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?

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: Global access to clean water supply by nation.

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 1950-2015. The graph shows total water withdrawals (blue curve), and the partitioning of those uses between sectors.
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) [12]

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: C.A Dieter, et. al., Estimated Use of Water in the United State in 2015, USGS [12].

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 [13]

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 [13]

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 [14]), 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 2015 water withdrawals for agricultural (irrigation) 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 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

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 2024) is approximately 8.05 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 9.7 billion people by the United Nations—a staggering 20% increase in the next, say, 26 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 20% 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. United Nations World Population Prospects showing past and projected human population growth, broken down by world region.

United Nations, Department of Economics and Social Affairs, World Population Prospects 2022 [15].

Figure 21. 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 [16].

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). survey by CircleofBlue.org for 2019 water pricing in 30 cities across the U.S. found an average increase of 3.2% 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 2018 to 2019, costing an average of $72.93 a month.

Monthly rates for some representative municipalities are shown in the table below, based on data in the CircleofBlue.org 2014 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 [17]

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 [17]

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 (Watergy Nexus: The Complex Relationship and Looming Crisis Between Our Thirst For Water and Our Hunger for Energy [18]) 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 [19] 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.

Percent of population using privatized water resources as of 2017.

EPA Safe Drinking Water Information System (SDWIS) [20]

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 [21] 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 [22]). 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 [23] 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.

Michigan lawmakers propose that water bills are capped at 3% of households income and $2 added to bills that can afford to make up for lost income. Claiming that shutting off water is a human rights violation. Read more here. [24]

A 2022 Cornell Chronical story explains that private ownership had the largest impact on annual water bills, which averaged $144 higher in privately owned systems than in public sector systems. Low-income households served by private operators spent 4.4% of their income on water service, about 1.5 percentage points more than in communities with public ownership.

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.)

Figure 7: USGS water cycle diagram [26].

Public Domain

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 [27], 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) [28]

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

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 (02:43)

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 [29]

Channel Networks and Watersheds

Streams naturally assemble themselves into surprisingly well-organized (quasi-fractal) networks. Figure 2 shows a typical channel network where many small streams converge to make progressively larger streams. The smallest streams in the network, which have no other streams flowing into them, are referred to as first order streams. When two first order streams meet, a second order stream is formed. When two second order streams meet, they form a third order stream, and so on. According to this conventional stream ordering system, first developed by Horton (1945) and refined by Strahler (1957), when a smaller order stream (e.g., first order) meets a larger order stream (e.g., second order), the resulting stream retains the order of the larger stream (in this case, second order).

Each stream has a watershed, also known as a ‘river basin’ or ‘catchment’ because it is the land that ‘catches’ precipitation and funnels it towards the stream. The watersheds of two first order streams are outlined with grey dashed lines in Figure 2. The watershed of a second order stream is outlined in black dashed lines and encompasses the two first order watersheds. The solid black outline in Figure 2 shows the watershed boundary for the fourth order watershed, which encompasses all other watersheds nested within it. The right side of Figure 2 shows the Mississippi River watershed highlighted in green, with the Missouri River watershed nested within it, highlighted in orange. By the time the Mississippi River reaches New Orleans, it is a tenth order stream (though only a few of its largest tributaries are shown in Figure 2), and drains more than one-third of the contiguous US.

The concept of connectivity between rivers and their watersheds will come up again towards the end of this module in the context of restoration. If a particular stretch of stream is impaired for one function or another (e.g., fish habitat has been degraded), in some cases it makes sense to ‘fix’ that specific stretch of river, while in other cases the impairment is simply a symptom of problems higher up in the watershed, so the ‘fix’ may need to be applied at that location in the watershed before human intervention or natural processes can begin to repair the impaired stream. Such is the way that watersheds and streams are connected.

Figure 2. Left panel shows a stream network colored and labeled according to stream order. Dashed grey, dashed black and solid black lines indicate watershed boundaries. Right panel shows entire Mississippi River watershed (green) with the Missouri River watershed (orange) nested within it.

Source: Images Patrick Belmont

Watersheds are Complex Systems

When you look around, you see that the world is full of systems…assemblages or combinations of things that form a functional unit. Some systems are human-made, others are made by nature. Some systems are simple, meaning the way they work is straightforward and the outputs from the system are easily predictable. Other systems are complex, meaning they often have many parts that interact, often in non-linear ways, making the outputs from those systems more difficult to predict.

For example, a coffee maker is a pretty simple system. You put in 8 cups of water and two cups of coffee grounds and (assuming you put them in the right places), you turn the machine on and get ~8 cups of coffee. If you change the amounts of either of the inputs, it is pretty easy to predict the impacts on the coffee you brew.

Watersheds are not such a simple system. They are incredibly complex. One example can be seen in how the relationship between rainfall and runoff changes throughout the year. In a simple system, you would expect a constant relationship between incoming rainfall and outgoing flow. For example, a 1-inch rain event should translate to a stormflow hydrograph that might last 2 days and peak at 1000 cfs. But this isn’t what we see. Figure 3 shows streamflow (blue line, values on the left axis) and precipitation (orange bars, values on the right axis) from March through September 2008 for the Maple River near Rapidan, Minnesota. Precipitation is relatively evenly distributed throughout the year. As you can see, in April and May, rainfall events that are 0.5 to 1 inch result in relatively high flows (1000 to 1500 cfs). However, in July, August, and September, similar rainfall events hardly elicit any flow response whatsoever! Why do we see such non-linear behavior?

Figure 3. Streamflow and precipitation measured for the Maple River near Rapidan, Minnesota.

Source: Data from the Minnesota Pollution Control Agency. Plot created by Patrick Belmont.

Watersheds comprise many interacting parts. Figure 4 (top panel) is one way to represent various ‘parts’ that might be considered to comprise the watershed. While this is clearly a very simple view of this complex system, it is useful to take a “crude look at the whole”, a term coined by Nobel Prize-winning Physicist Murray Gell-Mann, as a starting point. When one component of the system is systematically changed, it may have direct as well as indirect impacts that propagate through the system. For example, changes in precipitation, snowmelt regime, or water storage may change streamflow. This altered streamflow has direct effects on river channel morphology, sediment transport, riparian vegetation, water quality, nutrient processing, and biodiversity, as indicated by the yellow arrows in the middle panel of Figure 4. But there are other interactions within the system, feedbacks that are indicated by purple arrows in the bottom panel of Figure 4. So to predict impacts of the changes in flow on aquatic biodiversity you would have to take into account not only the direct effects (yellow arrow between flow and aquatic biodiversity, but also the indirect effects associated with changes in channel morphology. This concept is also relevant in the context of watershed ‘restoration’. If a particular stretch of stream is impaired for one function or another (e.g., fish habitat has been degraded), in some cases it makes sense to ‘fix’ that specific stretch of river, while in other cases the impairment is simply a symptom of problems higher up in the watershed, so the ‘fix’ may need to be applied at that distant location in the watershed before human intervention or natural processes can begin to repair the impaired stretch of stream.

These notions of complex feedbacks and cascading effects greatly complicate the process of predicting what impacts human activities or natural disturbances within a watershed might have downstream. We’ll come back to this theme of system dynamics and complexity throughout the course.

Figure 4. Top panel shows various components that can be considered part of a watershed system. The middle panel illustrates components of the system that might be directly impacted by a change in streamflow. The bottom panel highlights other linkages (feedbacks) within the system that may amplify or dampen effects of the streamflow changes on the various components.

Source: Patrick Belmont

Streams

Streams are the most obvious way that water is moved through a watershed because we see them all over. But there are many other means by which water moves, as discussed in module 2. Figure 5 illustrates the various stocks (places were water is stored, even if only temporarily) and fluxes (mechanisms by which water moves) of water that may exist within any given watershed. For example, one raindrop might fall onto vegetation (called interception) and subsequently be evaporated back up into the atmosphere. Another raindrop might fall onto the soil surface and then runoff the surface into the stream channel or it might infiltrate down into the soil. Once in the soil, the water might further percolate down into the groundwater, where the soil or rock is saturated with water. Alternatively, once in the soil, the water might travel downhill within the soil and runoff into the stream or it might be taken up by vegetation and transpired back into the atmosphere. Estimating and predicting which, and to what extent, water travels through these pathways is an active field of hydrologic research and is also vitally important for environmental management and policymaking, as certain pathways may be more or less prone to filtering or polluting water along its journey to the place where you might want to use it for drinking, irrigating, fishing, swimming or the myriad other purposes for which we need water.

Figure 5. The water cycle represented at the scale of an individual watershed, from incoming precipitation (top) to the outlet of the stream at the bottom of the watershed (lower right). Arrows represent fluxes of water that can be transferred through the various stocks (places where water is at least temporarily stored). Streams can receive water via direct inputs of precipitation, runoff from the surface or (soil) subsurface as well as seepage from groundwater.

Source: Patrick Belmont

Hydrograph

A hydrograph is a graph of discharge over time. The time period shown could be short, for example, the flow resulting from an individual rain storm, or it could be long, for example, a continuous record of flow over many decades. While numerous federal and state agencies, corporations, and individuals monitor discharge in streams throughout the country, the US Geological Survey is the chief entity charged with monitoring streamflow, maintaining over 9,000 stream gages, most of which record water discharge in 15 minute intervals and many of which also include water quality data. Visit the USGS Water Resource webpage (water.usgs.gov) and peruse the wealth of information compiled to assess water resources. Exercises utilizing these data are included below in module 3 as well as module 4.

The Figure 4 shows example hydrographs from the Logan River, near Logan, Utah for two different water years (2006 and 2012). The water year begins October 1 and ends September 30. Hydrologists often prefer to conduct analyses based on the water year rather than the calendar year to facilitate comparison of incoming precipitation and outgoing streamflow, and specifically to ensure that snow delivered in October, November, or December is accounted for in the same time period that it is likely to melt, which may be in spring or summer of the following calendar year.

The Logan River hydrograph shows a long (about 5 month) prominent peak in discharge, primarily driven by snowmelt, with many other smaller peaks superimposed (from accelerated snowmelt during warm periods or rain events). The hydrograph of the Logan River over a 50 year time period (Figure 6) shows the prominent peak from snowmelt each year, but provides little information about the smaller scale variability that is visible on the annual timescale. Note the non-linear y-axis of the plots. Such axes can be useful for visualizing detail in both high and low flow conditions, whereas the detail in low flows would not be visible on (typical) linear axes. The apparent shift in low flows circa 1970 on the Logan River was caused by removal of a water diversion upstream from the gauge. Note that there is a considerable amount of ‘noise’ (i.e., variability) in streamflow over the past 50 years. This variability is not random, but rather has some ‘structure’ to it, some of which is visibly obvious (annual peaks) and other portions that can only be quantified using advanced analytical or statistical techniques, which are beyond the scope of this course, but currently represent a vibrant facet of hydrologic research.

Figure 6. Hydrograph for the 2006 water year (October 1, 2005 through September 30, 2006) for the Logan River, near Logan Utah.

Source: USGS

Figure 7. Hydrograph for the 2012 water year on the Logan River.

Source: USGS

Figure 8. Hydrograph for approximately 50 years of flow on the Logan River, near Logan, Utah.

Source: USGS

Examples of Logan River Hydrographs 2022-2023

Figure 11. 2022 Logan River Hydrograph

Source: USGS

Figure 12. 2023 Logan River Hydrograph

Source: USGS

River Flow Regimes

The temporal patterns of high and low flows are referred to collectively as a river’s flow regime. The flow regime plays a key role in regulating geomorphic processes that shape river channels and floodplains, ecological processes that govern the life history of aquatic organisms, and is a major determinant of the biodiversity found in river ecosystems. There are five components that characterize the flow regime:

1. Magnitude: the total amount of flow at any given time
2. Frequency: how often flow exceeds or is below a given magnitude
3. Duration: how long flow exceeds or is below a given magnitude
4. Predictability: regularity of occurrence of different flow events
5. Rate of change or flashiness: how quickly flow changes from one magnitude to another

River in regions with similar climate, geology, and topography tend to have similar flow regimes. For example, rivers draining high mountains, such as the Logan River, tend to have relatively infrequent, high magnitude, long duration, and predictable flood events that have a slow rate of change (Figure 6 on the previous page). Rivers in many tropical climates have similar flow regime characteristics as mountain rivers, due to predictable rainy and dry seasons. In contrast, rivers in arid regions are often characterized by high magnitude, short duration floods of low predictability and high flashiness (e.g., Figure 11 on the next page).

