Water Source and Storage Calculations
Approximately 40 million cubic miles of water cover or reside within the Earth. The oceans contain about 97% of all water on Earth. The other 3% is freshwater: (1) snow and ice on the surface of earth account for about 2.25% of the water; (2) usable groundwater is approximately 0.3%, and (3) surface freshwater is less than 0.5%. In the U.S., average rainfall per year is approximately 2.6 feet (a volume of 5900 cubic kilometers, or km
). Of this amount, approximately 71% evaporates (about 4200 cubic km), and 29% goes to stream flow (about 1700 cubic km).