Water and Population Centers

Some cities are sited in areas where water is available - or was at the time they were settled - including Las Vegas, Los Angeles, Chicago, St. Louis, and Pittsburgh (Figure 4). In some cases, and as we will discuss in detail in later modules in the course, rapid development and growing demand can outpace the original and limited water source for a city or region, leading to a vicious cycle of water acquisition, growth enabled by water availability, and subsequent water stress.

Figure 4: Night image of the continental U.S. from data acquired by the Suomi NPP satellite in 2012.

Source: NASA Earth Observatory

Figure 5: Annual precipitation for 2013, overlain on the same nighttime image.

Source: NOAA [7]

Many of America’s major manufacturing centers (i.e. the rust belt) are located in areas where major rivers and canals provided a means for transport of raw materials and goods, power generation, water supply for processing and cooling, and conveyance of waste. At small scale, harnessing hydropower was accomplished by mills; at larger scales in modern dams, it is through hydroelectric power generation. Major rivers also provide the water supply for irrigation-based agriculture in some areas, where precipitation is not sufficient or consistent enough to support crops.

Figure 6: Nighttime view of the Nile River Valley and Delta, on Oct. 13, 2012, from the Visible Infrared Imaging Radiometer Suite (VIIRS) on the Suomi NPP satellite.

Source: NASA Earth Observatory/Suomi NPP

Indeed, for these reasons, rivers in many parts of the world are considered the “lifeblood” of society (Figure 6). For example, the Nile River valley in Egypt comprises ~5% of the land area, yet is home to nearly the entire population of 78 million, with a population density among the highest in the world (more than 1000 people per square km). Despite the obvious connection between water availability and human needs, the story of water resource distribution and population growth is not that simple! In some cases, major engineering projects in which millions of acre-feet of water are moved across states or continents have allowed cities and irrigated agricultural regions to flourish in water-scarce parts of the world. In others, major dams or new water sources (i.e. deep groundwater, reclaimed water, or desalination) have provided a means for cities to prosper in unlikely places. For example, take another look at Figure 4 above. The concentration of nighttime lights provides a reasonable proxy for population density. In many parts of the U.S., they follow the water: along the St. Lawrence, girdling the Great Lakes, and along the Mississippi River. Yet other major population centers have sprung up in perennially dry regions, mainly in the deserts of the southwest: Los Angeles, Las Vegas, Tucson, and Albuquerque.

Learning Checkpoints

1. Inspect Figures 4 and 5 and compare the two maps. Note 3 major cities that are near large water sources (rivers or lakes). View Figures 4 and 5above.

Click for answer.

ANSWER: Answers will vary, but examples include the cities around the Great Lakes (Chicago, Toledo, Milwaukee) as well as along the Mississippi River (St. Louis) and along the East Coast (New York, Philadelphia).

2. List 3 cities or regions of high population density that are not near major water sources, and/or lie in areas of low precipitation.

Click for answer.

ANSWER: Answers will vary, but most examples are in the Southwestern US – in Utah, Colorado, Arizona, California, and Nevada. Prominent examples include Los Angeles, Las Vegas, Salt Lake City, and Phoenix.

3. Do you know anyone who lives in one of these dry areas, or have you thought about moving there?

Click for answer.

ANSWER: 

Water Quality and Human Health

The distribution of water-rich and water-poor regions is of course not the whole story – access to clean water isn’t just about the amount of water that falls as precipitation. It’s also about the infrastructure needed to obtain, treat, transport, and deliver potable water. And that’s just the water supply. Disposal and sanitation of dirty water are equally important and require a means of transporting waste away from the distributed sources, collecting it and treating it, and discharging it safely. Ideally, both supply and waste conveyance systems should also be monitored for performance and for their impacts on water quality.

In some areas, water is plentiful, but access to clean water is not (Figures 7-8). The converse is also true, mainly in developed nations where water projects, desalination, or dams provide a water supply to regions that receive little precipitation. There is also a clear distinction between access to clean water in rural and urban areas (Figure 7), wherein access in rural areas, even in developed nations, lags behind that in urban areas.

Figure 7: Access to clean water supply and sanitation in urban and rural areas. Note that rural areas lag behind urban ones in access to clean water and sanitary disposal, and developing nations lag behind developed ones.

Click the link to expand for a text description of Figure 7.8

Access to clean water supply and sanitation in urban areas.

Year	Status	Water Supply	Improved Sanitation
1990	World	95%	79%
1990	Developing	92%	61%
2004	World	96%	80%
2004	Developing	91%	70%

Access to clean water supply and sanitation in rural areas.

Year	Status	Water Supply	Improved Sanitation
1990	World	63%	26%
1990	Developing	61%	19%
2004	World	72%	39%
2004	Developing	70%	35%

Source: United Nations Environment Programme Vital Water Graphics [8]

Access to clean water differs between rural and urban areas, and between developed and developing nations. In general, in rural areas, even in developed nations, access to water and sanitation lags behind that in urban areas. Globally, the areas with the poorest access to clean drinking water are in equatorial and sub-Saharan Africa, and parts of South America and southeast Asia (Figure 8).

One might imagine that access to clean water and sanitation would be strongly correlated with water-related illnesses and death. For example, compare the maps in Figures 8 and 9.

Figure 8: Global access to clean water supply by nation.

Source: World Health Organization, WHO [9]

Figure 9: Deaths attributed to water supply and sanitation problems.

Source: World Health Organization, WHO [10]

Water Use

How much and for what purposes?

Globally, there is a widely varied usage of water, as a result of differing total populations and population densities, geography and climate (i.e. water availability), cultures, economies, lifestyles, and water use and reuse efficiency. This can be described both in terms of total water abstraction from surface water and groundwater sources and as per capita water withdrawal. It can also be divided to consider the end uses (for example, as percentages of the total use), or to consider the source of the water. Each of these facets of water use illuminates different aspects of the “water story”.

In many industrialized nations, the dominant water uses are for industry (including thermoelectric power generation, manufacturing, etc…) and agriculture (Figures 10-11). In contrast, domestic and municipal water use generally constitutes less than 15-30% of the total. In developing nations, this is somewhat different – total water use is smaller, less is used for industry, and the proportion used for domestic water supply is larger.

In the U.S., the average per capita use of domestic or municipal water (i.e. the most direct uses – those that would be measured by the water meter at your home) is about 215 m³ per person per year, equivalent to 156 gallons per day (as of 2002). For comparison, the total abstraction of water from surface and groundwater sources in the U.S. is about 1700 m³/person/yr, or 1230 gallons per day. The difference in these numbers represents the large proportion of water that goes to so-called “indirect” uses: food production, manufacturing, power generation, and mining, among others.

