Estimates of global water resources based on several different calculation methods have produced varied estimates. Shiklomanov in Gleick (1993) estimated that:
- The total volume of water on earth is 1.4 billion km3.
- The volume of freshwater resources is 35 million km3, or about 2.5% of the total volume. Of these, 24 million km3 or 68.9% is in the form of ice and permanent snow cover in mountainous regions, and in the Antarctic and Arctic regions.
- Some 8 million km3 or 30.8% is stored underground in the form of groundwater (shallow and deep groundwater basins up to 2,000 metres, soil moisture, swamp water and permafrost). This constitutes about 97% of all the freshwater potentially available for human use.
- Freshwater lakes and rivers contain an estimated 105,000 km3 or 0.3% of the world’s freshwater.
- The total usable freshwater supply for ecosystems and humans is 200,000 km3 of water, which is less than 1% of all freshwater resources, and only 0.01% of all the water on earth (Gleick, 1993; Shiklomanov, 1999).