ABSTRACT
Approximately 40 million cubic miles of water cover or reside within the Earth. The oceans contain about 97% of all water on the planet. The other 3% is freshwater, comprised of snow and ice on the surface (2.25%), usable groundwater (0.3%), and surface freshwater (0.5%). In the United States, average rainfall is approximately 2.6 feet (a volume of 5900 km3). Of this amount, approximately 71% evaporates (about 4200 km3), and 29% goes to stream flow (about 1700 km3).
