2050 will increase the challenge to satisfy future water demands, especially for the group of developing countries.

     Table 5-13 presents total water consumption, which re­fers to the volume of water that is permanently lost (through evapotranspiration or flow to salty sinks, etc.) and cannot be reused in the water system, typically a river basin. In the reference world, by 2050 world water consumption is expected to grow by 14%. Regionally, by 2050, SSA is projected to more than double water consumption, LAC is expected to increase water consumption by 50%, and ESAP by 13%, while in the NAE region the increase is modest, at 6%. Only CWANA reduces its water consumption—as a re­sult of further worsened water scarcity. The IRW reduction of CWANA makes its global share of IRW decrease from 3.2% to 2.5% (Table 5-12). Combined with the increase of

population share from 10% to 13% CWANA is expected to face the largest challenge in meeting demands exerted by socioeconomic development and conservation demands to sustain ecological systems.

     Irrigation is expected to continue to be the largest wa­ter user in 2050 for all regions (Table 5-14). However, it is estimated that the share of irrigation consumption in total water depletion will decrease by about 8% from 2000 to 2050, largely due to the more rapid growth of nonirriga-tion water demands that compete for water with irrigation (Table 5-15), and also because of projected declines in ir­rigated areas in some parts of the world. Actual irrigation consumption will decline significantly in CWANA due to chronically worsening water scarcity in the reference run. For individual dry river basins the IWSR could be even lower than these spatially-averaged values since abundant water in some basins mask scarcity in the dry river basins. On the other hand, significant increases are expected in the LAC and SSA regions at 45% and 77% respectively.

    Constraints to water supply vary across regions. Water shortages today and in the future are not solely problems of resource scarcity, but are also closely related to stages of economic development. Three types of water scarcity constraints will become more important in the future: first is absolute resource scarcity, which will become more and more a feature of regions characterized by low and highly variable rainfall and runoff, often accompanied by high evapotranspiration potential. They include countries and subnational regions in CWANA, ESAP (for example, north­western China), and NAE (for example, southwestern US), among others. The second type is infrastructure constraints, typically in regions where water availability is not extremely low but infrastructures to store, divert/pump, and convey water is underdeveloped. Despite rapid development of ir­rigation-related and other water infrastructure in the SSA region, the region will remain infrastructure constrained out to 2050. The third type, water scarcity, is caused by water