Within regions of similar climate, local factors such as soil type, soil depth, vegetation cover, and watershed size influence the natural flow regime. For example, watersheds with deep, permeable soils will be able to absorb more precipitation than watersheds with thin, impermeable soils, and will thus tend to have less flashy floods of lower magnitude and longer duration. Large rivers tend to be less flashy than small streams, which respond more quickly to individual precipitation events. Thus, natural flow regimes can be somewhat variable between nearby watersheds. Also, although general patterns in flow regime can be determined from watershed characteristics, yearly variation in precipitation patterns means that many years of flow monitoring will be required to fully characterize the flow regime of individual rivers.

Temporary vs. Perennial Streams

Most large rivers are perennial, meaning they maintain flow throughout the year. However, many headwater streams or streams in arid regions sometimes run dry. A stream is considered temporary if surface flow ceases during dry periods. Temporary streams are often classified further as intermittent and ephemeral. An intermittent stream becomes seasonally dry when the groundwater table drops below the elevation of the streambed during dry periods. A spatially intermittent stream may maintain flow over some sections or surface water in deep pools even during dry periods due to locally elevated water tables or perched aquifers. An ephemeral stream only flows in direct response to precipitation such as thunderstorms. Thus, the flow variability of an intermittent stream is much more predictable than in an ephemeral stream.

In many parts of the world, such as the desert southwest, temporary streams may comprise a majority of the river network, >80% in some areas. However, even in wet regions, temporary streams at the head of river networks can account for >50% of the total stream network. Thus, river networks can be considered dynamic systems, with total miles of surface flow expanding and contracting in response to precipitation events.

Figure 15. The San Rafael River watershed, located in southeastern Utah. Left figure shows only perennial streams. Right figure shows perennial, intermittent, and ephemeral streams. Note the large proportion of the network comprised of temporary stream channels.

Source: Brian Laub

Why would we still call a channel that goes dry for much of the year a stream? In other words, how can we distinguish between a temporary stream and an upland terrestrial ecosystem? In short, a stream has characteristic hydrological, geomorphological, and ecological processes. However, as with many topics in environmental science, the distinction between stream channels and uplands and between perennial streams and temporary streams is often fuzzy and scale-dependent. Individual stream channels may hold water for decades and then become dry during exceptional droughts that occur infrequently (once every 50-100 years). Similarly, small gullies on hillsides may flow only a few days of the year and may transport sediment but not be resident to aquatic life. Are such systems part of the river network?

Flow Duration Curve

While it can be very informative to study hydrographs and the other flow metrics described above, often an important question often asked about rivers is ‘what percentage of time does flow exceed (or not exceed) a given value (e.g., 100 cfs)?’ It might be important to answer that question to determine the percentage of time when the flow is too low to support a particular fish species. Or it may be important to know what percentage of time the river exceeds a certain value known to cause flood damage. The proportion of time any given flow is exceeded can be determined by generating a flow duration curve. Figure 22 shows the flow duration curve for the hydrograph of Logan River for four different years. You can immediately see that the mid and lower flows (exceeded about 40% (or 0.4) of the year) are relatively similar in each year, but the larger flows exhibit quite a bit of variability. In 2007 the highest flow of the year was only a bit over 400 cfs, while it was over 1500 cfs in 2006. The flow that was exceeded 20% of the time (0.2 on the x-axis) was approximately 450 cfs in 2005, but only 200 cfs in 2007.

Figure 22. Flow duration curves for the Logan River, in northern Utah for four different years. As the Logan River is a snow-melt dominated system, observed differences in high flows each year (left side of plot) correlate to differences in snowpack in each of the years.

Source: Patrick Belmont

Note that this plot provides detailed information on different parts of the flow duration curve depending on whether you use linear or log scales for the x or y axes (see example from the Stilliguamish River, Washington below in Figures 22-25).

Figure 23. Stilliguamish River, Washington

Source: USGS

Figure 24. Stilliguamish River, Washington

Source: USGS

Figure 25. Stilliguamish River, Washington

Source: USGS

Figure 26. Stillaguamish River, Washington

Source: USGS

Figure 27. Root River, Minnesota

Source: USGS

Flow duration curves can be made for a given river over two different time periods to illustrate if/how the range of flows has changed over time. For example, Figure 27 shows flow duration curves for the Le Sueur River in southern Minnesota for two different time periods (1950-1970 in blue, 1990-2010 in red). Note that in these plots the fraction of year exceeded is labeled as ‘exceedance probability’. These two terms are interchangeable, both being computed as:

$$Ep=\frac{R}{\left(n+1\right)}$$

Where E_p is the exceedance probability or the fraction of the year that a given flow is exceeded, R is the rank, and n is the total number of values (365 if you are using daily-averaged flow values for a non-leap year). High flows (toward the left side of each plot) and low flows (toward the right side of each plot) appear not to have changed in the Elk and Whetstone rivers. In the Blue Earth River, low flows (exceeded more than 85% of the time) have not changed much, but mid-range and high flows all appear to have increased. In the Le Sueur River, the full range of flows appears to have increased. Note that the y-axis is plotted on a log scale, so even the modest difference between the two curves represents a significant increase in high flows (e.g., those that are only exceeded 5-10% of the time). The Root River, in southeastern Minnesota, has experienced significant increases in high and low flows within the past two decades, see example above.

Figure 28. Flow duration curves are shown for two different time periods in the Le Sueur River, southern Minnesota. Changes in flows are attributed to changes in artificial drainage of agricultural fields, as well as changes in precipitation and crop type. See Schottler et al., (2014) Hydrological Processes for more information.

Source: Patrick Belmont

Number of Channels and Sinuosity

While the variety of river types is best thought of as a continuum, rather than a bunch of discrete boxes, it is often useful in science to create a taxonomy to classify items for the purpose of description and communication. Figure 30 illustrates some of the most common characteristics by which rivers can be classified (see Brierley and Fryirs, 2005 or Montgomery and Buffington, 1997 for detailed discussions of channel classification). At the most basic level it is useful to classify rivers according to the number of channels they contain, from single-threaded to braided (with more than three interweaving channels that are frequently reorganized) to anastomosing (which typically have somewhat stable, vegetated islands between channel threads), to discontinuous streams that have un-channelized reaches). Wandering rivers are those that alternate between single-threaded and slightly braided reaches. Another useful metric, particularly for single-threaded channels is sinuosity, which is calculated as the length along the river divided by the straight-line distance along the river valley. Rivers can have sinuosity ranging from one up to three (i.e., the river length is three times longer than the valley). Bends in rivers are called meanders. Meanders can exhibit a variety of forms with some in nature being remarkably regular (see the Fall River in Rocky Mountain National Park in Google Earth) and others being irregular or tortuous (frequently folding back on itself).

Figure 30. The number of channels and degree/type of sinuosity is two common metrics for characterizing rivers. The number of channels includes: single, wandering, braided, anastomosing, and discontinuous. The Degrees of Sinuosity are straight, low sinuosity, and sinuous. The types of sinuosity are regular, irregular, and tortuous.

Source: Patrick Belmont, after Brierley and Fryirs, 2005.

Stream Power

While there is currently no generalizable equation or universal law describing what a river channel should look like, a vast array of field data and modeling has culminated in some useful generalities. Stream power, defined as the product of water density (about 1000 kg/m³), gravitational acceleration (9.8 m/s²), discharge (m³/s), and channel slope (m/m), is one useful predictor of channel form and dynamics because it quantifies the amount of ‘work’ that can be done by a stream, such as moving sediment on the bed or in the banks of the river (i.e., erosion or sediment transport). For example, braided rivers tend to have more stream power than single threaded meandering rivers because their channel slope tends to be higher as they often flow closer to mountains (on steeper topography).

Sizes of a River Channel

River channels are self-formed. Typically they are only partially filled, the water level is well below the tops of the banks. Sometimes, they are overfilled and water spills out onto the floodplain. These simple observations lead to the fundamental question, ‘what sets the size of a river channel?’ igure 31 conceptually illustrates the rationale supporting the empirical finding that an ‘effective discharge’, which occurs frequently enough and has sufficient power to do work, ultimately dictates the size of the channel. Specifically, the brown curve illustrates that the frequency distribution of discharge in a river is typically right (positively) skewed, meaning that relatively low discharges are quite common and increasingly higher discharges occur with diminishing frequency. There is some discharge below which sediment does not move on the river bed because there is insufficient power to move the sand or gravel, as indicated by the light orange line starting at some moderate discharge and increasing in a non-linear manner at progressively higher discharge. Multiplying the brown and light orange lines together yields the darker orange line, which has a peak at some relatively high discharge value. This ‘effective discharge’ tends to occur when the river is approximately full up to its naturally formed banks. Even very large floods, which greatly exceed the capacity of the channel, do not necessarily add a proportionate amount of power to the channel because much of the additional water (and therefore the energy to transport sediment) is dissipated on the adjacent floodplain.

As changes in climate alter precipitation patterns or as land and water management modulates the proportion of precipitation that becomes streamflow, the frequency curve in Figure 31 may change and thus change the effective discharge as well as the geometry of the channel. In this way, rivers are dynamic features in the landscape, growing bigger when more water is flowing through the landscape and smaller during extended drier periods.

Figure 31. Conceptual figure illustrating the concept of effective discharge, which occurs frequently enough and moves a sufficient amount of sediment such that it often determines the size (width and depth) of a river channel.

Source: Patrick Belmont, after Leopold and Wolman, 1960.

Forecasting and Predictions

Meteorologists have made excellent progress in the past few decades to improve our abilities to forecast when rain events might occur over the next week or so, which facilitates the short-term forecasting of floods and droughts discussed in the paragraph above. However, complex atmospheric dynamics prevent forecasts beyond more than a few weeks in advance. Nevertheless, we must have some basis for making decisions about development, infrastructure and agriculture (e.g., How big should we build a culvert under a road? What size retention basin is needed next to a new housing development? Which agricultural fields will require artificial drainage and which will require irrigation?).

For these longer-term predictions we can use statistics to determine how likely it is that a given location will experience, for example, more than 10 cm of rain in a day, or less than 5 cm of rain during a given month. Many million- and billion-dollar decisions about development and infrastructure are based on such predictions.

To make these predictions, hydrologists synthesize historical data and use a statistics-based approach to determine the likelihood that a given event might occur. While Figure 1 highlights the ‘messiness’ of precipitation events over time, reorganization of the data provides useful information. Figure 2 shows a histogram of the precipitation data presented in Figure 1. A histogram is a plot showing the number of events that fall within chosen data groups or “bins” (shown on the x axis). From these data you can quickly determine that Kingston, NY experiences no rain about 2 out of every 3 days (731 out of the total 1096 days in this record). Only 10 days in the record had rainfall that exceeded 6 cm, so from these data alone you would expect such large rainfall events to happen 10 days out of 1096, or about 1% of the time. On 210 days during this time period the amount of rainfall was between the minimum measurable (typically 0.025 cm or 0.01 inch) and 1 cm (0.4 inches).

Hydrologists tend to use the term ‘forecast’ when referring to a future projection for which we have a lot of information (and therefore relatively high certainty of when an event might occur and what magnitude it might be). In contrast, hydrologists use the term ‘prediction’ for future projections for which less information is available, and therefore uncertainty is greater.

Figure 2. Histogram showing the number of events (specifically, total rainfall measured for each day from October 1, 2010 to September 30, 2013) in each of the bins indicated on the x axis. Bin labels are simplified for illustration purposes. For example, the bin labeled 0-1 includes all 210 days where precipitation was between 0.025 cm (minimum measurable) and 0.999 cm.

Click for a text description of Figure 2

Daily precipitation categorized by precipitation amount.

