Water resides as a liquid, solid or vapor within one of several reservoirs constituting our planetary system. Incoming solar radiation provides the energy for changes in the physical phase of the water and an exchange of water between these reservoirs. This exchange is often called the hydrologic cycle. The earth's hydrologic cycle is an extremely important factor, not only 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 radiation out of the system, and the distribution of energy within the planetary system.
Various disciplines within the earth sciences focus upon certain aspects of the water cycle. Meteorologists usually focus primarily upon water in the atmosphere, precipitation and, to a certain extent, evaporation - the primary mechanisms involving the exchange of water between the atmospheric reservoir and those reservoirs that reside on the earth's surface. Besides considering 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 or a month or a year.
On the other hand, hydrologists, concerned with water on or under the earth's surface, typically focus upon exchange processes on land. Often, they are concerned with flow rates in rivers and the amount of soil moisture that affects the water table.
One method used to quantify the flow of water between various reservoirs on the planet is the 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 the earth's surface we can rephrase this statement to say that any water gained by precipitation would be balanced by the loss due to evaporation and surface runoff, plus any storage term which would be associated with either a change in the soil's water table or a change in lake or ocean levels.
We could apply this type of mass budget approach to any size system - ranging from the size of a corn field through the Mississippi River watershed that covers a large portion of the nation, to the entire planet.
Let us consider the entire planetary system on a long term basis as represented by annual averages. Over this time scale, we can assume that the gain to the surface, as represented by the annual precipitation averaged over the globe should equal the loss through evapotranspiration, since we cannot detect large changes in stored water over time.
The annual average precipitation for the entire planet is approximately 33 inches (83 centimeters), with the same amount of evaporation taking place annually. Obviously, any locale can depart greatly from these annual global averages, with some desert locales such as Death Valley, CA receiving minuscule amounts of precipitation annually, while some stations on Hawaii receive over 200 inches (5 meters) per year. Usually, more evaporation usually takes place over the oceans, while more precipitation typically takes place over the land. The differences between rates over land and ocean are reconciled by a net runoff of the excess rainwater from the continent by rivers, and a net onshore flow of moisture in the atmosphere.
The study of the anomalous atmospheric and oceanic circulation patterns associated with 1997-1998 El Niño episode reveals that since last summer some locales on the planet received much greater amounts of precipitation 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 locales that were undergoing severe drought conditions.
So 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 hydrologic reservoirs. 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 6.1 in the Online Weather Study Guide) and the rates at which water either replenishes or depletes the reservoir.
Applying mass budgeting techniques to each hydrologic reservoir, water molecules would cycle through the clouds most rapidly, spending an average of only 1.3 hours in a cloud as a cloud droplet or ice crystal and between 9 to 10 days as a water vapor molecule in the atmosphere. On the 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 oceans, the molecule could spend an average 3300 years in the world oceans. 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 would be on the order of 11,500 years.
These average residence times verify our observations 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 from the cloud base by precipitation (as rain or snow) and 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 appear on weather maps. The times needed for the water to pass through the ocean and the ice caps (sometimes called 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, 1999, The American Meteorological Society.