• Global Water Distribution
• Global Water Reverse Osmosis

Dr. Raymond W. Schmitt

Global Logger Software: Instructions for installing drivers on Windows 8 software Global Logger Software: Procedure for calibrating the bar graph to a higher or lower range Global Logger Software: Calibrating Level sensors for depth to water readings Global Logger II Communication protocol 407119 Hot Wire Anemometers Data Acquisition Software. It is projected that global water demand will reach 4,350 billion cubic meters in terms of withdrawal by 2040. In the last few decades, the growth in water demand has doubled that of population. Figure 05: Global and regional trends in water supply in 2008.13 Figure 06: Global and regional trends in sanitation practices in 2008.14 Figure 07: Change in water withdrawal and the global population, 1950-2025.15 Figure 08: Trends in urban population density. Global is a leader in the marine services industry with decades of experience managing complex, critical and technical projects in and around the water. We bring the expertise and resources necessary to get the job done quickly, reliably and safely – even in the most challenging environments.

A look at the Earth from space reveals a mostly blue planet. The domination of earth’s surface by the oceans is quite clear. As Arthur C. Clark once said, 'How inappropriate to call this planet Earth when clearly it is Ocean”.This deep blue mass of water has major implications for the climate of the Earth. It means that most of the Sun’s energy is absorbed into the ocean. It also means that this fluid body, with its huge heat capacity (1100 times that of the atmosphere) is the main reservoir for heat on the planet. And of course the ocean is the primary reservoir for water. Most of the water we experience as rain has come from the ocean, which is the principal source of water for the atmosphere.

A Traditional Depiction of the Global Water Cycle

Salinity and Evaporation - Precipitation

In contact with both the oceans and the continents, the thin, transparent atmosphere acts as a conduit connecting the terrestrial and oceanic components of the water cycle. Figure 2 is a traditional depiction of the global water cycle as viewed by hydrologists. It focuses on the processes of most interest to society, but gives a distorted picture of the role of the oceans. Relegating the oceans to a small corner of the diagram conveys a very inaccurate picture. In fact, the oceans cover the majority of the earth and hold 97% of the free (not chemically bound) water on the planet, comprising both the primary source of water evaporated into the atmosphere and the main site of precipitation. We estimate that 86% of global evaporation and 78% of global precipitation occurs over the oceans. Surely, the oceans deserve a more prominent place in any depiction of the water cycle!

A Better Perspective

Relative Roles

A look at the volumes associated with the components of the water cycle (Figure 3) leads to a better perspective on the relative roles of ocean atmosphere and land. To focus only upon terrestrial fluxes, ignoring the oceanic component, would overlook the ocean-atmosphere interface that plays a critical role in maintaining the terrestrial moisture balance (Figure 3). The oceans function as a reservoir and buffer in the planetary circulation of water. Storing 23 times the water on land and a million times the water in the atmosphere, the ocean’s air-sea fluxes are many times larger than the terrestrial equivalents.

Past Limitations

Until recently, the vastness of the world’s oceans and the absence of rainfall and evaporation sensors represented a barrier to the study of the oceanic hydrologic cycle. These limitations of the past left a void in observations of precipitation or evaporation over the ocean that made quantifying the water cycle over the ocean difficult. And mankind’s natural focus on its own water supply meant the large oceanic water cycle received little attention. However, new satellite observations of rainfall and improved data sets of meteorological conditions over the oceans have afforded us an improved understanding of the oceanic water cycle.

Evaporation and Precipitation

Modern Estimates

We have compiled modern estimates of evaporation and precipitation over the ocean to develop a more complete picture of water fluxes across the air-sea interface.Taking the difference between evaporation (E) and precipitation (P) tells us the net gain or loss of water by the ocean.A new map of E-P shows where water evaporates (red) and precipitates (blue) (Figure 4.) The mid-latitude regions of the oceans tend to be dominated by evaporation under the trade winds. These regions supply water to the atmosphere, which transports it to a low latitude rainfall region known as the “Inter-tropical Convergence Zone” (ITCZ). This is where rising warm, moist air drops its moisture burden back on the sea. The high latitude oceans also tend to gain more water from rain than is lost to evaporation.

An Alternative Schematic

This pattern of mid-latitude evaporation and high and low-latitude precipitation leads us to propose an alternative schematic for the “Global Water Cycle” (Figure 5). It more accurately depicts the dominance of the ocean-atmosphere processes, and delegates land processes to a more suitable minor role. It is drawn as a north-south section and shows the atmosphere carrying water vapor from evaporation to precipitation regions. It also shows the complementary return flows induced in the ocean. Without those oceanic return flows, regional sea level trends of up to 10 meters per year would develop! This diagram is an oceanographer’s view of the global water cycle.

Global Water Distribution

Oceanic Fluxes

Our estimates of the flux of water into and out of the ocean can be integrated over its surface all around the globe. This provides a view of the strength of the oceanic return flows (Figure 6). Similarly, measurements of the strength of north-south winds and their moisture content allow estimates of the meridional transport of water by the atmosphere. The two curves are largely complementary, showing that it is primarily the oceans that maintain the water balance. One can also see from the curve labeled “Mississippi” that north-south transports of water by rivers are quite small by comparison. Thus, in discussing the “Global Water Cycle”, it is truly misleading to neglect the dominant oceanic component.