In contrast, in sub-Saharan Africa, total water use is less than 200 m³ per person per year (less than 12% of water use in the U.S.). Total abstractions in Western Europe are about 600 m³ per person per year, about 850 m³ per person per year in the Middle East; and 1150 m³ per person per year in Australia. Among those nations with the highest water use, agriculture accounts for anywhere from <40% of use (U.S.), to 67-81% (India and China), to as much as 96% (Pakistan). Industrial use (including power generation) ranges from over 80% of total water use to less 1%. In the U.S. water use for power generation is near 50%; in China, it constitutes 25% and in India about 5%. Germany, Russia, Canada, France, and much of Western Europe use around 60% of withdrawn water for power generation. Municipal and domestic water use typically constitutes about 10-20% of the total and varies little among the worlds most populous countries (Figure 9). You can explore these patterns on your own via a useful interactive plotting engine at Gapminder. [11]

It is important to note that because many products are imported or exported across state and national borders, the total abstractions of water in a given place do not necessarily map to the distribution of water “consumption” there. Consider tomatoes that are transported from California to Massachusetts. The water withdrawal from rivers and aquifers needed to grow the tomatoes would appear on California’s “water tab”, but the eventual use of that water would be elsewhere. The same goes for agricultural and industrial products exported internationally. This flow of indirectly used water, embedded in products, is termed virtual water, and is defined as the amount of water used in generation of the product, or alternatively, the amount of water that would be needed to generate the product at the site where it is ultimately used. It is “virtual” because the water use is indirect; it is required to make or grow the item but is not actually physically contained in the item or transported with it.

Consumptive vs. Non-consumptive Use

Another important aspect of water use is the degree to which the water is available for recycling and/or reuse (Figure 12; cf. Figure 2). For some water uses, including industrial or domestic applications, the wastewater is captured, treated, and may be reused. These are termed nonconsumptive uses. For example, water used in homes is, for the most part, recaptured for treatment and discharged to surface water or groundwater systems – or for recycling of supply. In this sense, the water is not removed from the system (i.e. not “consumed”). In other applications, the water is effectively removed from the Earth’s surface environment and is not available to be re-captured. These are consumptive uses. Examples include water used for agriculture, which is mostly transpired by plants or evaporated and thus transferred to the atmosphere, or thermoelectric power generation, in which much of the water also evaporates (think of the steam you may have seen rising from power plants – this is consumptive water use, in action!).

Figure 10. Water use in the U.S., shown as five-year averages, from 1950-2005. The graph shows total water withdrawals (blue curve), and the partitioning of those uses between public/domestic supply, power generation, irrigation, and “other”.
Click here for a text description

Trends in total water withdrawals by water-use category, 1950-2005. Figure examines 5 different water users: public, rural domestic, irrigation, thermoelectric and other, and total withdrawals. Rural domestic water withdraws have stayed fairly consistent around 10 billion gallons a day. Public supply has slowly increase from 20 billion to 45 billion. Other uses have decrease by about 10 billion to around 35 billion. Irrigation has increased from 1950-1980 but then decreased slightly until 2005 to around 110 billion. Thermoelectric power increased steeply from 1950-1980 and then leveled out around 170 billion. The total withdrawals trend similarly increases from 1950-1980 and then levels out around 400 billion gallons of water withdrawn per day.

Source: J.F. Kenny et al., U.S. Geological Survey Circular 1344 (2009)

Figure 11. Percentages of water used for various purposes in the U.S. in 2005.

Click Here for Text Alternatie for percentages of water image

Percentages of water used for various purposes in the U.S. in 2005

Usage	Percentage
Public supply	11
Domestic	1
Irrigation	31
Livestock	Less than 1
Aquaculture	2
Industrial	4
Mining	1
Thermoelectric power	49

Source: N.L. Barber, U.S. Geological Survey Factsheet 2009-3098.

Figure 12. Proportions of water withdrawals used for agriculture (top), industry (middle) and domestic use (bottom) in 2000.
Click here for a text description

3 world maps. The first examining the percentage of water withdrawn for agriculture in a different nation. Europe and Canada withdrawal less than 16% for agriculture. The middle East, South East Asia, and Africa do the most agriculture with 63-100% of water withdrawals going to agriculture. The second map examines water withdraws for industry. Russia, Canada, and Europe withdraw over 50% of water for industry. Nations in the Southern hemisphere and the Middle East withdraw the less than 16% for industry. The third map examines water withdraws for domestic use. Greenland and Central African nations withdrawal over 45% of their water for this purpose. The United States and East Asia withdraw the least at lower than 15% for domestic purposes.

Source: United Nations Environment Programme Vital Water Graphics [12]

Figure 13. Proportions of water withdrawals used for agriculture, industry, and domestic use from 1900-2000, and projected for 2005. The gap between extracted and consumed water is shown by the gray band in each panel.
Click here to expand for a text description of Figure 13

Extraction vs consumption bar graphs for water in agriculture, domestic use and industry from 1900-2025. Extraction bars are always smaller than consumption bars. Agriculture both extracts and consumes the most water followed by domestic use then industry. All three graphs show an increase in consumption and extraction but the separation of between the two bars grows over time. The most dramatic difference appears in domestic use, followed by industry, then agriculture. The difference between consumption and extraction is highlighted with a grey band. Water may be extracted, used, recycled (or returned to rivers or aquifers) and reused several times over. Consumption is the final use of water, after which it can no longer be reused. That extractions have increased at a much faster rate is an indication of how much more intensively we can now exploit water. Only a fraction of water extracted is lost through evaporation.

Source: United Nations Environment Programme Vital Water Graphics [12]

Supply and Use on Multiple Scales: Units of Water

Units of measurement: volumes, fluxes, and concentrations

The uses of water for human activity vary immensely, and as a result, water resource management covers a wide range of temporal and spatial scales. In some cases, the timescales are short and volumes relatively small (i.e. domestic pumping of several gallons per minute, over timescales of minutes or hours). At the other extreme, water allocations for states or municipalities are often considered in the context of average annual flows in the billions of gallons. Because so many different scales of measurement are used to describe water flux or discharge (volumes of water) and flow rates (the velocity of flow), it is important to have some facility with the various units of measurement and get a sense for their relative magnitudes.

As one example, the total fluxes of water through river systems – commonly used to define allocations of water for states or nations - are measured and reported in acre-feet. This is a unit of water volume equal to the amount of water that covers an area of one acre, one foot deep. One acre-foot is equivalent to 325,851 gallons (see summary of unit conversions from the U.S. Geological Survey [13]), and is often considered as the amount of water needed for a family of four for about one year.

As we’ll discuss in Module 3, over shorter timescales, river discharges are reported in units of cubic feet per second (cfs), cubic meters per second (m3/s), or gallons per minute (gpm). As one example, on average, Spring Creek carries about 50 cfs at Houserville, PA; this increases downstream to about 90-100 cfs at Axemann as the creek is fed by springs and small tributaries. Short-lived peak discharge may exceed 500 cfs after storm events. For comparison, the flow of the Mississippi River at St. Louis, MO is typically about 400,000-600,000 cfs; in major floods the discharge is over 1,000,000 cfs. The flow rates of rivers and groundwater, as we will see in Modules 3-4 and 6, are reported as a velocity - units of length per time. These measures represent the velocity of the water itself, or of an object (stick, boat, person, etc…) carried by the river or stream.

