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GLOBAL WATER CYCLE
Introduction
The presence of water as solid, liquid, and gas is a feature that makes the Earth unique in the solar system and that makes possible life as we know it. The biosphere that has evolved on Earth has been strongly influenced by the rather special chemical and physical properties of water, including water's ability to store heat, act as an inert solvent, and transport nutrients. The transport of water and the energy exchanged as it is converted from one state to another are important drivers of our weather and climate system. The water cycle is the largest chemical flux on Earth (see Figure 1)
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.Figure 1.
Water is continually moving around, through and above the Earth as water vapor, liquid water, and ice. In fact,
water is continually changing its form. One should consider the Earth as a "closed system" for the most part, like a
terrarium. That means that the Earth, as a whole, neither gains nor loses much matter, including water. Although some matter, such as meteors from outer space, are captured by Earth, very little of the Earth's substances
escape into outer space. This is certainly true about water. Therefore the same water that existed on Earth millions of years ago is still here. The global water cycle dictates that the same water is continually being recycled
all around the globe.
In today's presentation, we will use a general approach to understanding the global water cycle that can be
applied to any element cycle. This approach consists of three parts: accounting ("where things are"), cycling
("where things are going"), and controls ('what factors control the distribution and the cycles). In addition, we will
summarize some of the tools that are currently being used to study and understand the dynamics of the global water cycle. Using this approach and gaining
knowledge of each of these three components enables you to answer the question of "How will things change?"
Gaining this kind of a predictive understanding of systems is the most important goal in basic scientific research.
Distribution of Global Water
As shown in Table 1 below, over 97 percent of all water on Earth is found in the oceans. Of the remaining 3 percent, most is locked up in glaciers and icecaps in Greenland and Antarctica, in saline inland seas or in the
atmosphere, and is not readily available for consumptive use. Less than 1 percent of available water is usable by humans and other members of the biosphere. Water is continually being shifted (recycled) from one of these
reservoirs to another in the water cycle. The total amount of water in the different reservoirs remains nearly constant with time on a short time scale, but it can change for various reasons. These changes have profound
effects on the biosphere. For example, it is known that the temperature of the Earth can fluctuate on time scales varying from years to centuries to thousands to millions of years. Therefore, both alpine and continental glaciers
have decreased and increased in size as a result of regional and worldwide climatic change. A consequence of these fluctuations in the cryosphere is that the amount of water in each reservoir of the hydrologic cycle has
changed over time.
Table 1. Water Reservoirs
Reservoir Volume (106 km3)*
Ocean 1370
Cryosphere 29
(ice caps and glaciers)
Ground water 9.5
Lakes and streams 0.125
Soils 0.065
Atmosphere 0.013
Rivers 0.0017
Biosphere 0.0006
Total 1408.7
(after Berner and Berner 1996)
km3 = 109 m3
Although oceans and ice caps contain over 99 percent of all water on Earth, the remaining fraction at any given time in the atmosphere, in lakes and streams, and in the soil plays unique and important roles. The flow of water
on the surface is a major determinant of the configuration of the physical environment. Soil moisture is essential to
most terrestrial plant life. The stocks and flows of surface and ground water are major links in the transport and cycling of chemical nutrients (such as nitrogen, phosphorous, and carbon) and important determinants of what
kinds and intensities of human activity can be supported in what locations. Water in the atmosphere has several functions that are central to shaping climate. Next we will turn to the important topic of cycling and describe the
global hydrologic cycle.
Global Hydrologic Cycle
The hydrologic cycle is driven by solar energy and involves water changing its form through the oceans,
atmosphere, land and vegetation, and ice and glaciers. The cycle includes all physical states of water – liquid, solid (ice and snow), and gas (water vapor). It also includes all of the possible transformations among these
states – vaporization or evaporation (liquid to gas), condensation (gas to liquid), melting or fusion (solid to liquid),
and sublimation (gas to solid, or the reverse). The principal flows in the hydrologic cycle are (1) evaporation of water from the surface of the oceans and other bodies of water, and from the soil; (2) transpiration of water by
plants, the result of which is the same as evaporation – namely, the addition of water vapor to the atmosphere; (3) horizontal transport of atmospheric water from one place to another, either as vapor or as the liquid water
droplets and ice crystals in clouds; (4) precipitation, in which atmospheric water vapor condenses (and perhaps freezes) or sublimates and falls on the oceans and the continents as rain, sleet, hail, or snow; and (5) runoff, in
which water that has fallen on the continents finds its way, flowing on and under the Earth's surface, back to the
oceans as return flow. Because it is difficult and not particularly useful to distinguish between the contributions of evaporation and transpiration on the continents, the term evapotranspiration (ET) is generally used.