Daily Precipitation (cm)	Number of events
0	731
0-1	210
1-2	70
2-3	43
3-4	14
4-5	8
5-6	10
6-7	2
7-8	3
8-9	2
9-10	1
10-11	1
11-12	0
>12	1

Source: Patrick Belmont, data from NOAA National Climatic Data Center, as used in Figure 1 on the previous page.

Predicting Frequency and Magnitude

We can do similar calculations to estimate the frequency and magnitude of floods. For example, Figure 3 shows the annual maximum series of flows for the Lehigh River at Bethlehem, PA from 1910 to 2013. The annual maximum series is simply the highest flow value recorded for each year (typically for the water year, October 1 – September 30 as discussed in module 3). The largest flow on record (92,000 cfs) occurred in 1942 (on May 23, to be exact). Note that these are daily flow values which average flow over the course of the entire day, so the actual peak that occurred on May 23 was probably slightly higher. Interestingly, the year that had the smallest peak discharge on record was April 6, 1941 (only 8210 cfs, less than 10% of the whopper flood that came through the following year). But note that we are still talking about the highest flow for that particular year, which is still considerably higher than the average annual flow which is about 1,300 cfs. How can we use this information to predict what might happen in the future to support decisions about development, flood risk, or factors that may influence biota in the river or floodplain (e.g., factors limiting the return of certain fish)?

Similar to the precipitation example above, it helps to reorganize the data. The histogram shown in Figure 4 illustrates the frequency of events in 19 different “bins”. For example, the Lehigh River at Bethlehem, PA has never experienced an annual peak flow less than 5,000 cfs, so there is no orange bar in that bin. But it has experienced 4 peak flows that fall between 5,000 and 10,000 cfs. It has experienced a total of 55 peak flows that fall within the range of 10,000 to 25,000 cfs. These are the most common peak flows for the Lehigh River. And you can see the two outliers at the high end (92,000 cfs in 1942 and 91,300 cfs in 1955).

Typically, hydrologists will approximate the distribution of events that you see in Figure 4 as a probability density function, represented by the smooth, dashed-grey curve superimposed on the plot. This allows them to integrate under the curve above a value of interest to determine the probability that a flood of that magnitude (or larger) will occur. For example, integrating to find the area under the grey dashed curve above 60,000 cfs in Figure 4 would tell you the probability in any given year (in the future) that a flood of that magnitude (or larger) will occur. From that information, you can build your bridge to the appropriate size, or set your flood insurance rates accordingly, etc. Calculating probabilities associated with floods of a given magnitude is discussed in greater detail in the exercise associated with this module.

The same general principles apply to studying droughts. However, as we discuss below, droughts are more difficult to define and quantify because they build up over time and vary immensely over any given area. Nevertheless, similar plots of drought frequency, severity and duration can be developed for droughts, similar to Figure 4, to make sense of all the messy variability we observe in water deficiencies. With this brief introduction hopefully you can appreciate the great challenges in predicting the occurrence and severity of floods and droughts, and you can begin to see the implications for socio-economic systems and ecosystems.

Figure 3. Annual maximum series of flows (a.k.a., annual peak flows) for the Lehigh River at Bethlehem, Pennsylvania. Specifically, the maximum daily flow recorded each year is represented.

Source: Patrick Belmont, data from US Geological Survey

Figure 4. Orange bars show the histogram of the annual peak flows shown in Figure 3 for the Lehigh River. Grey dashed line is a theoretical distribution (a probability density function, or PDF) approximating the complete distribution represented by the histogram of historical data. The PDF can be used to make predictions of flood frequency.

Source: Patrick Belmont, data from US Geological Survey

Hydrologic Versus Hydro-Geomorphic Perspectives

Scientists tend to think about floods in two different (but related) ways, one being strictly hydrologic and the other requiring an evaluation of the floodplain topography and how different flow regimes might impact the channel and surrounding areas. In module 3, we explored how hydrologic analyses (analyzing the patterns such as the frequency, duration, and magnitude of flood events) could be used to characterize the river flow regime (e.g., how often does the river exceed 800,000 cfs?). However, a hydrologic analysis does not provide information about the extent or duration of flooding across the landscape (e.g., which parts of the natural floodplain or streets will be flooded?). To predict how much of the floodplain might be inundated by a given flow, we need to consider the channel and floodplain topography (a hydro-geomorphic analysis). For example, a river system with low channel banks and a broad, flat floodplain will experience more frequent flooding of greater extent than a river system with tall banks and a narrow floodplain, given the same flow regime.

Because the size of a river channel can change over time, the relationship between the hydrologic flood frequency and hydro-geomorphic mapping of the area inundated may also change, as discussed in the later module section on hydrologic non-stationarity. For example, the Minnesota River (a major tributary of the Mississippi River) has widened by nearly 50% in the past 3-4 decades. Therefore a flood that may have inundated a significant amount of floodplain 50 years ago may now be entirely conveyed within the channel itself. Thinking back to the example of the Lehigh River in Figures 4 and 5 it is very likely that the 1941 flood (the lowest on record) did not fill the channel and inundate the floodplain. 1943 and 1944 had moderately high peak flows, but may also not have gotten out of the channel because the massive 1942 flood would have widened and deepened the channel. Over the following years, the channel would likely have narrowed again, in response to relatively smaller floods. Flood frequency analysis discussed below and in the exercise associated with this module, is strictly a hydrologic analysis. A hydro-geomorphic analysis is needed to estimate the risk of flood damage. Both types of analyses may be important for engineering plans and ecological studies. High-resolution topography data (elevation data with a vertical precision of about 15 cm and horizontal resolution of about 1 m, also known as ‘lidar’ for Light Detection and Ranging) is revolutionizing the way we make flood inundation predictions. Lidar data contains very detailed information about the ground surface, as well as vegetation on the floodplain, which exerts a strong influence on the velocity and depth of the water. Many states are revising their flood risk maps using this new high-resolution data.

Societal and Economic Implications of Floods

Floods are consistently ranked among the most costly natural disasters around the world, with many billions of dollars in damages reported annually. For example, the Centre for Research on the Epidemiology of Disasters International Disaster Database [43] (EM-DAT) reports that floods accounted for four of the ten most deadly natural disasters in 2013, with confirmed global fatalities exceeding 5000 people (EM-DAT, (2023)). The same 2023 report documents $20.4 billion in damages directly related to river floods.

Number of disasters by continent, comparing 2003-2022 average to 2023. Notice floods and storms are the largest proportion of natural disasters globally.

Downloaded from the Centre for Research on the Epidemiology of Disasters (CRED) [44]

During late spring and summer of 1993 heavy rains all throughout the Midwestern US resulted in flooding along the upper Mississippi and Missouri river systems. The floods were the most costly in US history, causing about $15 billion in damages and forcing about 75,000 people from their homes. Heavy rains from Tropical Storm Irene in August 2011 caused approximately $10 billion in damages throughout the Caribbean and eastern United States (including flooding Kingston, NY during the wet period indicated in Figure 1). Notably, the numbers of fatalities associated with these extreme events were relatively low (~50 deaths each) due to remarkably accurate flood forecasting, highly effective emergency response systems and regulations that limit development in flood-prone areas.

Figure 6. Flooding along the Mississippi River near St. Louis, Missouri on July 9, 1993.

Source: Photo by Andrea Booher, Federal Emergency Management Agency.

Rivers that cannot transport their sediment load (sand and gravel) are particularly susceptible to flooding because sediment settles out in the river bed, causing the river channel to become shallower relative to its banks, thus increasing the chances of flooding. The Yellow River in China is one example of such a river. While the Yellow River has played a pivotal role in the Chinese economy for thousands of years, sedimentation has repeatedly caused the river channel bed and banks to actually build up higher than the surrounding floodplain. This is an especially dangerous situation that can cause the river to catastrophically flood, breach its banks, abandon its channel altogether, and ultimately form a new channel elsewhere within the floodplain, a process known as an avulsion. The Yellow River has caused many devastating floods, including a flood in 1332-1333 that killed an estimated 7 million people. Another Yellow River flood in September of 1887 inundated an estimated 130,000 km² (50,000 square miles, an area approximately the size of Alabama!) and killed an estimated million people. Yet another flood in 1931 is estimated to have killed 1-4 million. Such catastrophic disasters have earned the Yellow River its nickname, ‘China’s Sorrow.’

Humans have made extraordinary efforts to reduce flood damages. In some cases, these efforts involve limiting development in flood-prone areas. In other cases, these efforts involve building structures meant to control the floodwaters. Flood control structures include dams and retention basins that store water and/or building levees, dikes, and floodwalls that attempt to keep floodwaters confined. Some of the most extensive flood control systems in the world include the floodway diversions on the Red River, which runs between Minnesota and the Dakotas and crosses the US-Canada border into Manitoba.

While we typically think of floods as dangerous and costly natural hazards, they can also provide benefits to society. For example, floods naturally deliver fresh, nutrient-rich sediments to their floodplains, which have historically benefited farmers in many places throughout the world. Yearly floods of the Nile River allowed the early Egyptian people to grow crops, which helped them thrive as a civilization for thousands of years. However, the severity of the floods was unpredictable and floods that were too large caused significant damage. Therefore, in the mid-1900s the Egyptians constructed a flood-control dam on the Nile River. The dam eliminated both the risks and benefits of annual flooding and therefore agricultural practices have had to adapt by using irrigation and petroleum-based fertilizers to replace the water and nutrients that are no longer delivered to the floodplain by the river.

Flood control is not always feasible, given the unpredictable nature of these events as well as geographic or economic constraints. Nor is flood control necessarily desirable in many situations, given the potential environmental benefits for the river and floodplain discussed at the beginning of this section and discussed in greater detail towards the end of this section. In such cases, efforts can be made to reduce economic losses from floods. For example, in many places regulations limit the construction of permanent buildings on floodplains. Emergency response programs, such as the National Weather Service and Federal Emergency Management Agency (FEMA) help flood victims by improving methods to warn and evacuate people from flood-prone areas and to provide relief aid. In an alternative approach, communities in Tonle Sap, Cambodia have constructed their houses on floats and stilts to deal with the annual flooding of 8-9 m (26-30 ft) from the Mekong River.

Figure 7. Houses on Tonle Sap lake in Cambodia built on stilts to accommodate annual flooding of the lake by the Mekong River.

Source: Colocho, shared under Creative Commons Attribution-Share Alike 2.5 Generic license.

In many places, flood insurance can be purchased to help cover costs associated with residential and commercial flood damages. However, the private insurance industry is somewhat limited because the number of potential claimants far exceeds the number of people who wish to ensure their property against flooding. As a result, the US Congress created the National Flood Insurance Program in 1968. The program is an effort to provide flood insurance to protect homeowners, renters, and business owners as well as an effort to encourage communities to adopt flood risk management policies established by FEMA.

Measuring the Severity of Drought

Many different indices have been developed over the past several decades to indicate the occurrence and severity of drought. The simplest index relates precipitation amounts during a specific period of time to the historical average during that same time period. For example, precipitation for the month of June 2014 was 15% below the historical average for Wenatchee, Washington. While this statement conveys some useful information, it is not possible to determine whether or not that 15% deficit qualifies for any of the definitions of drought. The number of days with no precipitation is another simple index, but again must be considered in the context of historical data or water demand, and there is no standard definition for what number of days without precipitation would necessarily qualify under any of the four types of drought. Also, if an area receives a very small amount of precipitation (< 0.1 cm) during an otherwise unusually dry time period, a strict interpretation of this index would ‘reset the clock’, but in reality, the severity of the water deficit remains essentially unchanged. Complex phenomena, such as drought, require somewhat complex metrics to be measured in a meaningful way.

The Standardized Precipitation Index (SPI) is a slightly more complex measure of precipitation deficit that compares measured precipitation to the median historical precipitation over multiple timescales, ranging from one month to 24 months. As dry or wet conditions become more severe, SPI becomes more negative or positive, respectively. Several different indices of varying complexity have been developed to assess drought based on both water supply and demand using multiple environmental criteria. The most common index used to define and monitor drought is the Palmer Drought Severity Index (PDSI), which attempts to measure the duration and intensity of long-term, spatially extensive drought, based on precipitation, temperature, and available water content data. PDSI ranges from values exceeding 4.0, which are considered extremely wet, to values below -4.0, which are considered extreme drought (see Figure 12). Weekly maps of PDSI for the entire US (current and historical) can be viewed on the web page maintained by the National Weather Service Climate Prediction Center [47].