Yet other key quantities in hydrology are reported in units of an equivalent depth (or length) per time. For example, rainfall rates are described in units of inches, cm, or mm per hour (for individual storm events) or per year (i.e. annual average precipitation). Evaporation rates are reported in the same way – but of course, represent water transport in the opposite direction (up!). The total volume of water these represent depends on the area over which they occur.

The Geographic Distribution of Water Uses

A deeper look: the geographic distribution of water uses

It is also instructive to look in more detail at the distribution of different water uses. For example, in the U.S., industry is concentrated East of the Mississippi, mainly in the “steel belt” (also known as the “rust belt”) and in Texas and Louisiana (primarily related to oil and gas extraction) – and thus water use for industry is as well (Figure 14). It’s worth considering whether this pattern is ultimately rooted in the timing of settlement and westward expansion in the U.S., availability of fuel (i.e. coal), or availability of water sources and rivers as a means of transportation for goods and raw materials. The pattern of water withdrawal for agriculture in the US is even more dramatic (Figure 15). Large agricultural water withdrawals from surface water and groundwater are dominantly West of the Mississippi. This is evident from a state-by-state map view and shown even more clearly when plotted simply from West to East (Figure 15, bottom panel).

Figure 14. Total water withdrawals for industrial uses shown by state in map view (top), and arranged from West to East (bottom).
Click link to expand for a text description of Figure 14

Water Withdraws million gal/day by State **approximate numbers

State	Water Withdraws million gal/day
Hawaii	100
Alaska	100
Oregon	250
Washington	500
California	200
Nevada	100
Idaho	100
Arizona	100
Utah	250
Montana	100
Wyoming	100
New Mexico	100
Colorado	200
North Dakota	100
South Dakota	100
Nebraska	100
Texas	2200
Kansas	100
Oklahoma	100
Minnesota	200
Iowa	300
Missouri	100
Louisiana	3200
Arkansas	250
Wisconsin	500
Mississippi	300
Illinois	400
Alabama	500
Tennessee	800
Indiana	2400
Kentucky	250
Michigan	700
Georgia	600
Ohio	650
Florida	250
South Carolina	300
West Virginia	1100
North Carolina	300
Virginia	500
Pennsylvania	900
Maryland	250
D.C.	100
New York	300
Delaware	100
New Jersey	100
Connecticut	200
Vermont	100
Massachusetts	200
Rhode Island	100
New Hampshire	100
Maine	250
Puerto Rico/US Virgin Islands	100

Source: J.F. Kenny et al., U.S. Geological Survey Circular 1344 (2009).

Figure 15 Total water withdrawals for agricultural (irrigation) uses shown in map view (top), and arranged from West to East (bottom).
Click link to expand for a text description of Figure 15

Total Water Withdraws for Agriculture **approximate numbers

State	Water Withdraws million gal/day
Hawaii	200
Alaska	200
Oregon	6000
Washington	3000
California	24000
Nevada	1500
Idaho	16000
Arizona	5000
Utah	4000
Montana	10000
Wyoming	4000
New Mexico	2000
Colorado	13000
North Dakota	200
South Dakota	200
Nebraska	9000
Texas	8500
Kansas	2000
Oklahoma	500
Minnesota	300
Iowa	200
Missouri	1500
Louisiana	900
Arkansas	9000
Wisconsin	300
Mississippi	2000
Illinois	500
Alabama	200
Tennessee	200
Indiana	200
Kentucky	200
Michigan	300
Georgia	750
Ohio	200
Florida	3500
South Carolina	200
West Virginia	200
North Carolina	200
Virginia	200
Pennsylvania	200
Maryland	200
D.C.	200
New York	200
Delaware	200
New Jersey	200
Connecticut	200
Vermont	200
Massachusetts	200
Rhode Island	200
New Hampshire	200
Maine	200
Puerto Rico/US Virgin Islands	200

Source: J.F. Kenny et al., U.S. Geological Survey Circular 1344 (2009).

The source of the water we use also provides clues about where water may be most readily available, and/or where typical rainfall and snowmelt cannot meet demand. Inspect the maps below (Figure 16). Surface water withdrawals are spread more or less uniformly across the U.S., and reflect overall water use reasonably closely. This is influenced in large part by total population, energy production, and industrial and agricultural activity (i.e. CA, TX, NY, and FL are the most populous states). However, groundwater withdrawals (obtained by pumping at wells) are a good indication that surface water flows alone are not sufficient to meet demand.

Figure 16. Total surface water abstractions (left) and groundwater abstractions (right) by state. The color scale is the same as for Figure 15.
Click the link to expand for a text description of Figure 16

text text

Surface Water Withdraws

Amount (million gals./day)	States (random order)
1500-3200	CA
600-1500	ID, TX, CO, IL, MI, OH, NY, NC, VA, TN,
300-600	OR, MO, WI, IN, PA, NJ, DE, SC, AL, LA, AK
300	All others

Ground Water Withdraws

Amount (million gals./day)	States (random order)
1500-3200	n/a
600-1500	CA
300-600	TX, NE, AK
<300	All others

Source: J.F. Kenny et al., U.S. Geological Survey Circular 1344 (2009).

Future Demand for Water

What will the future bring?

What will the future bring? Good question, right? How can we gauge what water demand and availability will be in the future, particularly with projected large increases in population and potential climate change superimposed? Not to alarm you, but to inform you, we will go through the exercise of making such projections, both for the U.S. and, on a more limited basis, for the world. What do we need to know for making such estimates? First, let's jot down some ideas. Then we will continue the process below.

First, here is an expert opinion as to how the future will go…

In Human Population and the Environmental Crisis Ben Zuckerman and David Jefferson write: “At a low population density, a society may be able to derive its water from rivers, natural lakes, or from the sustainable use of groundwater. As the population grows, so does the volume of water needed (we will assume demand is proportional to population size). Moreover, levels of waste discharge into the environment will grow as the population rises. Thus, the available unmanaged supplies deteriorate at the same time that demand on them is increasing…A destructive synergy is at work: population size affects the water resource in a manner that is not one of simple proportionality.”

What was it Yogi Berra (N.Y. Yankees catcher and later Manager) infamously said…"It's tough to make predictions, especially about the future." Well, that is a truism, but let's see what projections are being made regarding future population growth, because, clearly, that's one of the inputs we need to determine potential future water use globally. The present global population (as of 2014) is approximately 7.25 billion people. Interestingly, the top three countries, in terms of population, are China, India, and, yes, the United States, in that order (Figure 17). But, by 2050 the global population is estimated to be 9.6 billion people by the United Nations—a staggering 33% increase in the next, say, 35 years! So, at the minimum, if we assume that water use will increase linearly on a per person basis, we would expect that this rate of growth will require 33% more fresh water by 2050. Is that a problem? Do we have excess capacity to supply this water?