The magnitudes of these flows, averaged over all the continents and oceans and expressed in thousands of cubic kilometers (103 km3) of water per year, are shown in Figure 2. These magnitudes are based on the assumption
that the various components of the hydrosphere are in equilibrium, which at least is a first approximation. Two values are presented for each element of the hydrologic cycle: an observed value and a mathematical model
calculation. Surprisingly, these two estimates are
generally in good agreement. As shown in the figure, the inflows and outflows from the atmosphere, oceans, and
continents, on a year-round average, are in balance. For example, the atmosphere receives, in 103 km3, 71 + 425 = 496 as evaporation/transpiration from the Earth's surface and gives up 111 + 385 = 496 as precipitation.
The majority of these transformations (precipitation and evaporation) occurs over the oceans. In addition, it should be noted that about 40 x 103 km3 of the total water percolates through the soils as ground water flow or
runs off in rivers and returns to the oceans. Another 40 x 103 km3 is transported in the atmosphere as water vapor, which an important greenhouse gas.
One can gain an idea of how much heat is involved in the water cycle by calculating the amount of heat necessary to evaporate water from the Earth's surface annually. As shown in Figure 2, 496,000 km3 of water are
evaporated from Earth. This evaporation requires: 496,000 km3 x 1015 g/km x 590 calories/g = 2.9 x 10 23 calories of heat. This amount of heat is equivalent to 23 percent of the solar radiation reaching the Earth's outer
atmosphere. Of this heat, 85 percent is used in the
evaporation of the ocean water. When the evaporated water condenses into precipitation, the heat is released to
the atmosphere. The heat involved in the hydrologic cycle and its distribution globally are major variables in the development of weather patterns and climate.
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Figure 2.
Combining the information on equilibrium flows with the information on stocks in the hydrosphere summarized in
Table 1, permits us to estimate the average residence time of water in different parts of the cycle. For example, the ocean residence time: Rt = (1370 x 106 km3) / (0.425 x 106 km3/yr) = ~3,224 years. Using this
relationship, calculate the residence time of water in the atmosphere. These residence times are of great importance in analyzing the transport of pollutants, as well as nutrients, in the hydrologic cycle.
The balance between precipitation and evaporation varies widely from continent to continent, as shown in Table 2. The size of the runoff (the difference between precipitation and evapotranspiration) is a measure of how much
water is potentially available for domestic and industrial uses by society (including dilution and removal of wastes) and for other functions that flowing water performs, such as hydropower. Note in Table 2 the remarkable fact
that South America has a runoff per unit of surface area almost three times that of North America, the continent with the next greatest runoff. It is perhaps not so surprising then that the discharge of the Amazon River, which
drains the wettest third of South America, amounts to a seventh of the runoff of the entire world.
Table 2. Average Water Balance of the Continents
Precipitation Evaporation Area Runoff Total Runoff
Continent (cm/yr) (cm/yr) (cm/yr) (km3/yr)
Africa 69 43 26 7,700
Asia* 60 31 29 13,000
Australia 47 42 5 380
Europe 64 39 25 2,200
No. America 66 32 34 8,100
So. America 163 70 93 16,600
* includes entire Former Soviet Union
Much of the runoff on the continents takes place not on the surface, but beneath it. Although the quantities can only be estimated, it is clear that most rivers receive at least as much of their flow from seepage through the
ground as from flow over the ground. Water beneath the land's surface is called soil moisture or soil water, when
it is distributed in the first meter or so of the soil (a zone defined by the depth of penetration of the roots of most plants). Below the zone of soil moisture is an
intermediate zone where water percolates downward through open pores in the soil and rock; and below this is the water table, marking the surface of the body of ground water that saturates the soil or rock in which it finds
itself. Figure 3 illustrates this process. The ground water extends downward until it is limited by an impermeable layer of rock. In some instances, there are successive layers of ground water (as aquifers) separated by
impermeable layers of rock. The absolute lowest limit of ground water is probably about 16 km from the surface, where the pressure is so great that all pores are closed and any rock becomes impermeable. Most ground water
is flowing, albeit very slowly (10 meters per day) in coarse gravel near the surface and more commonly 1 meter per day and much more slowly at greater depths.