Figure 12. Palmer Drought Severity Index map of the conterminous US, segmented by climate divisions. This example is drawn from the week ending June 8, 2024.

Source: Climate Prediction Center, NOAA [48]

Related indices are the Palmer Z Index, which attempts to measure short-term drought on a monthly timescale, the Palmer Crop Moisture Index, which attempts to measure short-term drought and quantify impacts on agricultural productivity, the Palmer Hydrological Drought Index, which attempts to estimate the long-term effects of drought on reservoir levels and groundwater levels. An immense compilation of current and historical drought information for the entire US is freely available on the US Drought Monitor web page [46], maintained by the University of Nebraska National Drought Mitigation Center.

Increasingly, government and industry groups are using ‘cloud seeding’ techniques to induce precipitation and reduce the severity of a drought. One of the potentially limiting steps in the formation of precipitation is the presence of tiny particles (nuclei) on which water can condense and coalesce to form raindrops or ice crystals large enough to begin falling through the air. Cloud seeding is the practice of injecting nucleating agents, such as silver iodide (AgI), into clouds in an attempt to form precipitation. The effectiveness of these approaches is questionable, but under the right conditions, cloud seeding may increase the probability of rain and therefore it is practiced in some semi-arid regions, including the western US. However, questions remain regarding environmental and human health impacts as well as concerns regarding ‘stealing’ atmospheric moisture from would-be recipients downwind.

Floods and Droughts Impact Ecosystems

How do floods and droughts impact ecosystems?

Variation in river flow (i.e., the river flow regime – see Module 3) exerts a strong influence on river and riparian ecosystem function. In particular, floods and droughts control the creation and maintenance of river and floodplain habitats and the sustainability of the high biodiversity observed along river systems. The temporal pattern of floods interacts with channel and floodplain topography to create a highly heterogeneous landscape of depressions, oxbows, gravel bars, and terraces (Figure 13). The hydro-geomorphic diversity means that the inundation frequency varies strongly over short distances on river floodplains, and creates habitats for a diverse suite of organisms adapted to a wide range of flooding frequencies.

Figure 14. The Tagliamento River and floodplain in northeastern Italy. Note the diverse array of hydrogeomorphic surfaces, including vegetated islands, gravel bars, off-channel pools and non-flowing channels. The different surfaces will become inundated at different magnitude floods and thus will be flooded at different frequencies over time. The diverse array of flooding frequencies provides habitat for a diverse group of aquatic and riparian organisms.

Source: Brian Laub, aerial imagery from Esri

Both riparian and aquatic organisms have adapted to take advantage of flood-drought cycles in river ecosystems. For example, many fish species time spawning runs to coincide with predictable floods, because this allows large adult fish to access small streams that provide optimal habitat for egg development and growth and survival of young fish. In the Amazon River, many fish species can almost be considered forest-dwelling fish, because they feed directly on leaves, fruits, seeds, and insects that fall into the river when it floods surrounding forests during the annual rainy season. Trees of these seasonally flooded forests have in turn developed fruits and seeds that mature during the flooding season and that can survive fish digestive systems in order to take advantage of the seed dispersal ability of mobile fish species. In the western U.S., cottonwood trees time the release of seeds to coincide with the recession of flood peaks in order to access fresh sediment deposits with elevated water tables that provide ideal habitats for germination.

It may be less obvious that droughts could be beneficial for aquatic and riparian biota, but when coupled with periodic flooding, droughts play an important role in the survival of many river organisms. During droughts, resources such as organic material and nutrients can accumulate on floodplain surfaces, and when a flood does occur there is a pulse of greater resource availability than would occur under regular flooding, and this period of high resource availability can ensure the quick growth and survival of organisms, including young fish. In addition, periodic drying of rivers and floodplain wetlands eliminates competitors and predators for organisms that can quickly colonize areas when water returns. Such areas of refuge from predators are critical for the persistence of many aquatic organisms and would not exist without periods of drought.

The importance of floods and droughts to the integrity of river-floodplain ecosystems is apparent when alterations to the natural flow regime occur. Riverine organisms are often closely adapted to the local magnitude, frequency, duration, and predictability of extreme events, such that alteration of any one component can threaten species persistence. For example, recruitment of cottonwood trees along many dammed rivers in the western U.S. has essentially ceased, because the dams prevent flooding and creation of germination sites during the spring when cottonwood trees release their seeds. Excessive drought is also highly detrimental to river systems. One of the most famous examples of drought impacts is seen in the Colorado River delta in Mexico, which was once a highly productive floodplain forest and swamp, but due to prolonged drought conditions in the river basin and water infrastructure development, is now a dry desert.

Non-Stationary Hydrology

When ‘normal’ changes: non-stationary hydrology

As we have discussed at length above, floods and droughts can have significant impacts on society and the environment. However, as also discussed above, we can characterize the frequency, duration and magnitude of these rare or extreme events and both human and natural systems have mechanisms to deal with them. For example, we can estimate the magnitude of a 100-year flood and build a bridge sufficiently large to pass the flood with minimal risk of damage to the bridge or changes in water depth (i.e., from water backing up behind the bridge and therefore flooding additional land upstream). Similarly, we can compute the width of a floodplain that would be inundated from that 100-year flood and choose to not build within that flood corridor, or choose to require those who do build within the flood corridor to purchase flood insurance to cover costs of potential damages.

Similarly, as we discussed in module 3, river channels naturally adapt their width and depth to accommodate common floods. Thus, society and the environment naturally develop some amount of resiliency to historical climate extremes. However, all of the prediction methods we have discussed so far in this module rely on the assumption that the future will look statistically similar to the past (i.e., the distribution of events will not change and occurrence of events in the future will be consistent with the probabilities computed from the histograms or probability density functions such as those shown in Figures 3 and 4). This assumption is known as the Stationarity Assumption in hydrology. Specifically, stationarity implies that while there is considerable variability in precipitation and streamflow, that variability is bouncing around a relatively constant average value and has a relatively constant spread, as shown in the hypothetical plot of peak streamflow over time in the left panel of Figure 17. From left to right, the mean (µ) and standard deviation (s, i.e., the spread of the data around the mean) don't change. However, as climate changes, the magnitudes, durations, and frequencies of floods and droughts may occur that are outside the historical range of observations, resulting in a change in the average magnitude of floods (Figure 17, middle panel) or a change in the variability of floods (Figure 17, right panel). In either of these situations, the statistics from the left side of the graph don't provide a good basis for making predictions about the right side of the graph (or predicting what will happen in the future!). Understanding and accounting for such non-stationary patterns in precipitation and streamflow are among the greatest challenges in hydrology today because we need to make accurate future predictions for many decisions about flood and drought risk, infrastructure design (roads, bridges, culverts, ditches, parking lots, detention basins, sewers, etc.!) and water availability. So this is a hot topic in the field and many new techniques are emerging!

Non-stationarity will be common in the future as regional climates systematically change. According to the Intergovernmental Panel on Climate Change, climatic warming will increase the risk of both floods and droughts (Table SPM2 in IPCC, 2007; see also IPCC, 2014 and IPCC 2023). The multitude of factors that combine to ultimately cause floods and droughts are exceptionally difficult to predict over the next few decades. Nevertheless, there is a high level of agreement among the competing IPCC climate simulation models regarding the general trends of several metrics. For example, precipitation intensity increases are expected in most places and especially at mid- and high-latitudes where mean precipitation also increases. Summer droughts are also expected to increase over low and mid-latitude continental interiors. Snowpack is expected to decline overall as more precipitation will fall as rain rather than snow, especially in areas with temperatures near 0°C in fall and spring. Relatedly, snowmelt is projected to occur earlier. The combination of less snowpack and earlier snowmelt increases risk of summer to fall drought in snowmelt dependent regions, such as the western US. Some regions outside the US are highly dependent on meltwater from glaciers for water supply. Accelerated melting due to climatic warming increases the risk of flooding downstream in the near-term. In the long-term, glaciers in many of these areas will shrink and ultimately cease to exist, posing a serious threat to the downstream water supply. This poses a serious risk to the hundreds of millions of people in China and India who depend on glacial meltwater from the Hindu Kush-Himalayas. In closing, there is a good reason to expect floods and droughts to become more severe in the coming decades, increasing the urgency for improved predictions, mitigation efforts, and adaptation strategies.

Nutrient Supply to Floodplains

Flooding is a natural process that replenishes soil and nutrients to floodplains. Of course, floodplains are ideal sites for agriculture – they are flat, water is accessible, and – at least prior to modification of the system by levees or dams – the soils are among the most fertile on Earth due to recurring flooding that deposits nutrient-rich fine-grained sediments. Historically, these are the sites of major agricultural and population centers, including the “Fertile Crescent” along the Tigris-Euphrates floodplain (now largely barren due to long-term effects of irrigation-based agriculture and flood prevention), and the Nile Floodplain (see The Nile River and Aswan Dam below). Likewise, most major modern agricultural production is localized to floodplains - including the Central Valley of California, the Susquehanna River Valley, the upper Tigris-Euphrates basin, the Nile Valley, and the floodplains of the Mississippi and Missouri Rivers.

Prevention of flooding through the combination of dams (which control river discharge) and levees (which artificially channelize flow and shunt it downstream so that it cannot spill onto the floodplain) is a strategy to limit the loss of crops and property, and allow development in otherwise flood-prone areas. This approach, while generally effective in limiting short-term losses, affects soil fertility, groundwater systems, and the health of downstream waterways in the longer-term. For example, flood prevention eliminates a major source of recharge to aquifers in valley-fill sediments that lie below the floodplain. Recurring floods also serve to flush salts that accumulate naturally in soils due to evaporation and transpiration (i.e. water is transported to the atmosphere by these processes, but even small amounts of dissolved salts remain in the soil). Reduction or elimination of this flushing can lead to soil salinization, with negative effects on soil fertility.

Perhaps most notably, by eliminating or limiting the replenishment of nutrients to the floodplain, imported fertilizer is required to grow crops. Excess fertilizer application, in turn, leads to runoff enriched in Nitrogen and Phosphates that affects aquatic species and can cause eutrophication of lakes and estuaries downstream. This is a longstanding problem that leads to algal blooms at river mouths, consumption of Oxygen by organic matter (dead algae), and ultimately to “dead zones” in these regions that affect fisheries (Figure 2).

Figure 2: Dissolved oxygen levels in the Gulf of Mexico in Summer, 2011. Note clearly demarcated “dead zone” extending along the Texas-Louisiana coastline.

Source: NOAA (Public Domain) [51]

The Nile River and Aswan Dam

Large rivers are difficult to control. The Nile River, so important to Egypt's populace, is no exception. But since the late 1960s, the Nile River has been under the control of humans because of the construction of the Aswan High Dam. Part of the rationale for this dam was to manage the natural cycles of flood and drought to produce dependable water supplies for farming and other uses. The consequences of cyclic climate variations on a decadal scale were buffered by the large storage capacity of the Nile Valley behind the High Dam, which is nearly six trillion cubic feet (157 km3) of water! This is about four times the amount of water stored behind Hoover Dam (USA, Lake Mead) and Three Gorges Dam (China) (Chao et al., 2008). In addition, the Aswan High Dam initially produced a significant amount of electrical power (about 50%, now less than 15% of Egypt's needs) that allowed electrification of "rural" Egypt.