Population Growth vs. Water Needs

Do we have excess capacity to supply this water? That is an important question, but you have probably already determined that the real issue is where the population growth occurs and what water resources are available there. The major growth is projected to occur in developing countries (Figure 17). African nations are likely regions for greater than average growth. Interestingly, much of Africa is estimated to have significant groundwater resources (BGS, 2013) that could be developed if necessary. In fact, Nigeria is projected to surpass the population of the U.S. by 2050 (Figures 17-19). One must examine the population density and rate of projected growth vs. water needs. In addition, climate change impacts must be considered.

Figure 17. The distribution of population by country scaled by China (largest red dot) at 1.36 billion people in 2010. (World plot)

Source: Gapminder

Figure 18. Top 10 countries by population from 1950 to 2050, according to UN data.br>
Click link to expand for a text description of Figure 18

1950- total population 2.5 bn

 

1. China- .5 billion
2. India
3. United States
4. Russia
5. Japan
6. Indonesia
7. Germany
8. Brazil
9. Britain
10. Italy

2013- total population 7.2 bn

 

1. China-4 bn
2. India
3. United States
4. Indonesia
5. Brazil
6. Pakistan
7. Nigeria
8. Bangladesh
9. Russia
10. Japan

2050 forecast- total population 9.6 bn

 

1. India-7 bn
2. China
3. Nigeria
4. United States
5. Indonesia
6. Pakistan
7. Brazil
8. Bangladesh
9. Ethiopia
10. Philippines

Source: United Nations

Figure 19. Fertility index (children per woman) by country as a function of per capita income for 2012. Note the higher fertility for African countries. China and the U.S. are well below 2 children per woman.
Click the link to expand for a text description of Figure 19

Chart with GDP/Capita on the X-axis (low to high) and Children per woman (total fertility) on the y-axis for 2012. Different colors and dot sizes are used to represent different countries. The line of best fit would look like a negative exponential function. General trends include high fertility in African countries followed by India, (2-8 children). China, the Americas, and Russia are well below 2 children per woman.

Source: Gapminder

Increased Impacts of Climate Change on Demand  

We would probably be better off examining the impacts of climate change on water availability that would increase "water stress," then compare these stresses with those caused by increasing demand, either by population growth in a given region (personal or agricultural demands) or increased water usage resulting from new demands (e.g., energy production) (Figure 20). A number of studies have predicted water supply vs. water demand relationships resulting from climate change. A study by MIT (Massachusetts Institute of Technology) researchers (Schlosser et al., 2014) compared the potential impacts of climate change, on the basis of projected greenhouse gas emission increases in a complex Earth-system model, on water stress in 282 assessment regions (large or multiple watersheds) globally, holding demand constant, to the potential impacts of population growth in the same regions.

Figure 20. Some estimates of total population growth (UN assessment) from 2010 to 2050. Not all countries experience growth, but note Nigeria and Kenya as examples of increasing population in Africa.
Click link to expand for a text description of Figure 20

Population % change by country

Country	Increase/Decrease	Percent
US	Increase	28
Mexico	Increase	32
Brazil	Increase	18
Germany	Decrease	13
Nigeria	Increase	176
Kenya	Increase	138
India	Increase	34
China	Increase	2
Japan	Decrease	15
Russia	Decrease	16

Source: United Nations, Department of Economics and Social Affairs, World Population Prospects: 2012 Revision, June 2013 [14].

They found that, in most regions, projected population growth with increased demand to 2050 was the greater stressor. These researchers use a Water Stress Index (WSI) defined as WSI = TWR/RUN+INF (TWR is total water required for a given watershed region, i.e. all consumptive uses, RUN is available runoff within the watershed, and INF is inflow to the watershed from adjacent regions. The cutoffs used for interpreting water stress are: WSI<0.3 is slightly exploited, 0.3≤WSI<0.6 moderately exploited, 0.6≤WSI<1 heavily exploited, 1≤WSI<2 overly exploited, and WSI≥2 extremely exploited as originally set out by Smakhtin et al. (2005).

It appears that a substantial proportion of Africa, all of the middle East, India, and central Asia will see increased water stress in the next few decades, largely due to projected population increases. Even the southwestern U.S. is projected to experience expansion and intensification of water stress, but, in this case, mostly as the result of climate change and longer-term drought. Interestingly, the major central U.S. groundwater source, the Ogallala Aquifer, does not appear to be a candidate for significant stress except at its southern end in Texas. However, other studies (see Module 7) suggest that depletion of this aquifer will be more severe.

Possible Solutions for Meeting Water Demand in Stressed Regions

There are a number of possible methods to enhance supplies of fresh water, each of which has an economic, political, and/or environmental impact.

Some of these strategies have been alluded to previously (e.g., encouraging transfers from agricultural use to drinking water supplies). Water storage behind dams is an old strategy and problematic in a number of ways (see Module 6), including high costs, environmental impacts, and political issues that arise when major rivers flow through multiple countries. Nonetheless, there is still major proposed and ongoing dam building in China and other countries.

Groundwater banking is a newer strategy that requires replenishment of aquifers with treated wastewater and/or with runoff available during times of excess. Costs are associated with treating, impounding, and injecting the water (see Module 7). This will mainly benefit regions with significant groundwater resources.

Recycling and reuse are gaining support with successful projects in the U.S. and elsewhere. Penn State University recycles and reinjects nearly 98% of its treated wastewater and has done so since the 1960s. Orange County, CA, has another successful system (see Module 8). Such systems must overcome consumer opposition, however, because of the perception that consumers will be drinking, well, toilet water! Nonetheless, the water quality in such systems is as good or better than that in municipalities that draw water from rivers downstream from other municipalities that discharge treated wastewater into the same river. Another form of reuse is to employ "gray" water (only partially treated) for irrigation of golf courses in arid to semiarid, water-stressed regions. Las Vegas, NV, has implemented such a system, coupled with the removal of water-hungry turf, for which the economics work and conservation is encouraged.

Desalination may be a last resort because of the costs of energy required to remove salts from seawater or water pumped from saline aquifers in non-coastal regions. However, in water-poor but hydrocarbon-rich middle-Eastern countries the economics may support the desalination of seawater. Alternative energy sources (e.g., solar) or emerging processes such as chemical reverse osmosis may be economical in the future as they become more efficient and less costly. And, of course, if water is deemed to have significant value in the future, the high costs may be more acceptable.

Finally, there are still proposals to import or export water from regions replete with fresh water resources (e.g. Alaska) to severely water-stressed regions (e.g. India). However, the costs of transporting such a commodity across the oceans would appear to exceed the value of that water at its terminus.