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Figure 3.
Factors Affecting Distribution and Cycling of Global Water
The hydrologic cycle is being affected by human activities, such as water resource exploitation, urbanization, and deforestation. All these control the distribution of water and the cycles. Below we will consider three of these
factors: human consumption, effects of global climate, and land use changes.
1.Human Consumption
Of the water resources listed in Table 1 only the water in fresh water lakes, stream channels, and ground water is directly available for humankind's needs. Whether these resources provide enough water for the daily use of the
present population depends on their local or regional availability (water like other natural resources is not a uniformly available resource), on the style of living, and on the state of industrial development.
Global water demand and consumption by sector are shown in Table 3. You will note that agricultural and industrial uses withdraw about 87 percent of the world's fresh water resources. Irrigation also consumes most of
the globe's fresh water. That is, it generally does not have return flows that can be reused directly. In the United States, fresh water withdrawals in 1995 totaled 341,000 million gallons per day (mgd) or about 471 km3/yr
(Solley, et al., 1998). In Figure 4, the U.S. fresh water withdrawals by sector are shown for 1999. As shown,
average withdrawals are about equally shared by agriculture and thermoelectric power plants. The remaining
sectors make up a much small fraction of the total water use. Figure 5 shows the U.S. consumptive use for the same year. Here you will see that irrigation consumes the vast majority of the U.S. fresh water supply.
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Figure 4.
Table 3. Global Water Demand and Consumption
Demand Consumption
Sector (km3/yr) Percent (km3/yr) Percent
Agriculture 2880 65 1870 82
Industry 975 22 90 4
Domestic 300 7 50 2
Reservoir Losses 275 6 275 12__
Total 4430 2285
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Figure 5.
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Figure 6.
Per capita water demand illustrates another issue as shown in Figure 6. Namely that water usage on a per capita
basis varies greatly across the globe with the U.S. currently withdrawing about 2.5 times more water per capita than Japan, greater than 6 times more than Brazil, and 9 times more than the average for Africa. In the U.S.
about 150 gal are used per person a day for drinking, washing, household needs, cooking, and some irrigation of lawns and gardens (average annual per capita use of heated water for washing, clothes washing, etc. is 64
gal/day). This level of consumption, however, does not include the water used to irrigate the crops grown for
food nor does it include water used in mining, manufacturing, petroleum refining, and electric power plant cooling.
Nationally, it is estimated that the agricultural sector adds about 700 gal/day and the industrial sector about 650 gal/day for a total average per capita of 1500 gal/day. The regional demands may be quite different. For example
in California, the agricultural sector uses about 85 percent of the total demand; municipal is about 10 percent and the remaining is for the industrial sector. California is unique because it is the major producer of many crops for
the U.S. and the world, and because its electric power plants are located along the Pacific Coast and therefore generally do not use fresh water for cooling. Water demand for cooling electric generating plants in inland states
is over 30 percent of total U.S. demand.
During times of low rainfall and streamflow, farmers have relied on ground water sources when available
resulting, in many cases, in "ground water mining" where withdrawals from underground aquifers exceeds replenishment. Ground water overpumping and aquifer depletion are now occurring in may the world's important
crop-producing areas such as the High Plains of Texas, Central Valley of California, Arabian peninsula, African Sahara, India, northern China, and other parts of southeast Asia. This overpumping can also cause salt intrusion
into fresh water aquifers and other salinization problems. Other potential environmental problems include water pollution from sewage treatment, industrial wastes, mining wastes, or other surface drainage such as, non-point
source pollution (mercury from streets, oil and grease, nitrates and pesticides from agricultural operations), special pollutants including detergents, toxic chemicals, trace metals, and insecticides; and acidic water from acid
rain and dry acid deposition.
