The Water Cycle

Much of environmental chemistry takes place in aqueous systems. Water itself takes part in a cyclic process of evaporation, precipitation, and transport, shown schematically in Figure 9-1. Approximately 5 x 1014m3 (5 x 1017kg) of liquid water evaporates and precipitates annually, with most of this being over the oceans. About 4 x 1013 m3 (4 x 1016 kg) more water falls on land as precipitation than evaporates from it, and this is the annual global volume of freshwater runoff.

Less than 2 x 1013 m3 of water is permanently present in the atmosphere as vapor or cloud. The bulk of all water, more than 97%, is found in the oceans, where it contains much dissolved inorganic material and small but important amounts of organic substances. For many purposes ocean water can be considered in two categories: surface water, in which there is relatively rapid mixing, and a larger amount of deep water, with mixing on much longer time scale. Surface freshwaters (lakes, streams, etc.) are of highly variable


^ Precipitation ^


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