All of these strategies will be explored in later modules in more detail.

Pricing Varies

You are likely all too familiar with bottled water—that convenient liter-sized plastic bottle containing some sort of water, commonly tap water, or filtered spring water, sometimes treated…it appears that, in the U.S., we pay for the convenience of "grab-and-go." For that convenience, we typically pay about $4/gallon, more than we presently pay for a gallon of gasoline! In most municipalities; however, the cost of water delivered in pipes to taps in homes costs far less ($0.003-0.006/gallon). A survey by CircleofBlue.org for 2014 water pricing in 30 cities across the U.S. found an average increase of 6.2% in monthly bills for a family of four using an average of 100 gals/day each (12,000 gals/mo or 45.4 m3/mo) from 2013 to 2014.

Monthly rates for some representative municipalities are shown in the table below, based on data in the CircleofBlue.org survey and information from water authority websites for some municipalities not covered in that survey (Pittsburgh, PA and State College, PA). Note the large range in rates that do not seem to make sense geographically. For example, arid Phoenix, AZ has the lowest rate, with Las Vegas, NV not far behind, whereas high precipitation, seemingly water-rich regions such as Seattle, WA and Atlanta, GA top the rate list. Note that Los Angeles, CA, Phoenix, AZ, and Las Vegas, NV all depend on Colorado River water, although Los Angeles also draws on northern California sources and all require significant transport infrastructure. So, in part, this disparity in rates results from the costs of maintaining infrastructure and the numbers of households served, as well as the local abundance of water.

Chicago, IL, for example, has nearby Lake Michigan as a source and a large number of users and its rates are relatively low. Little State College, PA has a significant, sustainable groundwater resource (see Module 6), even though the user base is relatively small. Many municipalities have higher rates because they are financing necessary improvements in infrastructure, which can be quite costly.

Municipalities have adopted different methods for scaling water prices.  Some, such as Philadelphia and Detroit, provide cost reductions for larger users (decreasing block), some, including New York, have uniform pricing, whereas others, such as Las Vegas and Atlanta, have implemented tiered pricing (block increases) that encourage conservation while trying to maintain the user base. The objective of all municipalities is to sustain income and provide for future infrastructure requirements.

Internationally, pricing varies even more than in the U.S. Figure 22 illustrates average water prices (Kariuki and Schwartz, 2005) and the impact of non-public water suppliers on the cost to the consumer.  Where public utilities are not available, the cost to the consumer can be a factor of 10 higher.  In part, this occurs because of increases in cost to the water supplier to purchase water from a public or private supplier because of the large volumes purchased with prevailing block pricing increases.  Figure 23 shows the step increases for several African and Indian cities. Recall that the average family of four in the U.S. would use about 45 m3/month, but average usage is probably much lower in many developing nations with lower standards of living. Step increases in block pricing appear to be a fair method of pricing to allow low cost for low-volume users and encouraging conservation by imposing higher costs for larger-volume users.

Figure 22. Data for 2005 based on a survey of 47 countries and 93 locations (Kariuki and Schwartz, 2005).

Source: Circle of Blue [15]

Figure 23. Variations in block pricing in several large cities in Africa and India.v($U.S./m3).
Click the link to expand for a text description of Figure 23

Block water tariffs increase in a step-like fashion as water usage (cubic meters/month) increases. For example, take Dakar. From 0-20 cubic meters, there is a \$0.35 tariff but from 20-40 cubic meters there is a \$1.05 tariff and from 40-110 cubic meters it costs \$1.20. Another example is Nairobi. From 0-10 cubic meters, there is a \$0.15 tariff but from 10-30 cubic meters, there is a \$0.25 tariff. From 30-60 cubic meters, there is a \$0.38 tariff and from 60-110 it costs \$0.48

Source: Circle of Blue [15]

The Water and Energy Nexus

Considerations of water pricing are complicated because of the multitude of factors that must be taken into account. These include availability and dependability of water supply locally, state of the distribution infrastructure, and the distribution and size of the user base. Dependability is related to climate impacts, such as prolonged droughts that deplete water reserves. A recent study ( [16]Watergy Nexus: The Complex Relationship and Looming Crisis Between Our Thirst For Water and Our Hunger for Energy [16]) highlights an additional factor—the amount and cost of energy to acquire, transport, and treat water. This study argues that the cost of energy (usually electricity, but including fuel if the water is trucked in) must be considered in pricing water. The study uses data for 2013, a year of severe drought in much of the western and central U.S. to show how water prices should be adjusted to guarantee supply and cover costs of acquisition. Although the U.S. Geological Survey indicates that the average energy required to provide 1000 gallons (1 kgal) of water is 1.9kWh of electricity, water-stressed regions such as northern California (3.5 kWh/kgal) and highly stressed southern California (11.1 kWh/kgal) require far more (Figure 24). However, the study suggests that municipalities are not taking this factor into consideration in providing a durable and resilient water supply. During times of water stress, municipalities may have trouble meeting costs, and begin to examine other strategies, such as privatization. Of course, when water availability becomes restricted, costs can go up as in California with severe drought conditions (e.g., see the news article In dry California, water fetching record prices [17] about California water pricing: ).

Figure 24. Variations in water price and examples where water is overpriced or underpriced in consideration of transport costs.
Click for a text description

Drought Map of the Us examining water prices in comparison to source energy per kgal of water to determine over/underpricing. The figure shows that the driest areas of the country are from the Mississippi River westward. The east is also dry but not considered in a drought except for Georgia and South Carolina. On top of the drought map, are several cities with a white ring indicating the price and a black dot indicating energy for every kgal of water. When the price ring is much larger than the black dot then the water is overpriced like in Burlington Vermont. When the energy dot is much greater than the price ring the water is underpriced. The Southwest and up the California coast are either well matched or underpriced. The Midwest is generally fair pricing and the east is generally overpriced.

Source: EnergyPoints

Privatization

Cash-strapped municipalities with failing water systems might be tempted to contract with private companies to manage their water and sewer systems. Economic drivers, such as the collapse that occurred in 2008, affect users ability to pay for public water systems. In the U.S., about 20% of water supplies are privatized at present. In evaluating this option, one must keep in mind that corporations are for-profit entities, and will need to recoup the full costs of providing water while adding a margin for profit to benefit the corporation and/or their stockholders. Public utilities can be held responsible for controlling costs and providing clean water supplies, whereas it is more difficult for the public to do so for private companies.

There are a number of examples where privatization has apparently failed consumers. Pushed by the costs of renovating their failing water supply infrastructure, Atlanta, GA, for example, handed over control of their water system to United Water, which took over in 1999, with a 20-year contract. Atlanta had long-deferred most maintenance because their revenue was insufficient to cover the full cost of providing service, and because of rapid population growth and their aging system, expansion and improvement were required. Their sewage system was more of a problem than the water supply system, and they were being sued under the U.S. Clean Water Act for that problem as well.