Globally, the greatest withdrawals of fresh water support croplands for food supplies. Irrigated land currently
accounts for more than 16 percent of the world's cropland. Growing global populations with growing food requirements can only increase the impact on scarce water resources. For example, in 1995 the world consumed
an average of 300 kg of grain per person per year and used about 1 m3 of water per kg of grain produced. If 90 million people are added each year globally, this population growth would require an additional 27 x 109m3/yr or
27 km3/yr. By 2025, an additional 700 km3 of water would be required to maintain the 1995 level of grain production. Looking again at Table 1, what source do you believe will provide this additional demand?
2.Effects of Changing Global Climate
The hydrologic system is potentially very sensitive to changes in climate. Changes in precipitation affect the
magnitude and timing of runoff and the frequency and intensity of floods and droughts. Changes in temperature results in changes in evapotranspiration, soil moisture, and infiltration.
There is some disagreement within the scientific community about surface temperature trends, especially the temperature observations since 1979. On one hand, surface observations for the U.S. with conventional
thermometers show a rise of about 0.1° C per decade over the past century. These data include a detailed record of observations since at least 1850 and are archived in the U.S. Historical Climate Data Network (HCN)
maintained by NOAA's Climatic Data Center in
Asheville, NC. On the other hand, more recent satellite data show no significant warming trend between 1979
and 1997 in the lower troposphere (Christy and Spencer, 1999). Some scientists argue that the satellite record is too short to show a definite trend. This discrepancy in estimating surface temperature trends is one of the major
areas of contention between those who support the global warming hypothesis and those who do not.
Global climate models are being used to estimate the possible range of future climate (temperature and
precipitation) that might result from emissions of various greenhouse gases including carbon dioxide, nitrous oxide, and methane. The temperature range being considered is from 1.5° C to 4.5° C (global warming rates of
between 0.1° and 0.4° C per decade) over the next century. Even with the uncertainty mentioned earlier, it is known that increasing temperature will increase the
rates of evaporation and ice melting, which in turn, can affect future levels of precipitation and sea levels. These
potential climate-related changes will certainly affect the global water cycle. In the U.S., for example, the last two decades have been the wettest this century and since 1900 precipitation has increased about 5 percent across the
country (see Figure7). The increase in precipitation is reflected primarily in the number of extreme daily precipitation events (i.e., greater than 2 inches per
day). This trend toward more intense precipitation is consistent with recent flooding events, such as the 1993 Mississippi floods, the New England floods of 1996, the 1997 spring floods along the Ohio River, and this year's
flooding in North Carolina.
Figure 7.
The El Niño/Southern Oscillation (ENSO) phenomenon contributes seasonal-to-interannual variations in temperature and precipitation that complicate longer-term climate change analysis in certain parts of the world.
Climate anomalies (i.e., departures from the norm) associated with ENSO extremes vary both in magnitude and spatial distribution. For example, the 1990 to 1995 persistent warm-phase of ENSO (which causes droughts and
floods in many areas) was unusual in the context of the last 120 years. Although a relationship has not been found between increasing global temperatures and the occurrence of warm- and cold-phase ENSO events, this climate
phenomenon will certainly affect the content and nature of the global water cycle. This is especially true at the local and regional levels. All major climatechange analyses to date have concluded that if temperature does
increase as a result of increased greenhouse gases, the global mean hydrological cycle will be enhanced and increased precipitation and soil moisture will occur, especially in high latitudes especially during the winter (IPCC,
1996). All these changes are associated with identifiable physical mechanisms.