Salinity

The oceanographer does have one tool to help understand the poorly sampled water cycle over the ocean: the oceanic salt content or salinity. Evaporation leaves salt behind, thereby increasing salinity; precipitation dilutes the salt content and freshens surface waters. Salinity can be thought of as analogous to humidity in the atmosphere. The general patterns of surface salinity reflect the workings of the global water cycle, the mid-latitude evaporation zones have high salinity, while precipitation zones at high and low latitudes have low salinity (Figure 7). Salinity is much harder to measure than temperature, so there is not nearly so much historical data to examine for climatological trends. However, we have reason to be optimistic about the future; the ARGO program of profiling floats is rapidly expanding the number of salinity profiles available around the globe ( http://www.argo.ucsd.edu/ ), a new salinity-sensing satellite (AQUARIUS https://www.nasa.gov/mission_pages/aquarius/overview/index.html ) is due to be launched in 2010, and here at WHOI we have been developing new foul-proof salinity sensors for deployment on surface drifters and moorings. One of the most striking features of the surface salinity pattern is the higher salinity of the Atlantic Ocean. This may be largely due to loss of water vapor to the Pacific across Central America, and the lack of water supply from the Sahara. These higher salinities mean higher seawater densities, so that the North Atlantic hosts a main sinking site of the meridional overturning circulation. There surface waters flow north to become cooled by the atmosphere to become dense water which sinks to depth and flows southward. A great deal of heat transport is associated with this overturning circulation, which is believed to have been disrupted by glacial meltwater in the past. Thus, salinity is intimately tied to the general circulation of the ocean and the climate of Earth.

We acknowledge the National Science Foundation for generous support of this research.

The Water Cycle and Global Warming

One aspect of concern for the future is how the water cycle will respond to global warming. This is dictated by the physics of water vapor in the air. A warm atmosphere can hold much more water vapor than a cold atmosphere (something we all experience with seasonal humidity changes). The water carrying capacity of the air is governed by the vapor pressure dependence on temperature, plotted here in Figure 8. We see a strong non-linear dependence of vapor pressure on temperature. At the present mean global surface air temperature of 14 degrees C, the vapor pressure increases about 7% for each one degree increase in temperature. Other things being equal (winds, cloudiness etc) we may expect an intensification of the water cycle of 7% per degree of global warming. This means more intense droughts in some regions, stronger rainfall in others. In many ways, society is more vulnerable to changes in the water cycle than to temperature changes alone. Since most of the water cycle is between atmosphere and ocean, we expect that the oceanic salinity field will be an excellent indicator of a changing water cycle. We thus view it as an especially important climate variable to monitor.

'Water, Water, Everywhere...' You've heard the phrase, and for water, it really is true. Earth's water is (almost) everywhere: above the Earth in the air and clouds and on the surface of the Earth in rivers, oceans, ice, plants, and in living organisms. But did you know that water is also inside the Earth? Read on to learn more.

• Water Science School HOME • Water Basics topics • The Water Cycle •

Where is Earth's Water?

'Water, Water, Everywhere....'
You've heard the phrase, and for water, it really is true. Earth's water is (almost) everywhere: above the Earth in the air and clouds, on the surface of the Earth in rivers, oceans, ice, plants, in living organisms, and inside the Earth in the top few miles of the ground.

For an estimated explanation of where Earth's water exists, look at this bar chart. You may know that the water cycle describes the movement of Earth's water, so realize that the chart and table below represent the presence of Earth's water at a single point in time. If you check back in a million years, no doubt these numbers will be different!

• Left bar: All water, freshwater and saline, on, in, and above the Earth.
• Center bar: All freshwater
• Right bar: Only the portion of freshwater residing in surface water (rivers and lakes, etc), snow and ice, and relatively-shallow ground water.

Here is a bar chart showing where all water on, in, and above the Earth exists. The left-side bar chart shows how almost all of Earth's water is saline and is found in the oceans. Of the small amount that is actually freshwater, only a relatively small portion is available to sustain human, plant, and animal life.

Notice how of the world's total water supply of about 332.5 million cubic miles of water, over 96 percent is saline. And, of the total freshwater, over 68 percent is locked up in ice and glaciers. Another 30 percent of freshwater is in the ground. Fresh surface-water sources, such as rivers and lakes, only constitute about 22,300 cubic miles (93,100 cubic kilometers), which is about 1/150th of one percent of total water. Yet, rivers and lakes are the sources of most of the water people use everyday.

• In the first bar, notice how only 2.5% of Earth's water is freshwater - the amount needed for life to survive.
• The middle bar shows the breakdown of freshwater. Almost all of it is locked up in ice and in the ground. Only a little more than 1.2% of all freshwater is surface water, which serves most of life's needs.
• The right bar shows the breakdown of surface freshwater. Most of this water is locked up in ice, and another 20.9% is found in lakes. Rivers make up 0.49% of surface freshwater. Although rivers account for only a small amount of freshwater, this is where humans get a large portion of their water.

One estimate of global water distribution
(Percents are rounded, so will not add to 100)

Source: Igor Shiklomanov's chapter 'World fresh water resources' in Peter H. Gleick (editor), 1993, Water in Crisis: A Guide to the World's Fresh Water Resources (Oxford University Press, New York).

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