But, in 2003, the city of Atlanta withdrew from the agreement because a number of issues with United Water (see What Can We Learn From Atlanta's Water Privatization [18] for the full story), which included costs, viewed as excessive, and poor performance in maintenance, meter installation, and bill collection.

The situation in Detroit has been much in the news of late (for example – see this MSNBC article, Detroit residents and national allies protest water shutoffs [19]). As a result of the economic downturn, the Detroit Water and Sewer Department has recently gone on a campaign to force users to pay their outstanding water bills with the threat of cutoffs. In addition, the city of Detroit is pursuing the possibility of privatizing its water and sewer systems. Although clearly having its own perspective and position, an interesting argument against water privatization in Detroit [20] is found on The Blue Planet Project website. One common argument against privatization is the rapid increase in the costs of water to consumers. Of course, in many cases, this may occur because the public purveyor was not charging for the full cost of providing the water in the first place.

The Configuration of the Water Molecule

A molecule of water is composed of two atoms of hydrogen and one atom of oxygen. The one and only electron ring around the nucleus of each hydrogen atom has only one electron. The negative charge of the electron is balanced by the positive charge of one proton in the hydrogen nucleus. The electron ring of hydrogen would actually prefer to possess two electrons to create a stable configuration. Oxygen, on the other hand, has two electron rings with an inner ring having 2 electrons, which is cool because that is a stable configuration. The outer ring, on the other hand, has 6 electrons but it would like to have 2 more because, in the second electron ring, 8 electrons is the stable configuration. To balance the negative charge of 8 (2+6) electrons, the oxygen nucleus has 8 protons. Hydrogen and oxygen would like to have stable electron configurations but do not as individual atoms. They can get out of this predicament if they agree to share electrons (a sort of an energy "treaty"). So, oxygen shares one of its outer electrons with each of two hydrogen atoms, and each of the two hydrogen atoms shares it's one and only electron with oxygen. This is called a covalent bond. Each hydrogen atom thinks it has two electrons, and the oxygen atom thinks that it has 8 outer electrons. Everybody's happy, no?

Figure 1. Water Molecule

Source: Maureen Feinman

However, the two hydrogen atoms are both on the same side of the oxygen atom so that the positively charged nuclei of the hydrogen atoms are left exposed, so to speak, leaving that end of the water molecule with a weak positive charge. Meanwhile, on the other side of the molecule, the excess electrons of the oxygen atom, give that end of the molecule a weak negative change. For this reason, a water molecule is called a "dipolar" molecule. Water is an example of a polar solvent (one of the best), capable of dissolving most other compounds because of the water molecule's unequal distribution of charge. In solution, the weak positively charged side of one water molecule will be attracted to the weak negatively charged side of another water molecule and the two molecules will be held together by what is called a weak hydrogen bond. At the temperature range of seawater, the weak hydrogen bonds are constantly being broken and re-formed. This gives water some structure but allows the molecules to slide over each other easily, making it a liquid.

The Structure of Water: Properties

Studies have shown that clustering of water molecules occurs in solutions because of so-called hydrogen bonds (weak interaction), which are about 10% of the covalent water bond strength. This is not inconsiderable and energy is required to break the bonds, or is yielded by the formation of hydrogen bonds. Such bonds are not permanent and there is constant breaking and reforming of bonds, which are estimated to last a few trillionths of a second. Nonetheless, a high proportion of water molecules are bonded at any instant in a solution. But this structure leads to the other important properties of water.

We will consider, for the purposes of this course, only six of these important properties:

1. Heat capacity
2. Latent heat (of fusion and evaporation)
3. Thermal expansion and density
4. Surface Tension
5. Freezing and Boiling Points
6. Solvent properties

As mentioned above, these properties have importance to physical and biological processes on Earth. Effectively, large amounts of water buffer Earth surface environmental changes, meaning that changes in Earth-surface temperature, for example, are relatively minor. Thus, the high heat capacity of water promotes continuity of life on Earth because water cools/ warms slowly relative to land, aiding in heat retention and transport, minimizing extremes in temperature, and helping to maintain uniform body temperatures in organisms. However, there are other effects of water properties as well. Its low viscosity allows rapid flow to equalize pressure differences. Its high surface tension allows wind energy transmission to sea surface promoting downward mixing of oxygen in large water bodies such as the ocean. In addition, this high surface tension helps individual cells in organisms hold their shape and controls drop behavior (have you seen "An Ant's Life"?). Also, the high latent heat of evaporation is very important in heat/water transfer within the atmosphere and is a significant component of transfer of heat from low latitudes, where solar energy influx is more intense to high latitudes that experience solar energy deficits.

Video: Water - Liquid Awesome: Crash Course Biology #2 (11:16)

Take a few minutes to learn why water is the most fascinating and important substance in the universe.

Systems Thinking and the Hydrologic Cycle

Throughout this course, we will be dealing with complex systems and “Systems Thinking”. What is Systems Thinking, you may ask? According to Peter Senge, author of The Fifth Discipline Fieldbook, “Systems thinking is a way of thinking about, and a language for describing and understanding, the forces and interrelationships that shape the behavior of systems”. Some systems are very complex, but all systems can be simplified to help understand the relationships between systems components. Systems can be "modeled" to help investigate their dynamics. We do not expect you to become system modelers, per se, but simple models can begin to help you understand how changes in one parameter might influence changes in another.  Let's consider a simple system in which we have a bathtub, fed by a faucet, and drained at its lower level. We could diagram this simple system as follows…

Simple System Diagram

Source: Mike Arthur and Demian Saffer, The Pennsylvania State University

In this system there is a reservoir (the bathtub), an input (the faucet), and an output (the drain). The relationships in this system are simple and, hopefully, intuitive. If you want to run water into the tub for a long time to keep it quite warm, but not have it run over, what are your choices? You could keep the drain closed and run a very slow trickle of warm water into the tub from the faucet, letting it fill gradually, or, you could fill the tub quickly to some level, then open the drain to allow water to leave the tub at the same rate as it is being added to prevent further rise in the water level. Cold water is more dense than warm, so perhaps cooler water would drain preferentially and this would keep the tub water warmer overall. You could also evaluate the time it would take to fill the tub, or drain it, knowing the tub volume (gallons), the maximum input rate through the faucet (gallons/minute), and the maximum drain rate (gallons/minute).

Hydrologic Cycle

The movement of water between these reservoirs, primarily driven by solar energy influx at the Earth’s surface, is known as the hydrologic cycle.

Figure 6. Diagram showing the main components of the hydrologic cycle, including evaporation, transpiration, precipitation, runoff, infiltration, and groundwater runout.

