Hydrologic Cycle

Water is continuously involved in transport and exchanges between ocean, land, and atmosphere: this is the hydrologic cycle having no beginning or end. Earth is covered by a world ocean energized by solar energy and causing the cycling of water out of and back into the oceanic reservoir. Water’s ready ability to gain and lose energy and change the spatial arrangement between its molecules makes this possible. As water is able to circulate around the planet it is easy to appreciate the importance of the hydrologic cycle that provides precipitation to continental interiors. Out of sight to the casual observer is water’s role in transporting latent heat thus having a major impact on the global energy balance and providing energy needed to create atmospheric disturbances such as thunderstorms and hurricanes. Of tremendous importance is water vapor’s major role in causing the greenhouse effect.

Homer’s Iliad, written in the eighth century BCE, provided the first recorded notion of the connected nature of water over the planet. It remained for modern science to measure the various components with instruments. Although all of non-ocean water is about 3 percent of the world’s supply, the hydrologic cycle moves huge quantities of water over time. Annually, the cycle has been estimated to have over 400,000 cubic kilometers of water entering the atmosphere. The hydrologic cycle is not to be viewed as a smooth transference of water but works in “fits and starts.” One week the Great Plains of North America might receive flooding rains from the Gulf of Mexico and the next several weeks can be very dry with the steering patterns of the upper troposphere blocking the flow of lowlevel moisture. So, too, the hydrologic cycle has geographic variations of its components.

For instance, the evaporation may be prodigious from a tropical ocean surface, whereas cold temperatures and ice in polar waters make the hydrologic cycle much more sluggish. At any slice of time, 97 percent of Earth’s water is in the ocean. The ocean is large compared to the total surface of the planet and also quite deep. The deep ocean water is away from the direct influences of the atmosphere. It is in the ocean for thousands of years at a time because of its very slow circulation; in spatially limited upwelling areas these bottom waters come to the top. Near the surface the waters are mixed by wind and heated by solar radiation so these waters have a shorter average residence time.