Figure 3 shows the narrow Nile River Valley slicing northward through the Egyptian desert. The narrow green band of the Nile River Valley represents farmland irrigated by waters of the Nile River. Prior to the completion of Aswan High Dam, the Nile River would flood its valley annually during the rainy season in its higher altitude headwaters (Ethiopia, Sudan, Kenya, Uganda), bringing nutrient-rich silt to fields and renewing fertility. In addition, a substantial volume of sediment was carried down the Nile River Channel to its large delta, building out the delta into the Mediterranean Sea, providing additional fertile land for farming. This no longer happens because the Aswan High Dam effectively (an unintended consequence) traps sediment carried from the highlands behind it. Now, the delta region, which subsides naturally as the result of compaction of sediment (newly deposited sediments have water contents of 70% or more that are reduced by compaction by overburden), is diminishing in size because rates of coastal erosion exceed supply of sediment. Currently, almost 1/3 of the Nile Delta’s land area sits within a meter of sea level. Subsidence rates vary across the delta, but in some areas, the land surface is sinking as fast as 1 cm/yr. Control of the Nile’s flow has also lead to water quality problems. Because once-regular floods no longer flush salts, sewage, fertilizers, and waste from the delta, surface waters are polluted and those living near the Mediterranean coast increasingly rely on groundwater to meet demand for drinking water and domestic use. Extraction of groundwater, coupled with land subsidence, has led to saltwater intrusion in the aquifer as far as 30 km inland.

There have been other unintended consequences of the Aswan High Dam including the spread of disease (Schistosomiasis), a decrease in water quality and increase in algal blooms resulting from fertilization of farm fields and irrigation runoff, flooding of historical sites, and displacement of people from the regions flooded by the reservoir.

Figure 3. Egypt and the northern Red Sea (Google Earth image) and the Nile River Valley coursing through the desert region. Much of the green swath in the image is the fertile Nile valley with agriculture. The Aswan Dam and its reservoir (darker blue), however, can be seen in the lower part of the image.

Source: Google Earth

Figure 4. Satellite image showing the northern Nile River and Nile Delta on the southeastern margin of the Mediterranean Sea, as well as the Gulf of Suez, Gulf of Elat and northernmost Red Sea in the Egyptian Desert.

Source: NASA / MODIS

Sediment Trapping

Because large reservoirs behind major dams are areas where water flow velocity is slowed (also often called “slackwater”), sediments are deposited where rivers enter the water body (Figure 5). Sedimentation in reservoirs behind dams has several consequences. Sediment deposition reduces reservoir water storage capacity and therefore limits the useful lifetime of the dam for flood control, water supply, and hydropower generation. Recent detailed studies of storage capacity and sedimentation rates for reservoirs in the U.S. suggest that average annual storage losses range from less than 0.5% to more than 2% (see supplemental reading: Graf et al., 2010; “Sedimentation and sustainability of western American Reservoirs, Water Resources Research”). The highest rates of storage loss are occurring in the American West, and the lowest in the Northeast.

For example, almost 20 million tons of sediment are deposited annually in reservoirs along the Mississippi River (UNESCO, 2011). China’s Three Gorges Dam alone (one of several along the Yangtze River) traps 34 million tons of sediment per year, or 31% of the river’s sediment load (Hu et al., 2009). Globally, the amount of sediment trapped in dams is estimated to be 73 km3 (Syvitsky & Kettner, 2011) and storage loss to trapped sediment has reached 16% of initial storage capacity with an expected 26% loss by 2050 (Perera et al., 2023).This sediment accumulation slowly reduces reservoir capacity behind dams and is one factor that limits their useful life expectancy. Recent studies of sediment accumulation suggest that the life expectancy of Lake Powell is ~300-700 yr, and that of the Three Gorges Reservoir in China is ~150 yr.

Figure 5. Aerial photo (taken in 2004) of delta and sediments deposited into Upper Las Vegas Bay by Las Vegas Wash where it flows into Lake Mead. The deposition of sediments has already filled in much of the bay, and as a result, the boat ramp and marina (part of the Lake Mead National Recreation Area) have been abandoned.

Image from US Geological Survey [52] (USGS) (Public Domain)

Figure 6. Sediment discharge in the Mississippi River and major tributaries before (left) and after (right) large-scale damming of the river system.

US Geological Survey Circular 1133 (1995)

The concomitant reduction in sediment delivery to downstream areas also has several consequences (Figures 5-6). Ultimately the decreased sediment supply to the river mouth translates to net erosion of beaches and loss of land in coastal regions, as natural coastal erosion by currents and subsidence caused by compaction of delta sediments is not offset by delivery of sediment. For example, prior to construction of the Aswan High Dam began in 1960, the annual sediment flux to the Nile Delta was ~100 million tons. This sediment supply was enough to offset erosion and natural subsidence.

Other Impacts

In this section, we will consider other impacts of dams.

Three Gorges: A “Mega-Dam” and Its Impacts

The Yangtze River is the longest river in Asia and is the world’s 3rd longest (only the Nile and Amazon are longer). It flows for ~6300 km from its headwaters on the Tibetan Plateau to its delta at Shanghai, where it discharges to the East China Sea. The Yangtze watershed encompasses approximately 1/5 of China’s land area. The river serves the water demand of millions of people and the delta alone supports almost 20% of China’s GDP. However, the Yangtze is also notorious for its frequent and devastating floods. Floods in the twentieth century alone led to the loss of an estimated 300,000 lives, including 145,000 drowning deaths in a 1931 flood, and 30,000 deaths in 1954 from flooding and diseases that followed. In addition to loss of life, these floods inundated hundreds of thousands of acres of productive farmland and caused billions of dollars of damage.

To protect over 15 million people in Shanghai and the lower Yangtze floodplains, and control flooding of almost 15,000 square km of land, construction of the Three Gorges Dam began in 1994. The dam, constructed at a cost of between ~$28-60 billion dollars (exact cost is not known because the project has been funded by a combination of government subsidy and private investment), is nearly 200 m high, spans more than 2 km across the river, and was engineered to withstand a magnitude 6.0 earthquake (Figure 11). The long, narrow Three Gorges Reservoir extends ~600 km upstream and has a capacity of almost 40 billion m3 of water (equivalent to about 32 million acre-feet) (Figure 12). At the time of construction, the dam was the largest hydroelectric power plant on Earth, with a generating capacity of over 20,000 megawatts - more than 20 times that of Hoover Dam, equivalent to 18 nuclear power plants, and enough to supply almost 10% of China’s power demand. According to the Chinese government, if this amount of electricity were generated using coal-fired power plants instead, 100 million tons of additional carbon dioxide would be released into the atmosphere. The dam also increases the navigability of the Yangtze, allowing large freighters to transport goods far into China’s interior.

Despite the obvious benefits of the dam for the economy and generation of renewable energy, the Three Gorges Dam has been mired in controversy since its inception. Concerns about the dam include an array of environmental impacts, the forced relocation of over a million residents, initiation of large landslides and earthquakes by the rising reservoir, and flooding of important historic and cultural sites in the gorge upstream of the dam. The chief environmental issues center on impacts to river ecology and already threatened species increased chances for waterborne diseases, and water quality degradation associated with the slowed flow of the river in the backwaters of the dam, in tributaries, and in downstream regions. Indeed, in the wake of pollution concerns, during construction, an additional $4.8 billion was budgeted for new treatment plants and garbage disposal sites along the river’s upstream reaches.

Figure 11. The Three Gorges Dam in the final phases of its construction.

Source: "ThreeGorgesDam-China2009" by Le Grand Portage Derivative work: Rehman - Licensed under Creative Commons Attribution 2.0 via Wikimedia Commons

Figure 12. Image of the Three Gorges Reservoir, taken by astronauts on the International Space Station on April 15, 2009, when the reservoir was filled to near its capacity (maximum water level was reached for the first time in October of 2010)

Source: NASA

Enhancement of Populations of Anadromous Fishes

Dam removal has become increasingly popular, particularly as regards enhancement of populations of anadromous fishes--salmon and steelhead in the western U.S. and shad, herring and other species in the eastern U.S. Figure 13 illustrates the dams that have been removed in the lower 48 states (Figure 14 expands the eastern U.S.) from 1912 to 2023 on the basis of data tabulated by American Rivers [57]. These dams have been removed largely to improve the ecological conditions of river systems and to allow migratory fish to pass unimpeded for spawning in the upper reaches of rivers. Dam removal is commonly cited as a way of increasing stocks of imperiled fishes (Chesapeake Quarterly: Those Dammed Old Rivers [58]), allowing them to spawn in rivers that have been inaccessible to fish migration for as much as a century. Others argue that dam removal from coastal rivers is only part of the solution for fisheries improvements. Although numbers of anadromous fish may increase, the size of individual fish may not as long as fishing pressure remains the same (Oregon State University: Department of Fisheries and Wildlife [59]). This has been argued in the case of Elwha Dam removal in the Olympic National Park watershed (Crosscut: Elwha dams: Will bringing down NW dams really help salmon? [60]). In some cases, the benefits will accrue to native Americans who formerly depended on fishing for their nutrition and livelihood.

Figure 13. Map of the lower 48 states showing locations of dams removed from 1912 to 2023. At American Rivers [61], the locations are clickable to show information on each dam removed.

Source: A [62]merican Rivers [61]. Used with permission. All rights reserved.

Figure 14. Blow up of eastern U.S. portion of the dam removal map.

Source: American Rivers [61]. Used with permission. All rights reserved.

Esthetic Improvements

Arguments have also presented for esthetic improvements as the result of dam removal. An example of this approach is the argument for the removal of Hetch Hetchy (O'Shaughnessy) Dam on the Tuolomne River in the Sierra Nevada Mts. of California (Restore Hetch Hetchy [63]). This dam and its reservoir flooded a canyon much like Yosemite just to its south beginning in 1923, even though Hetch Hetchy was included in Yosemite National Park in 1890 by President W.H. Harrison. President Wilson, in 1913, signed the Raker Act that allowed San Francisco to dam the valley. Proponents of this dam removal argue for ecological improvements as well as access to once-spectacular scenery eliminated as the result of the water project. Hetch Hetchy provides some 20 percent of hydroelectric power generation to San Francisco as well as significant water supplies. Nonetheless, proponents of the dam removal argue that impacts on water or electrical power availability would be minor (Hetch Hetchy Today [64]).

Issues Accompanying Dam Removal

One of the issues accompanying dam removal is the potential impact of the large volume of sediment that accumulated behind some dams over time. This so-called "legacy sediment" is commonly very fine-grained and contains stored nutrients and organic matter, among other possible pollutants. When dams are destroyed, efforts must be made to avoid a large flux of this sediment downstream as the newly released river cuts down to its natural base level. This requires careful engineering and significant funding. Merritts and Walter (2010) have suggested that most rivers and streams in eastern Pennsylvania and Maryland run through floodplains and levees of legacy sediment, not natural river valleys, created by the plethora of small dams built to impound water for hydropower in the past. And, even now, some large dams serve as a buffer against sediment transport that would create broad mudflats and high turbidity in coastal bays such as the Chesapeake Bay. Conowingo Dam in Maryland is one such structure that has reached its sediment capacity. During high flow events, sediments carrying nitrogen, phosphorus, and other pollutants enter the Chesapeake Bay. As of 2021, Maryland is working to implement a multi-million dollar strategy to reduce pollutants from entering the reservoir and reduce their flow downstream. For an example of the engineering and costs of removing a dam, check out the San Clemente Dam Removal & Carmel River Reroute Project [65] in central California.

Figure 13. Map of the lower 48 states showing locations of dams removed from 1936 to 2013. At American Rivers [66], the locations are clickable to show information on each dam removed.

Source: AmericanRivers.org Used with permission. All rights reserved.

Figure 14. Blow up of northeastern U.S. portion of the dam removal map.

Source: AmericanRivers.org Used with permission. All rights reserved.

Many dam removal projects are proposed, but await funding from federal sources—Congress must appropriate funds. Projects on the Snake River and the Klamath River in the west remain controversial, but at least in the case of the Klamath, the removal of four large dams has begun and it’s estimates that this will have lasting benefits on the river, community, and salmon populations. Figure 9 illustrates the consequences of waiting for funding of dam removal using the Olmstead Dam example.

Figure 15. The costs of waiting to remove and replace dams once approved using the Olmstead Dam and Locks system on the Ohio River. Lambrecht, Bill (16 September 2013). "Costly Dam, Illinois dam costing billions more than expected as delays mount". St. Louis Post-Dispatch. Retrieved 9 March 2014.

