Water occurs as a liquid, solid, or vapor within one of many reservoirs in the Earth system. Incoming solar radiation provides the energy for bringing about changes in the physical phases of water and the exchange of water between these reservoirs. This exchange constitutes the global water cycle. Earth's water cycle is an extremely important factor, not only in directly sustaining life, but permitting the planet to remain a habitable place, by moderating the flow of solar energy into the planetary system, the flow of long wave (infrared) radiation out of the system, and the distribution of energy within the system.
Various disciplines within the Earth sciences focus upon certain aspects of the global water cycle. Meteorologists usually are concerned primarily with water in the atmosphere, precipitation, and to a certain extent, evaporation--the primary mechanisms involved in the exchange of water between the atmospheric reservoir and reservoirs on Earth's surface. Besides monitoring the extent of clouds and the amount of water vapor in the atmosphere, meteorologists routinely measure precipitation, the depth of water accumulated on a unit area from rainfall or snowfall in 24 hours, a month, or a year.
On the other hand, hydrologists, concerned with water on or under Earth's surface, typically focus upon exchange processes operating on land. Often, they are concerned with stages and flow rates in rivers and streams and the supply of soil moisture and groundwater.
One method used to quantify the flow of water between various reservoirs on the planet is mass budget. Like the financial budget that many of us try to maintain in our personal lives, the mass budget can simply be stated as "the gain (input) is equal to the loss (output) plus storage." On Earth's surface we can rephrase this statement to say that any water gained by precipitation in a given time interval is balanced by the loss due to evaporation and surface runoff, plus any storage associated with a change in the water table, glacial ice cover, lake or ocean levels.
We could apply the mass budget approach to any size system-- ranging from a cornfield covering several hectares to the Mississippi River watershed enveloping a large portion of the nation east of the Rockies, or to the entire planet. An energy budget is also associated with the mass budget of water. Because energy can neither be created nor destroyed (although changed in form), an energy budget is also an integral part of the global water cycle. Solar energy powers evaporation at Earth's surface and that energy in the form of heat is released to the atmosphere during condensation, deposition, and the formation of clouds. Potential energy at cloud level becomes kinetic energy as raindrops or snowflakes fall from clouds to Earth's surface and replenish terrestrial and oceanic reservoirs of water.
Let us consider the entire planetary system on a long-term basis as represented by annual averages of the flow rates of precipitation and evaporation. On this time scale, we can assume that the gain of water substance at the surface, as represented by the annual precipitation averaged over the globe, should equal the annual loss through evapotranspiration (the combination of direct evaporation plus transpiration by plants). This assumption is based on the fact that we cannot detect large changes in the planet's stored water over time.
The annual average precipitation for the entire planet is approximately 33 in. (83 cm), with the same amount of evaporation taking place annually. Obviously, a specific locale can depart greatly from these annual global averages, with some desert locations such as Death Valley, CA receiving minuscule amounts of precipitation annually, whereas some highland stations on Hawaii receive over 200 in. (5 m) per year. Usually, more evaporation takes place over the ocean, while more precipitation typically takes place over the land. (See Table 3.2 of the DataStreme WES textbook). The differences between rates over land and ocean are reconciled by net runoff of the excess rainwater from the continent by rivers, and net onshore flow of moisture aloft in the atmosphere.
The study of the anomalous atmospheric and oceanic circulation patterns associated with 1997-1998 El Niño episode reveals that some locales on the planet received much greater amounts of precipitation compared to average while others received considerably less. Specifically, winter storms that battered the West Coast were responsible for winter precipitation totals that were more than double the long-term climatological average for these locales. However, locations in Australia, Indonesia, and Korea were but some of the many places that experienced severe drought.
How fast does water cycle through a particular reservoir? Let us trace an "average water molecule" that is free to move between any of the reservoirs in the global water cycle. The average residence time that this water molecule would spend in any particular reservoir depends upon the combined effect of reservoir size (as indicated in Table 3.1 of the DataStreme WES textbook) and the rates at which water either replenishes or depletes the reservoir.
Applying mass budgeting techniques to each reservoir in the global water cycle, water molecules would cycle through clouds most rapidly, spending an average of only 1.3 hrs in a cloud as a cloud droplet or ice crystal and between 9 and 10 days as a water vapor molecule in the atmosphere. On Earth's surface, our water molecule would only remain for approximately 2 weeks, but as ground water, it would remain in the top soil for approximately 3 months and as much as 10,000 years in deep aquifers. Because of the immense size of the ocean, the molecule could spend an average 3300 years in the world ocean. The longest recycling time for a water molecule would be in the polar ice caps and glaciers of the world, where the average residence time is on the order of 11,500 years.
These average residence times verify our observation that clouds are indeed short lived, with new water molecules from the atmosphere rapidly replenishing the water droplets and ice crystals that are continually removed either from the cloud base by precipitation (as rain or snow) or by evaporation (or sublimation) into the atmosphere. The water vapor in the atmosphere recycles on the same time scales as those average time scales that we associate with weather systems that are plotted on weather maps. The times needed for the water to pass through the ocean and the ice caps (part of the cryosphere) are on the same time scales as those usually associated with long term climatic change, such as experienced since the last large scale glaciation of the Northern Hemisphere some 11,000 years ago.
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Prepared by Edward J. Hopkins, Ph.D., email
hopkins@meteor.wisc.edu
© Copyright, 2008, The American Meteorological Society.