ABSTRACT
Fresh water is a scarce resource on a worldwide basis. This becomes apparent when looking at the basic items of the global freshwater balance (Postel et al., 1996). Of the 110,000km3/a of precipitation on the landmass of the earth, 50,000 are returned to the atmosphere via evapotranspiration by the planet’s natural plant cover. Another 21,000 are used by man-made ecosystems (18,000km3/a by rain fed agriculture and 3,000km3/a by irrigated agriculture). This shows that agriculture and natural vegetation are already fierce competitors for the available freshwater. Of the accessible runoff of 13,000km3/a about 4,000 are appropriated by mankind. 70% of those go into irrigated agriculture. This means that a global water crisis would above all be a global crisis in food production. Compared to the 13,000km3/a available the abstracted 4,000 appear small. One should, however, not forget that these figures are averaged in time and space and therefore hide the real problem, e.g. droughts and floods. One can still use the ratio a of withdrawals and available renewable resources as an indicator. Due to the variability of the quantities involved, it is the experience that a value of a>0.4 already reflects severe scarcity. On a global scale a=0.31 is found (Alcamo et al., 2003). This indicates that scarcity on a global level is a reality today, with the arid world already experiencing very severe scarcity problems.