Click Here for Text Alternative for Figure 6

Component	Fluxes in 103 km3/ year
Evaporation	436.5
Precipitation	391
Groundwater Runout	45.5
Evapo-transpiration	65.5

Source: Michael Arthur and Demian Saffer

The hydrologic cycle is a conceptual model that describes the fluxes of water between the oceans, surface water bodies (lakes, rivers, and streams), groundwater in subsurface aquifers, the atmosphere, and the biosphere. One important aspect of the cycle is that no water is gained or lost: water moves between reservoirs but the total mass remains the same. Another way to say this is that the water that currently exists on Earth is the same water that has been here since the time the Earth formed. (Technically, there are small fluxes of water from the Earth’s interior to the surface and atmosphere through volcanism and venting, and small influxes of water from comets and debris, but these are negligible in comparison to the mass of water in the primary reservoirs shown above.)

The movement of water between reservoirs, or the “limbs” of the hydrologic cycle includes five primary processes:

• Evapo-transpiration: the movement of water from oceans or land to the atmosphere, through the combined processes of evaporation and transpiration. Evaporation and transpiration both involve a change in state, from liquid to vapor, which requires an input of energy. Evaporation is simply the change from liquid to vapor as a result of molecular motion, and is affected by temperature and ambient humidity. Transpiration is the movement of water to the atmosphere by plant respiration. In most terrestrial basins, transpiration is the dominant process by which water moves from the Earth’s surface to the atmosphere, whereas over lakes and the oceans, evaporation dominates.
• Precipitation: the movement of water from the atmosphere to the land surface or oceans, in the form of rain, snow, sleet, ice pellets, etc... Precipitation involves a change in state from vapor to liquid, known as condensation. This change in state releases heat energy. After precipitation falls on the land surface, it may flow into surface water bodies (lakes or streams), or percolate through soils and rock into the groundwater system.
• Runoff: the movement of water from the land surface to the oceans in streams or rivers.
• Infiltration: the percolation of water from the land surface or from surface water bodies through soils and into the subsurface. Water that infiltrates becomes part of the groundwater system, and is also known as groundwater recharge.
• Groundwater outflow, also known as subsea outflow: the seepage of water from the groundwater system directly into the oceans. The flux of groundwater outflow is the least constrained component of the hydrologic cycle, and is often estimated by balancing the other fluxes in the cycle.

Because the changes in state that accompany evaporation and precipitation also take in and release energy, the movement of water through the hydrologic cycle is paralleled by redistribution of heat and energy.

Uneven Distribution

Why is water distributed unevenly across the Earth’s surface?

As you probably know, things are far more interesting than a hypothetical case of evenly distributed precipitation! Both precipitation and evaporation vary widely over the Earth’s surface. This unequal distribution of water on the planet drives a diversity in climate and ecosystems (or biomes); water availability for human life, industry, and agriculture; and is fundamentally and intimately tied to the history of politics, economics, food production, population dynamics, and conflict – both in the U.S. and globally.

The abundance of water in some areas and scarcity in others follows systematic and predictable patterns. As part of this module, we’ll explore the physical processes that shape the overall distribution of precipitation - and thus water resources.

Figure 7 Map of average annual precipitations for the U.S. for 1981-2010

Source: PRISM Climate Group, Oregon State University [22], map created 2013.

Figure 8. MODIS satellite image renderings of vegetation coverage for July 2013 (top) and January 2013 (bottom). Gradations of green indicating leafy vegetation with the darkest green reflecting highest coverage of plants. Brown colors are ice or desert and black is "no data" (largely the oceans). Note the seasonal differences most pronounced in the northern hemisphere.

Source: NASA Earth Observations (NEO) [23]

Note the contrasting patterns in the two images in Figure 8 above, based on global satellite coverage. Vegetation in the southern hemisphere, which has relatively more ocean area (and less land area) than the northern hemisphere, changes little seasonally, whereas vegetation distribution in the northern hemisphere undergoes large changes. Why is that? There are probably two impacts on vegetation distribution—precipitation and temperature. Examine the figure below that illustrates the available moisture seasonally (summer vs. winter) and compare to the distribution of vegetation for the same seasons. Think about the role of temperature, precipitation, and soil moisture (water availability to plants), as well as the availability of sunlight for photosynthesis. Yes, there is a more complex relationship between plant growth and other factors, but the hydrologic cycle plays a major role.

Figure 9. Contours of average atmospheric water vapor calculated as equivalent rainfall in millimeters on the basis of satellite observations for July 2003 (top) and January 2003 (bottom). Again note the strong seasonal differences in the northern hemisphere in particular. Can this explain the vegetation differences entirely?

Source: NASA AIRS The Encyclopedia of Earth [24]

The Orographic Effect

To take the concept of relative humidity outdoors, let's consider why it rains in some areas and we have deserts in others. There are two primary reasons for this. Both are related to the transport, rise, and fall of air masses that lead to temperature changes, and ultimately in the amount of water vapor that the air can hold. These are the orographic effect, and atmospheric convection.

In both cases, cooling and warming of air masses occurs because they are forced upward or downward in the atmosphere. The decrease in air temperature with elevation is known as the atmospheric (or adiabatic) lapse rate, as shown below, and is related to decreasing air density and pressure with increasing altitude (as air rises, it expands due to decreased pressure, leading to lower temperature). A typical average lapse rate is around 7° C per km of altitude change. If an air mass begins rising and has not reached the dewpoint temperature, it follows a dry adiabatic lapse rate, with the rate of cooling due nearly entirely to decreasing pressure, as shown in Figure 14. Once the airmass temperature reaches the dewpoint during continued rise, water droplets begin to condense (forming clouds) and the airmass follows a moist adiabatic lapse rate (Figure 14), for which the rate of cooling with elevation decreases because of the addition of some offsetting heat to the airmass from the process of condensation (termed latent heat).

Figure 14. An example of the rate of cooling of an airmass rising from ground level to higher altitudes, and the effect on rate of cooling when reaching the point of saturation with respect to water vapor (level of condensation).
Click to expand for a long description

A graph of atmospheric temperature with altitude in meters on the y-axis and temperature in degrees Celsius on the x-axis. One line with two different decreasing slopes separated at 2000m. The moist adiabatic lapse rate (~0.6C/100m) occurs above 2000m. The dry adiabatic lapse rate (1C/100m) occurs below 2000m.

Source: Mike Arthur and Demian Saffer

The orographic effect occurs when air masses are forced to flow over high topography. As air rises over mountains, it cools and water vapor condenses. As a result, it is common for rain to be concentrated on the windward side of mountains, and for rainfall to increase with elevation in the direction of storm tracks. With continued cooling past the dewpoint, the amount of water vapor in the air cannot exceed saturation, so water is lost from the air via condensation and precipitation.

