The hydrologic cycle describes the complex and dynamic distribution, transfer and temporary storage of water over and under the Earth's surface. As the term cycle suggests, there is a recurring succession of events (evaporation, condensation, precipitation, surface runoff, subsurface infiltration, and evapotranspiration) that keep nearly all near-surface water in an endless circulation pattern (Figure H5).
Figure H5 The hydrologic cycle. Numbers indicate the amount of water transferred annually (× 103 km3).
All of Earth's surface and near-surface water has accumulated through a gradual release of water from volcanic activity ( juvenile water). The primary reservoir of water is the ocean, which contains greater than 97% of all surface water on Earth. Solar energy evaporates water from the ocean; approximately 13 million metric tons of water per year per average square kilometer are converted to atmospheric