Measures of sustainable water supply generally consider only surface
and subsurface runoff, measured as river discharge (Vörösmarty et al.,
2000). The importance of the dynamic nature of water supplies is revealed
in the fact that the world’s rivers contain about 2,000 km3
of water,
whereas total global annual water demand is almost double this quantity,
at 3,800 km3. However, the important number in this equation is the total
annual fl ow of water, or discharge, which is estimated to be 45,500 km3
per year