<P> The total volume of water in rivers is estimated at 2,120 km3 (510 cubic miles), or 2% of the surface fresh water on Earth . Rivers and basins are often compared not according to their static volume, but to their flow of water, or surface runoff . The distribution of river runoff across the Earth's surface is very uneven . </P> <Table> <Tr> <Th> Continent or region </Th> <Th> River runoff (km3 / year) </Th> <Th> Percent of world total </Th> </Tr> <Tr> <Td> North America </Td> <Td> 7,800 </Td> <Td> 17.9 </Td> </Tr> <Tr> <Td> South America </Td> <Td> 12,000 </Td> <Td> 27.6 </Td> </Tr> <Tr> <Td> Europe </Td> <Td> 2,900 </Td> <Td> 6.7 </Td> </Tr> <Tr> <Td> Middle East and North Africa </Td> <Td> 140 </Td> <Td> 0.3 </Td> </Tr> <Tr> <Td> Sub-Saharan Africa </Td> <Td> 4,000 </Td> <Td> 9.2 </Td> </Tr> <Tr> <Td> Asia (excluding Middle East) </Td> <Td> 13,300 </Td> <Td> 30.6 </Td> </Tr> <Tr> <Td> Australia </Td> <Td> 440 </Td> <Td> 1.0 </Td> </Tr> <Tr> <Td> Oceania </Td> <Td> 6,500 </Td> <Td> 14.9 </Td> </Tr> </Table> <Tr> <Th> Continent or region </Th> <Th> River runoff (km3 / year) </Th> <Th> Percent of world total </Th> </Tr> <Tr> <Td> North America </Td> <Td> 7,800 </Td> <Td> 17.9 </Td> </Tr>

Where is 70 of earth's fresh water stored in lakes in rivers in groundwater in glaciers