On the leeward side of mountain ranges, the opposite occurs: the air descends and warms. As it does so, it is capable of holding more water vapor (recall the saturation line in the relative humidity plot above). However, there is no source of additional water, so the descending air mass increases in temperature but the amount of water vapor remains constant. Because the air has lost much of its original water content, as it descends and warms its relative humidity decreases. These areas are called rain shadows and are commonly deserts. You’ve probably noticed this same process in action when you heat your house or apartment in the winter – warming the cold air leads to dry conditions – one of the reasons people often put water pots or kettles on their wood stoves.

Orographic Effect In Action

The animation below shows an airmass trajectory superimposed on a Google Earth image of western North America. The point of this animation is to provide an explanation of the orographic effect and the changes in temperature and water content of an airmass passing over several mountain ranges. The animation shows the "rain shadow" effect that results in desert regions behind large mountain ranges. An inset graph at bottom right illustrates combinations of temperature (x-axis) and moisture content (y-axis) in grams per cubic meter of the air mass as it passes over various topographic features on the land surface.

Orographic Effect Animation. The sequence of frames portrays a westerly wind, blowing onshore from the Pacific Ocean, driven by a large low-pressure system over the northwestern US. At point 1, the airmass is relatively warm (about 23 degrees C) and moisture-laden (relative humidity about 80%) blowing over the ocean surface. At point 2 the airmass rises over the California Coast Range, cools to about 17 degrees C, and its relative humidity reaches 100% so that clouds form and it rains, losing some of the moisture it is carrying. At point 3, the air has sunk into the Central Valley, warming nearly to its original temperature. However, because the airmass lost moisture over the Coast Range, it now has a lower relative humidity. At point 4, the airmass is forced to rise over the higher Sierra Nevada range, cooling progressively as it rises in elevation from 3000 feet (12 degrees C) to over 14000 feet (freezing point). Initially, moisture is lost as rain at lower elevations and then snow at the high elevations. Much of the moisture is wrung out over the Sierra Nevada such that when the air sinks into the low-lying (near sea level) Owens Valley to the east, it warms (to about 16 degrees C) and consequently has a very low moisture content and relative humidity. Position 6 illustrates rising air over the White Mountains, about 10,000 feet high, over which the air again cools and loses what little moisture it has as snow. As the air descends into the desert region of Nevada, it warms again with a very low moisture content and relative humidity. To watch the animation again from the beginning, just refresh your browser.

Source: Mike Arthur and Demian Saffer

Atmospheric Convection: Hadley Cells

There is a second, larger-scale effect that also plays a key role in the global distribution of precipitation and evaporation. Fundamentally, these patterns are also explained by the rise and fall, and cooling and warming of air masses – as is the case with the orographic effect – but in this case, their movement is a result of atmospheric convection rather than transport over topographic features.

As you have seen, there are regular climate and precipitation bands on the Earth – latitudes where most of the Earth’s tropical and temperature rainforests, deserts, polar deserts (also known as tundra) tend to occur. This global pattern – along with prevailing global wind patterns and storm tracks, are driven by atmospheric convection.
It all starts with solar radiation. Because of the Earth’s curvature, the tropics (between 23.5° N and S latitude) receive a larger flux of solar radiation per unit area on average than higher latitudes. Because the Earth’s axis is tilted, during Northern hemisphere summer, the peak influx of solar radiation occurs at 23.5° N latitude. During the Southern hemisphere summer, the maximum occurs at 23.5° S. (Incidentally, these latitudes define the tropics of Cancer and Capricorn.) Annually, the highest flux of solar energy per unit area occurs at the equator, as shown below.

As a result, the air around the equator becomes warmest. It holds quite a bit of water, too – based on the fact that, as you’ve seen above, warm air has a higher capacity to carry moisture.

Video Review: Global Atmospheric Circulation (2:24)

Take a few minutes to review the video below to help you understand Global Circulation a little better.

Energy Balance

Figure 15 - How Earth Receives light

Click Here for Text alternative of Figure 15

On average regions near the equator receive light at 90°. high latitudes receive light at low angles. Light energy is more concentrate near the equator. In other words, there is a greater flux per unit area (W/m²)

Source: Mike Arthur and Demian Saffer

Figure 16 - Solar Energy Concentration. Solar energy is concentrated near the equator.

Source: Mike Arthur and Demian Saffer

Figure 17 - Energy Absorbed and Emitted at varying latitudes. 

Source: Mike Arthur and Demian Saffer

Figure 18 - Radiation deficit and radiation surplus by latitude.

Source: Mike Arthur and Demian Saffer

The differential heat input from solar radiation input and loss by infrared radiation is a critical part of maintaining equability (relatively low gradients in temperature from low to high latitudes) on the Earth. The energy balance figures indicate that above about 40 degrees North and South (e.g., the latitude of New York City) of the equator the loss of heat by radiation (infrared), on average, exceeds the input of heat from the sun (visible). What does that imply for our climate? One might think that this should result in permanent snow or ice above this latitude. Right? Indeed, during the last glacial epoch, about 21 thousand years ago, thick continental ice sheets extended to nearly 40 degrees North in North America (just north of I-80). But normally, because of the heat gradient created by the imbalance between solar input and infrared radiation, the atmosphere (and ocean) is set in motion to redistribute heat from low to high latitudes. Otherwise, the tropics would be excessively hot and the high latitudes excessively cold—at all times. Next, we will see how this circulation works.

Global Wind

As this warm air rises due to its lower density, it cools. Once it cools past the dewpoint, condensation occurs and clouds form. With continued rise and cooling, the air cannot hold the moisture and precipitation falls.

In response to that rising air, surface air must flow in to fill the vacated space. The rising air results in a low-pressure center. This is why when you hear about low pressure in the forecast, is typically associated with rising air masses and therefore with crummy weather. The air rushing in toward the equator defines the trade winds. These winds converge on the equator but blow to the West because of Earth’s rotation. This rotational effect is known as the Coriolis effect. We won’t get into that in detail here, but if you are interested, check out the video below.

Video: The Coriolis Effect (03:05)

These flows drive convection cells, with dimensions that are controlled by the viscosity and density of air, and by the thickness of the atmosphere. The air that rose from the equator flows North and South at the top of the cell and eventually descends at around 30° N or S latitude. As the cool, now dry air descends it warms. Sound familiar?

Just as occurs when air descends on the leeward side of mountain ranges and causes rain shadows, the amount of water that the descending and warming air could hold increases. But there is no additional moisture to be found, so the actual amount of water vapor in the air mass remains more-or-less fixed. These descending limbs of the Hadley cells form high-pressure centers and would be regions where persistent dry conditions should prevail – leading to the Earth’s desert belts that include the Gobi, Sahara, Arabian, and the Australian Outback (not just a steakhouse!).

The equatorial convection cells are known as Hadley Cells. There are two more in each hemisphere, also driven by the uneven distribution of incoming solar radiation density; these are Farrell and Polar cells. Check out the diagram of this process below.

Figure 19. Global Winds

Source: DWindrim - Own work. Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons [25]