The Water Cycle

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The Water Cycle

The earth is the only planet in the solar system where liquid water is found on the surface. Water is the only substance which, under the ranges of pressure and temperature experienced on earth, is present in solid, liquid and gaseous phase. The water cycle is therefore of fundamental importance to many processes unique to earth. In comparison, the outer planets of our solar system (Saturn, Jupiter, Uranus, Neptune and Pluto) and their moons are too cold to contain water in any form other than ice, the inner planets (Mercury and Venus) are too hot to hold water in any form other than water vapor, and Mars is presently too cold but may have had liquid water on its surface at some point in its history. In the current stage of development of the solar system Earth is the only planet that contains water in all its phases.



Figure 2.1: The solar system.

 

Like many other cycles, the water cycle links processes acting in the living and non-living world: Precipitation and oceanic evaporation link ocean and atmosphere; evaporation from land and transpiration from vegetation link the atmosphere with the biosphere.

In the context of meteorology and oceanography the effect of the biosphere is quantitatively expressed as a single process, evapo-transpiration. The water cycle then describes a basic component of the combined system ocean-atmosphere.

Associated with every cycle is a budget. Cycles represent a qualitative description of processes; budgets turn them into quantitative statements. We distinguish between static budgets, which summarize how much of a particular material is available and how it is distributed between the different compartments, and dynamic budgets, which quantify how rapidly the material is moved between compartments. Cycles define the process; budgets allow answers to questions such as; "How is the water cycle affected if a given percentage of the existing bush land in a particular place is cleared and replaced by wheat farming?"




Figure 2.2: A sketch of the water cycle

Water cycles from the ocean to the atmosphere through evaporation, is transported in condensed form as clouds with the winds and returns to land and water as precipitation. The biosphere plays an important role in the water cycle. Evapo-transpiration from plants is the major component of the water cycle's pathway from the land to the atmosphere; direct evaporation from land is comparatively small. Evaporation from the sea, however, is quite significant. The link between land and ocean is represented by run-off from rivers. This closes the water cycle.

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