Click Here for Text Alternative of Olmsted dam spending graph

In 1988, Congress authorized spending 775 million dollars for a 7-year project to build Olmsted Locks and Dam. But the cost has more than quadrupled to 3.1 billion dollars, and 25 years later, the project is barely half done.

 

Year	Funds in millions
1991	5
1993	60
1995	35
1997	70
1999	55
2001	60
2003	70
2005	75
2007	115
2009	115 + 5 in stimulus funds
2011	140
2013	145
2014	163

Source: Army Corp of Engineers

Human Intervention in the Global Water Cycle

On the other hand, as discussed above in Ponding the Waters: Impacts of Dams, the effects of large-scale alteration of river systems and the hydrologic cycle have become increasingly clear in the past few decades. The scale of human intervention in the global water cycle is also becoming apparent, including restriction of river flows such that they no longer reach the ocean in many years, associated two- or three-fold increases in the residence time of runoff, decreased sediment delivery to the oceans, and a long-term measurable effect on global sea level caused by the impoundment of thousands of cubic km of water (Vörösmarty et al., 2004). As a result, new large dam projects have been heavily scrutinized and faced political and environmental opposition. At the same time, the efficiency and economics of energy production, and the net offset of greenhouse gas emissions from increased hydropower generation have been increasingly questioned (World Commission on Dams, 2000). One way to minimize environmental impacts is to design “run of the river” systems, in which no reservoir is created and instead the natural flow of the river in its channel is harnessed to generate power. However, these systems have several drawbacks: they rely on natural flows, so the power generating capacity fluctuates dramatically as a function of seasonal rainfall patterns and climate change; and there are no added benefits of flood control or water supply.

Dams and Economic Development

Despite the controversy, in many developing nations, major dam projects remain important engines for economic development and hold substantial potential for renewable energy generation. As of 2020, hydroelectric power constituted as much as 17% of global electricity production (and 50% of estimated renewable energy production). Of this, ~23% is in China, ~12% in Brazil, ~10% in Canada, and ~7.5% in the United States; combined, these four nations generate over half of the world's hydropower!

Moreover, globally, estimates suggest that up to two-thirds of economically viable dam sites have yet to be exploited. Undeveloped sites are especially abundant in Latin and South America (79% of renewable water remains unused), Africa (96%), India and China (48% is unused in Asia) (UNEP, 2013). Rapidly growing energy demand in India, China, and the Amazon Basin have driven the construction of hundreds of large dams as of 2002 (Figures 10-11; Table 1). This development may be a harbinger of things to come on the African continent. Africa has the second-highest population (after Asia), and the fastest-growing (See Module 1.3); it also has the lowest per capita energy use (UNEP, 2013). Looking to the future as demand for energy, water, and food in developing nations continue to grow – both per capita and in total as populations swell - it seems inevitable that demand for large dams will persist well into the 21st century. Hydroelectric power provides 50% of electricity for 28 emerging and developing nations reaching over 800 million people.

Figure 16. Number of large dams by geographic region, as of 1998 (World Commission on Dams, 2000).

Click Here for Text alternative of dams by geographic region graph

Region	Number of dams
China	22000
Asia	9000
North and Central America	8000
Western Europe	4000
Africa	1000
Eastern Europe	1000
South America	800
Austral-Asia	500

Source: World Commission on Dams

Data taken from CIG ICOLD [67]

Figure 17. Map showing existing and planned hydropower projects in the headwaters of the Amazon

Source: Finer M, Jenkins CN (2012) Proliferation of Hydroelectric Dams in the Andean Amazon and Implications for Andes-Amazon Connectivity. PLoS ONE 7(4): e35126. doi:10.1371/journal.pone.0035126

Goals and Objectives

Goals

• Explain the distribution and dynamics of water at the surface and in the subsurface of the Earth
• Interpret graphical representations of scientific data

Learning Objectives

In completing this lesson, you will:

• Identify the properties of artesian wells and describe the conditions under which they form
• Explain the difference between porosity and permeability
• List and describe the properties of aquifers that control the movement and storage of groundwater
• Explain the role of fractures in determining the transmission properties of aquifers
• Use Darcy's Law to explain the roles of aquifer properties and driving forces in governing the rate of groundwater flow

Aquifers Explained

Aquifers come in many shapes, sizes, and “flavors”. For example, some aquifer systems span hundreds – or even thousands – of kilometers across several states or nations and may include multiple individual layers of rock or sediment that total thousands of meters in thickness. Other aquifers may be restricted to a few kilometers within a stream valley, and be only a few meters thick.

Nonetheless, there are some important common features of different kinds of aquifers. In this section of the module, we provide a basic overview of aquifers: What is an aquifer? What are the different types of aquifers, and what is their anatomy?

Types of Aquifers

In more detail, there are three main classifications of aquifers, defined by their geometry and relationship to topography and the subsurface geology (Figures 6-9). The simple aquifer shown in Figure 6 is termed an unconfined aquifer because the aquifer formation extends essentially to the land surface. As a result, the aquifer is in pressure communication with the atmosphere. Unconfined aquifers are also known as water table aquifers because the water table marks the top of the groundwater system.

A second common type of aquifer is a confined aquifer, which is isolated from pressure communication with overlying or underlying geologic formations – and with the land surface and atmosphere – by one or more confining layers or confining units. Confined aquifers differ from unconfined aquifers in two fundamental and important ways. First, confined aquifers are typically under considerable pressure, which may be derived from recharge at a higher elevation or from the weight of the overlying rock and soil (known as the overburden). In some cases, the pressure is high enough that wells drilled into the aquifer are free-flowing. This condition requires that the water pressure in the aquifer is sufficient to drive water up the wellbore and above the land surface, and such wells are called artesian wells (Figure 7). Second, confined aquifers typically remain saturated over their entire thickness, even as water is removed by pumping wells. Water extracted from the aquifer comes only from the depressurization of the aquifer – a combination of depressurization and expansion of the water itself, and relaxation of the aquifer formation upon reduction in pressure (Figure 8).

Figure 6. Schematic cross-sectional diagram showing a layered system with an upper unconfined aquifer above a confining unit, and underlain by a confined aquifer. Note the water level in the two wells: In the unconfined aquifer, the water level in the well is the same as the height of the water table. In the confined aquifer, the water level is higher than the top of the aquifer – indicating that the aquifer is fully saturated and that the water is under pressure.

Source: USGS Water Science Photo Gallery [71]

Figure 7. Artesian well in Southern Georgia

Source: USGS Water Science Photo Gallery [71]

Figure 8. Schematic illustrating the effects of pumping from a well in an unconfined aquifer (left) and a confined aquifer (right). When unconfined aquifers are pumped, the water withdrawal leads to a drop in the water table, and the pore spaces become unsaturated. Pumping in confined aquifers decreases the water pressure, but the pore space remains fully saturated.

Source: W.M. Alley et al., 1999, USGS Circular 1186: Sustainability of Groundwater Resources [68]

The third main type of aquifer is a perched aquifer (Figure 6). Perched aquifers occur above discontinuous aquitards, which allow groundwater to “mound” above them. Thee aquifers are perched, in that they sit above the regional water table, and within the regional vadose zone (i.e. there is an unsaturated zone below the perched aquifer). The dimensions of perched aquifers are typically small (dictated by climate conditions and the size of aquitard layers), and the volume of water they contain is sensitive to climate conditions and therefore highly variable in time.

Figure 9. Schematic cross-section showing the occurrence of perched aquifers above an unconfined aquifer.

Source: D.T. Snyder, U.S. Geological Survey Scientific Investigations Report 2008–5059, Estimated Depth to Ground Water and Configuration of the Water Table in the Portland, Oregon Area [72]

Aquifer Properties

In the previous section, you’ve learned about the different types of aquifers, and the basic characteristics that define an aquifer – namely the ability to store and transmit water. But what, exactly, about a rock or sediment beneath the ground determines whether the rock can hold water, or whether water can percolate through it? In the following section, we will explore this question in more detail, to define the important individual properties of the rock.

Regional Aquifer Systems: Examples

Ground water flow systems extend over a wide range of scales, from small perched aquifers that may supply water for a single-family, to regional rock formations that span thousands of km and cross several states (Figure 18). These regional systems supply water for irrigation and domestic uses in many areas, especially in semi-arid and arid parts of the American West and coastal population centers along the East coast (remember Module 1, figures 10-12?). These regional systems commonly consist of several layered sedimentary formations and may extend to several kilometers in depth. The U.S. Geological Survey has compiled detailed studies of regional aquifer systems across the U.S., with useful information about climate, recharge, subsurface geology, use, and problems related to water quality or quantity (a list and links for each of the principal regional aquifers in the U.S can be found at USGS Groundwater Information [78]. A detailed atlas with information about the major aquifer systems in particular regions of the U.S. can be viewed at USGS Ground Water Atlas of the United States [79]. In this module, we will focus on a few example regional aquifer systems of particular relevance to the Northeastern and mid-Atlantic U.S. and the Central Valley of CA.

Figure 18. Principal aquifers of the conterminous United States.

Source: U.S. Geological Survey, Water Resources [80]

Darcy’s Experiments and Darcy’s Law

In 1855, Henri Darcy, a French hydraulic engineer (Figure 24), oversaw a series of experiments aimed to understand the rates of water flow through sand layers, and their relationship to pressure loss along the flow paths. Darcy’s experiments consisted of a vertical steel column, with a water inlet at one end and an outlet at the other. The water pressure was controlled at the inlet and outlet ends of the column using reservoirs with constant water levels (Figure 25) (denoted h₁ and h₂). The experiments included a series of tests with different packings of river sand, and a suite of tests using the same sand pack and column, but for which the inlet and outlet pressures were varied.  For part of our in-class activity this week, we will perform our own set of “Darcy Tube” experiments, and also work with the original dataset generated by Darcy in his experiments.

Figure 24. Portrait of Henri Darcy

Source: Glenn Brown, Biosystems and Agricultural Engineering, Oklahoma State University.

Darcy’s findings laid the foundation for the modern science of hydrogeology by quantifying the relationships between volumetric groundwater flow rate, driving forces, and aquifer properties. Specifically, Darcy’s experiments revealed proportionalities between the flux of water, Q, through the laboratory “aquifer” and different characteristics of the experimental system (refer to Figure 25 above):

1. Q was directly proportional to the difference in water levels from inlet to outlet, $h_1-h_2=\mathrm{\Delta }h$:

   $$Q\propto \mathrm{\Delta }h$$
2. Q was directly proportional to the cross sectional area of the tube:
   $$Q\propto A$$
3. Q was inversely proportional to the length of the column:
   
   $$Q\propto 1/\mathrm{\Delta }L$$
   Combining these proportionalities leads to Darcy’s Law, the empirical law that describes groundwater flow:

   $$Q=KA(\mathrm{\Delta }h/\mathrm{\Delta }L)$$

   where K is a constant of proportionality that defines the water flux for a given hydraulic gradient $(\mathrm{\Delta }h/\mathrm{\Delta }L)$. The above equation can also be recast in terms of the water volume flux per unit area, Q/A (also called "Darcy flux" or "Darcy velocity" with units of length per time):

   $$Q/A=K(\mathrm{\Delta }h/\mathrm{\Delta }L)$$

Goals and Objectives

Goals

• Describe the two-way relationship between water resources and human society
• Explain the distribution and dynamics of water at the surface and in the subsurface of the Earth
• Thoughtfully evaluate information and policy statements regarding the current and future predicted state of water resources
• Interpret graphical representations of scientific data

Learning Objectives

In completing this lesson, you will:

• Apply the concept of hydraulic head to draw flowlines on maps and cross-sections
• Interpret the current and historical balance between groundwater recharge and water extraction from well hydrographs
• Propose a course of action to address overdraft in an aquifer

Driving Forces for Groundwater Flow

The driving forces that control groundwater flow are a bit more complicated than those controlling flow in rivers and streams. As you learned in Module 3, surface water flows downhill due to gravity, and the flow direction is defined by the topography. Water flows downhill because gravity is a form of potential energy – and the water, or anything that falls or rolls downward – flows in response to differences in potential energy (from high to low).