3. Land Use Changes
Changes in land cover patterns can directly impact energy and mass fluxes. For example, when large areas of forests are cleared, reduced transpiration results in
reduced cloud formation, less rainfall, and increased drying. Changes in land cover can alter the reflectance of the Earth's surface and induce local warming or
cooling; generally as albedo (reflectivity) increases, surface temperature declines. Desertification can occur when overgrazing of savanna vegetation alters surface albedo and surface water budgets, and thus changes the regional
circulation and precipitation patterns. Overgrazing can also increase the amount of suspended dust that, in turn, cause radiative cooling and a decline in precipitation.
Of particular importance is the rate of deforestation across the globe. Deforestation is the process of clearing forests from the land by burning or logging practices that initially were solely for agriculture or settlement
purposes. Although land has been deforested by people throughout the world for thousands of years, most of the destruction occurred after World War II, with much of the intensive deforestation happening since the 1980s.
Most of the deforested land (~60 percent) is associated with tropical wet rain forests in developing countries. In Table 4 are shown estimates of annual forest area changes in developing regions for the period 1980-1995
(FAO, 1997). It was also estimated that the total wooded area in 1995 was about 3450 million hectares (ha).
Table 4. Annual Forest Area Change (1980-95)
Natural forests Total forests*
Regions (million ha**) _ (million ha**)
Africa -8.03 -7.87
Asia-Oceania -8.58 -5.17
Latin America -12.58 -12.25
Developing world -27.54 -25.29___
*Total forests represent differences between deforestation
and establishment of new plantations
** hectare (ha) = 10,000 m3 = 2.471 acres
(FAO, 1997)
Since deforestation of the tropical rain forests represents the largest impact on the world's forested land, we will
briefly consider the ramifications of these reductions on the global water cycle. The amount of water that falls in the rain forest exceeds that of the temperate forests. The water cycle in the rain forest is essentially a closed
system. For example, in the Amazonian rain forest, 75 percent of the water falling as rain evaporates or is transpired directly back to the atmosphere, only to fall again as precipitation. One acre of rain forest releases
approximately 76,000 liters of water per day into the atmosphere to form clouds. These clouds precipitate the water directly back to the forest and provide the abundance of water characteristics of the rain forest. The
elimination of a tropical rain forest will therefore disrupt the regional water cycle. The loss of forests results in the
loss of soil moisture. In deforested areas, less water is evaporated than before deforestation and the recycling of water between land and atmosphere and return is lessened by as much as 75 percent. Thus, deforestation keeps
water from returning to the atmosphere, resulting in changes in a number of characteristics of the watershed.
Harvesting of timber or changing land use from farmland to housing developments can also increase runoff and
cause the magnitude of flooding to be increased.
More development in flood plains and drainage basins can also damage the pattern of water flow by blocking the
flow of water of water and increasing the width, depth, or velocity of flood waters. Ponds, lakes, reservoirs, and other sinks in the watershed also prevent or alter runoff from continuing downstream. Covering land surface's
with asphalt and other impervious surfaces, as evidenced by worldwide trends toward urbanization and urban sprawl, both increase runoff and inhibit replenishment of the ground water reservoirs, and thus affects the overall
water cycle.
Now that the major components of the global water cycle have been briefly summarized we will turn to some of
the modern tools used to study these processes. The emphasis will be on the satellite-based sensors and accompanying models that the National Aeronautics and Space Administration (NASA) and National Oceanic
and Atmospheric Administration (NOAA) have launched since the mid-1970s.
Tools to Understand the Global Water Cycle
Over the years, several NASA missions have studied the effects associated with the global water cycle including El Niño and La Niña, such as changes in sea
surface temperature, cloud cover, ocean surface winds, and rainfall. These studies are augmented by data from operational satellites of NOAA. We provide a few examples below.
Initial efforts at mapping sea surface temperature and cloud cover were conducted using data from NASA's Nimbus series of satellites. Also this series included the first ocean color scanner (Coastal Zone Color Scanner),
which provided the first estimates of phytoplankton productivity (chlorophyll concentrations) from space. The Advanced Very High Resolution Radiometer (AVHRR) instrument flown on NOAA's TIROS-N weather
satellite in 1978 and on the NOAA-6 satellite in 1979 greatly enhanced the accurate measurements of factors related to climate variability. Still further increases were added to the AVHRR instrument and on subsequent
NOAA satellites.