About 15 million km2 of the land surface is covered by crop-land,[REF] and about 16% of this area is equipped for irrigation (Siebert et al. 2005[REF]).
During the last century, the rate of increase in `blue' water withdrawal (from rivers, lakes, and aquifers) for irrigation and other purposes was higher than the growth rate of the world population (Shiklomanov 1998[REF]).