In contrast to surface water, groundwater is separated from the atmosphere, and as a result, it can be under considerable pressure. Therefore, the potential energy that drives groundwater movement includes both pressure and gravity. In this section, you will learn about these driving forces, how we define them, and how they translate to the direction and rate of groundwater movement in the subsurface.

Effects of Pumping Wells

Groundwater is accessed by either pumping from wells drilled into the aquifer (Figure 33), or by developing natural springs where the potentiometric surface intersects the land surface (Big Spring in Bellefonte, PA is one example of a relatively large spring that is used for municipal supply). Although springs are relatively inexpensive to develop, they are not always present, nor are the flow rates always sufficient to support demand. As a result, most groundwater extraction occurs by pumping wells, or in many cases “fields” of wells concentrated in a small area.

Figure 33. Example of groundwater extraction well in the Garden of Cambremer, France

Source: Philippe Alès (Own work) CC-BY-SA-3.0 [87], via Wikimedia Commons (top)

Figure 34. Example of an irrigation well in Walker Lake, NV

Source: USGS Nevada Water Science Center, photo by Lindsay R. Burt (bottom)

Cones of Depression: Pumping at a well, or at a wellfield, pulls water toward the well from all directions – in other words, it induces radial flow (around the radius of the well). In doing so, pumping causes a reduction in hydraulic head, known as drawdown. This drawdown generates a cone- or funnel-shaped depression called a cone of depression (Figure 35). The reduction in hydraulic head drives groundwater flow to the well (in the down-gradient direction), as shown in the example from Long Island in Figure 36.

Figure 35. Schematic cross-section showing the effects of pumping and generation of a cone of depression in an unconfined aquifer between a lake and a stream. Pumping and the ensuing cone of depression reverse the hydraulic gradient from pre-development (A) and induce flow from the stream into the groundwater system and to the well(B)

Click for more information on Figure 35

Two similar images. Image A is under natural conditions while image B shows the effect of pumping. Both diagrams show the same piece of land with a lake on the right, a stream on the left, a ground-water divide at a water table, with a surface-water divide right above it, and a confining unit beneath the groundwater flow. Diagram A shows an equal groundwater flow direction to the stream and the lake split at the ground-water divide. Diagram B has a high capacity pumping well close to the lake causing a cone of depression. Water flows from the ground-water divide to the stream as before but also to the well. Water is also pumped from the lake to the well instead of flowing out to the lake.

Source: USGS: Groundwater in the Great Lakes Basin: the case of southeastern Wisconsin

Figure 36. Potentiometric surface for southwestern Long Island, New York, ca. 1903 (pre-development), 1936 (near the peak in pumpage for combined municipal and industrial uses), and 1965, well after the municipal supply was switched to the upstate supply system described in Liquid Assets, and after reduction in withdrawals for industrial use.

Source: USGS [88]

Both the width and the depth of the cone of depression scale with the rate of pumping, the aquifer permeability, and storativity, and the duration of pumping. In general, larger cones of depression result from larger pumping rates, higher permeability or lower storativity, and longer elapsed time. If cones of depression from two separate pumping wells grow large enough to overlap, it is known as well interference. When well interference occurs, the respective drawdowns are added together. The result is that drawdown is accelerated when multiple cones of depression interact. This is generally not desirable, and is one important consideration in the design, permitting, and operation of wells.

Not only does the cone of depression draw water to the well, but if the pumping rate is large enough or pumping is sustained for a long time, it can reverse the natural hydraulic gradient hundreds of meters to several tens of km away from the well(s). In some cases, this may result in interception of groundwater that would normally feed a stream or river as baseflow, and even in the interception of streamflow itself by inducing infiltration in the stream bed or banks (Figure 35B). In other cases, large cones of depression (up to a few miles wide!) associated with industrial or municipal well fields may reverse regional topographically-driven hydraulic gradients and lead to problems like saltwater intrusion (Figure 35B).

Groundwater Budgets

Now that you are an expert on how groundwater flows, we will apply that knowledge to the important problem of groundwater budgets. As groundwater flows through and exits an aquifer, for example at springs or at extraction wells, those losses of water may be balanced by recharge that percolates from the land surface. As you’ll investigate in the following section, and through a case study of the famous Ogallala aquifer in the American Midwest, understanding the budget of inflows and outflows to an aquifer is critical to evaluating the sustainability of groundwater use.

The High Plains Aquifer

The High Plains Aquifer system consists of Tertiary sedimentary rock, dominantly sandstone and gravel (Figure 45), eroded from the ancient Rocky Mountains and deposited in the Tertiary period (from about 31 to 5 million years ago). The Ogallala Formation is the primary aquifer unit in the system. The aquifer underlies almost 175,000 mi2 and spans eight states, with most of its area in Nebraska, Texas, and Kansas. This region is among the largest and most productive croplands in the U.S. and is the source of almost 20% of our corn, wheat, and cotton production, as well as a significant portion of our soybeans, sorghum, and alfalfa. It is also host to almost 20% of the cattle raised in the US. Because the climate is semi-arid, with mean annual precipitation ranging from 12 inches in the West to 33 inches in the East, growing economically viable crops requires substantial irrigation. If you have ever flown over the Midwest on a clear day you may have seen circular “patches” of irrigated land - the hallmark of center-pivot irrigation systems (Figure 46).

Although farming has been a major part of the economy in the region since the late 1800s, in the 1960’s new technology in electrical pumps allowed access to deeper groundwater and ushered in an era of rapidly expanding irrigated acreage (Figure 47). Accompanying this expansion, aquifer-wide groundwater withdrawals increased from a few million acre-feet (M-AF) per year to almost 20 M-AF. Water level declines began in the 1950s, with the onset of intensive groundwater extraction for irrigation-based agriculture. The total overdraft of the aquifer is almost 287 million acre-feet, from pre-development (ca. 1950) to 2019 – this is over 17 times the annual flow of the Colorado River.

Figure 45. Outcrop of the Ogallala Formation in Texas.

Source: USGS High Plains Water Level Monitoring Study [92]

Figure 46. Satellite image of center pivot irrigation systems in Nebraska. Each circle is ~1/4 mile in diameter.

Image generated in Google Earth.

Figure 47. Map showing single-pivot irrigation systems in Nebraska in 1972 (upper map) and 1984 (lower map).

Source: USGS National Groundwater Atlas [93]

Figure 48. Map showing decreases in groundwater level from pre-development through 2015.

Source: USGS High Level Monitoring Study [94]

As a consequence of sustained overdraft for several decades (see Figure 44 above), water levels in the aquifer have dropped substantially, by more than 100 feet in many areas (Figure 48). Over half of this decline has occurred since 2000. Water level declines are not evenly distributed, however. The highest rates of decline are focused in the southern reaches of the aquifer system, where recharge rates are low and irrigation demand is highest. In the northern portions of the aquifer system, water level declines are considerably smaller – and in some cases, the water table has actually risen – primarily due to locally higher recharge focused along the Platte River (Figure 48).

Serious Arsenic Problem in Groundwater: Bangladesh

An example of a very serious arsenic problem in groundwater is that of Bangladesh. The issue there is related to high rates of groundwater extraction through shallow wells in conjunction with shallow groundwater pollution that caused anoxia at shallow depth (see Fig. 5). The arsenic is associated with the anoxic zone which has been tapped by hundreds of thousands of shallow "tube wells" since the 1980s (Fig. 4), an innovation that saved millions from potential disease, including death by cholera, associated with getting their water from shallow pits. Ultimately, the new deeper water source began poisoning them with arsenic (Bhattacharjee, et al., 2007, Science 315, p.1659) liberated from iron oxides that were "reduced" under anoxic conditions, thereby liberating adsorbed As into dissolved form in the groundwater.

Figure 4. Tube well emptying into an open reservoir. Replacement of shallow tube wells in the 1980s  with wells that draw from deeper reservoirs reduced the risk of disease, but unintentionally tapped into arsenic-contaminated anoxic deep aquifers.

Source: Wikipedia [109]

Figure 5. A pit in Bangladesh illustrating the oxidation (red)/reduction (gray) front in the subsurface. Note that As concentrations in water>10ppm are considered toxic to humans.

Source: USGS International Program.

Goals and Objectives

Goals

• Describe the two-way relationship between water resources and human society
• Explain the distribution and dynamics of water at the surface and in the subsurface of the Earth
• Identify strategies and best practices to decrease water stress and increase water quality
• Thoughtfully evaluate information and policy statements regarding the current and future predicted state of water resources

Learning Objectives

In completing this module, you will:

• Compare land use in areas with contrasting access to fresh water
• Calculate the water needed to support a given population and compare with available resources
• Analyze water supply (scarcity) problems and solutions in the Western US
• Evaluate the policy of annexing water rights from both scientific and ethical perspectives
• Assess the sustainability of water banking as a solution to water scarcity in the event of sustained drought
• Assess the long-term effectiveness and scientific basis of the Colorado River Compact

Surface Water Allocation and Management

In the U.S. there are some differences regionally in how surface water allocations are handled. In large part, these differences arose historically and have been modified and given legal standing.

Cities in Peril: LA

The Giant Straws of Los Angeles

To see Los Angeles, with its lush landscaping and common swimming pools, one would never believe it to be water limited. Los Angeles is a sprawling agglomeration of towns and neighborhoods spread over nearly 470 sq. miles (1220 sq. km) of semiarid hills and valleys (precipitation is about 15 in--38 cm-- annually). One river, the Los Angeles River, runs through the city to the sea, but this watercourse flows only intermittently and--mainly for flood control--has now been straightened and confined to a concrete channel. The City of Los Angeles now has nearly 3.9 million people living within its borders, a far cry from the estimated 1600 people that lived there in 1850 when (a smaller footprint) LA was first incorporated (Fig 1). By 1900, LA's population had grown to over 100,000, and the local water supply was deemed inadequate. Thus began LA's quest for additional water resources. The subsequent history of water acquisition, especially that of Owen's Valley water and the LA aqueduct (see L.A. Aqueduct Centennial 2013 [113] for pics) engineered by William Mulholland, makes very interesting reading ("Cadillac Desert" by Marc Reisner, p. 54-107). Controversy still surrounds this acquisition. Table 1 shows the major aqueducts that now supply water to LA. If you aren't familiar with the term, an aqueduct is an artificial channel for conveying water, typically in the form of a bridge across a valley or other gap.

Figure 1. City of Los Angeles population and rate of growth 1850 to 2014 (estimated).

Source: U.S. Census Bureau, 2013

*California State Water Project: note that the length and cost is for the entire system, not just LA, and the cfs for the West Branch is not what LA alone receives. Source: California State Water Project At a Glance [114]

Figure 2. Map of aqueduct systems serving the State of California

Source: Wikimedia Commons [115]

Cities In Peril: Las Vegas

The Survival of Las Vegas

View of Las Vegas from the air

Source: Wikipedia [117]

It’s hard to think about Las Vegas without images of stereotypical excess: gambling, bachelor(ette) parties, luxurious hotels, swimming pools, golf in the desert, posh fountains, celebrities, major music, and entertainment acts, and famous restaurateurs. On the one hand, it may seem incongruous that Las Vegas and the surrounding Clark County, which receive only 4 inches of rain per year on average and lie within one of the driest regions on Earth (Figure 5) (as discussed in Module 1), are also home to one of the fastest-growing populations in the U.S. (Figure 6; See also the interactive link in the caption below). On the other hand, it may be surprising that Las Vegas is among the most water-conscious cities in the nation, and as discussed below, despite rapid economic and population growth over the past two to three decades the city has managed to live within the limits of its relatively meager allocation of water from the Colorado River, the main water source for the region (see Colorado River Compact).

Figure 5. Major rivers in the US, with widths scaled by average water discharge. Note that the American Southwest, and Nevada and Arizona in particular, have no major surface water flows other than the Colorado River.

Source: Map from the Pacific Institute [118], prepared by Matthew Heberger, 2013. Creative Commons License.

Figure 6. The population of Clark County, Nevada from 1970 to present, showing Centre County, PA over the same time period for comparison.

Source: Figure constructed using Google’s public data analysis site [119]

The Colorado River Compact

The Colorado River flows almost 1500 km from its headwaters in Wyoming, Colorado, Utah, and New Mexico, through Nevada, Arizona, and California, before crossing the border to Mexico and flowing to the Gulf of California. It is the lifeblood of the American Southwest, serving almost 30 million people and enabling cities, industry, and irrigation-based agriculture to thrive in one of the direst climates on Earth (see Figure 1 in Module 8.2). The river also provides hydroelectric power that spurred much of the 20th-century development of the Southwestern U.S.

In 1922, these seven western states and the federal government negotiated an agreement, the Colorado River Compact (Figure 15) to allocate water rights on the river. First and foremost the compact partitioned water between Utah, New Mexico, Wyoming, and Colorado (the Upper Basin States) where most of its discharge originates as snowmelt); and Arizona, Nevada, and California (the Lower Basin States), where population growth and water demand were increasing rapidly (Figure 16).

The compact was borne in part out of the Upper Basin States’ unease that water projects and use of the river (e.g., by construction of the planned Hoover Dam) by the Lower Basin States at the time would, if interpreted through the lens of the doctrine of prior appropriations, impact their future claims to water from the river.
The compact specifies that the Upper and Lower Basin would each have the rights to 7.5 million acre-feet of water per year. To accomplish this while recognizing that not all years would be the same, the delivery of 7.5 million acre-feet per year to the Lower Basin is evaluated based on a ten-year running average (i.e. the Upper Basin must deliver 75,000,000 acre-feet for any span of ten consecutive years). In fact, the primary purpose of Glen Canyon Dam, unlike Hoover Dam, which generates hydroelectric power and serves as the distributary dam for the Lower Basin States, is to serve as a large “capacitor” in the river system to help ensure that this agreement can be met. Later amendments to the agreement included the 1928 Boulder Canyon Project Act, the 1944 Mexican Water Treaty, and the 1948 Upper Basin Compact. In combination, these amendments spelled out the allocation of water between the individual states, and also allocated 1.5 million acre-feet for Mexico (Table 1).

Of course, the specification of an absolute amount of water to each of the states and Mexico has raised a few serious problems that remain contentious. First, the river is over-allocated. The 1920’s – coincidentally the time that the Compact was negotiated was an anomalously wet period with annual flows as high as ~20 million acre-feet (Figures17-18). In contrast, the long-term mean discharge of the river is about 15 million acre-feet, yet 16.5 million are allocated. Furthermore, the river flow is highly variable and based on historical data and tree ring reconstructions, it seems that decades-long dry periods with flows less than 13-14 million acre-feet may be common. Second, climate projections indicate that the region will become drier in the long-term, and some have suggested that we have already entered an era of steadily declining river flows along the Colorado. Fourth, improved understanding and renewed interest in the environmental impact of decades of dramatically reduced flow have spurred new pressures to allocate some discharge for the natural system. Finally, demand is likely to increase as populations in the region continue to grow, further stressing the already over-allocated river (Figure 18).

Figure 15. (left) The signing of the Colorado River Compact in 1922, with Herbert Hoover, the Secretary of Commerce at the time, serving as the chairman. (right) Cover page of the original Compact.

Sources: Left: U.S. Bureau of Reclamation [124]. Right: National Archives and Records Administration [125]

Figure 16. Map of the Colorado River drainage basin, showing the Upper and Lower Basin States.

Source: Arizona Department of Water Resources [126]

Colorado River Allocations (Million Acre-Feet per year, ten-year running average)

Total of Colorado River Allocations(in Million Acre-Feet per year) = 16.5

Figure 17. The reconstructed river flows along the Colorado River from analysis of tree rings (blue & black), and historical record (red) for comparison. The red dashed line at 100% corresponds to 15 million acre-feet per year, the long-term average flow of the river.

Source: Southwest Climate Change Network [127]

Figure 18. Historical and projected water use and demand along the Colorado River.

Source: U.S. Bureau of Reclamation Colorado River Factsheet. [128]

Summary and Final Tasks

Summary

In the first part of Module 8, you’ve learned about the water appropriation laws that have shaped access to water in much of the U.S. As you’ve seen, cities, especially in the arid American West, now must operate within the limits of these water appropriations, regardless of population or economic growth they have accommodated in recent decades. The tension between finite water allocation (i.e. from the CO River) and continued growth has motivated a diverse portfolio of strategies in place to cope with water scarcity and potential shortage. You are now well versed in these approaches, and should be able to describe them, and discuss the costs and benefits of each. In the second part of Module 8 (Module 8.2), we will build upon this knowledge and introduce another risk factor for water supply - that of climate change.

Reminder - Complete all of the Module 8.1 tasks!

You have reached the end of Module 8.1! Double-check the to-do list on the Module 8.1 Roadmap [129] to make sure you have completed all of the activities listed there before you begin Module 8.2.

References and Further Reading

The Big Thirst Chapters 5 and 7

Southern Nevada Water Authority 2015 Water Resource Plan [130]

Goals and Objectives

Goals

• Describe the two-way relationship between water resources and human society
• Thoughtfully evaluate information and policy statements regarding the current and future predicted state of water resources
• Communicate scientific information in terms that can be understood by the general public
• Predict how human interaction with water on Earth is expected to change over the next 50 years
• Interpret graphical representations of scientific data

Learning Objectives

In completing this module, you will:

• Argue one of the many viewpoints on climate change.
• Identify the causes of global warming and climate change over the past ~250 years, including anthropogenic and natural influences.
• Assess whether mathematical models are a sound basis for making policy decisions.
• Evaluate the implications of climate change on future water resources for specific locations in the US.
• Devise a water plan for Phoenix, AZ, projecting forward from 1915

Water Use, Water Stress, and Population Growth

Module 1 discussed the who, how and where of water use throughout the US and the world. In the US and most industrialized countries, the dominant water uses are industry and agriculture. Domestic and municipal water use typically comprises only 15-30% of water use. In developing countries, per capita, water use tends to be lower in general, with a smaller proportion dedicated to industrial use and a larger proportion dedicated to domestic uses (see Module 1, Figures 8 and 9).

It is also useful to remember that we don’t actually see most of the water needed to sustain our daily activities. In the US, average per capita ‘direct’ use of water (domestic or municipal, for watering your yard, taking a shower, flushing the toilet, etc.) is 156 gallons per day, but the per capita ‘indirect’ use of water (including water used for energy production, manufacturing, food production, etc.) is 1230 gallons per day. So we really only ever see about 12% of the water that is used to sustain our quality of life. This ‘invisibility’ (as Charles Fishman refers to it in “The Big Thirst”) of our dependency on clean, reliable water is one of the challenges in planning for the future. Often we’re not even aware of what we stand to lose!

Population growth was also discussed in Module 1. The population is expected to grow by nearly a third of what it is today, to around 9.7 billion by 2050.For an engaging look at population increase in real-time, see the US Census Bureau Population Clock [131]. It is all the more concerning that some of the most rapid population growth in the world (India and Africa) is expected to occur in places that are already experiencing water stress. Add to this the legacies of past policies and infrastructure as well as future projections of climate change and it seems that we have a lot of work and planning to do!

Climate Change

What do and don’t we know about climate change?

Global warming and climate change: Both of these phrases have been used, often interchangeably, to discuss what is currently happening to our climate system. The term ‘global warming’ was coined by a Columbia University geochemist and climatologist by the name of Wallace ‘Wally’ Broecker in a 1975 Science article entitled “Climatic Change: Are we on the brink of a pronounced global warming?” Global warming, in the strict definition, refers to the observation that Earth’s average surface temperature is rising due to increased levels of greenhouse gases. The term ‘climate change’ includes global warming, but also considers the myriad other changes to Earth’s climate system that are caused by rising temperatures, including changes in precipitation and evaporation, movement of air currents (be they frontal systems or convective systems, hurricanes or a polar vortex), etc..

There is virtually no disagreement among climate scientists that both global warming and climate change are happening and is primarily due to human emissions of greenhouse gases. Broad agreement on these points among the science community is not because scientists tend to be an agreeable group. To the contrary, scientists are typically quite quick to disagree with one another and discuss their disagreements ad nauseam, in great detail and based on all available evidence, from empirical observations or theoretical physics and chemistry. Scientists also have large incentives to prove one another wrong. If, for example, a scientist was able to provide compelling evidence that increased greenhouse gases are not causing a systematic change in Earth’s climate system (or that evolution is not the driver of biodiversity, or that the Earth is not 4.6 billion years old), he or she would be famous as the likes of Galileo, Darwin or Einstein (all of whom toppled earlier scientific understanding), their work would be well funded (we would consequently have a lot of new questions that would need to be answered!), their book would be a best-seller, they would probably pick up a Nobel Prize and most notably, they would be interviewed by all of the most reputable talk show hosts. But no scientist has made such a compelling case. To the contrary, the case for significant climate change is compelling in both the empirical observations as well as the theoretical predictions. Those who proffer the opinion that climate change is not happening or is a hoax presumably do so out of sheer ignorance and/or because they have a financial incentive to believe (or to have others believe) that to be the case.

Distinct from the question of whether or not climate change is occurring, many questions remain regarding the effects of climate change on societies and economies. Certainly, there are positive effects. Warmer temperatures and increased carbon dioxide levels mean increased plant and crop productivity. Some places are expected to receive increased amounts of precipitation, potentially relieving water stress (though perhaps also increasing flood risk). Other places will most certainly not be so lucky and generally speaking, the risks and expected losses associated with climate change are expected to far outweigh the benefits. A comprehensive review of climate science and climate change is not possible within the scope of this course, but we will review a few of the key points as they relate to water, science, and society. We refer students to the most recent reports from the Intergovernmental Panel on Climate Change for more detailed and updated information.

All Water Problems Are Local

It is useful to know how climate change is likely to impact the water cycle at the global scale and IPCC reports represent our best understanding of those impacts over the next few decades to a century. But as we have discussed elsewhere, all water problems are local. In very few situations is it even feasible, let alone prudent, to transfer water long distances. Every place has its own set of challenges, institutional and infrastructure legacies, financial or other resource constraints, and concepts of social acceptability.

Generally speaking, places currently experiencing water stress or expecting to experience water stress in the foreseeable future have only a few basic options: a) have fewer people, b) force/incentivize people to use less water, c) increase storage and/or minimize losses within the system, d) reuse water, or e) get water from elsewhere. The capacity to cope with water stress (short or long-term) generally increases with wealth, though in wealthier countries more infrastructure is potentially at risk. As major population centers have already begun to struggle with water shortages it has become clear that massive investments in water technology and security infrastructure can allow wealthy nations to offset higher levels of water stress without remedying their underlying causes. Less wealthy nations, on the other hand, remain vulnerable and have fewer options in water development.

The Nile River "Conflict"

There are many examples of water disputes involving cross-boundary uses of shared resources. For example, in Module 5, we discussed the damming of the Nile River in Egypt and the Nile River's importance to development and water supply in Egypt. The construction of the Aswan Dam, which was authorized by a Nile Waters Agreement of 1959, was of little immediate concern to countries in the source regions of the Nile (Figs. 1 and 2), but guaranteed water rights allocated by earlier agreements were. Egypt initially negotiated the Nile Waters Agreement of 1929 with, what was then, a number of East African colonies of Britain (British as signatories). Through this agreement, Egypt was assigned rights to 48 billion cubic meters/year (bcm/y), including all dry-season flow (mainly from the White Nile), and Sudan, just to Egypt's south, was initially apportioned 4 bcm/y. In addition, Egypt had the right to veto upriver water projects. A later treaty, the so-called 1959 Nile Waters Agreement between Egypt and Sudan, allocated 55.5 bcm/y to Egypt and 18.5 bcm/y to Sudan—the total allocation was nearly 90% of the estimated average annual Nile River flow (84 bcm/y, mostly from the Blue Nile)! This was accomplished prior to independence for the other countries within the watershed and failed to include the monarchy of Ethiopia in negotiations. Interestingly, at the time of the agreement, the White Nile was considered, in error, the source of most of Nile water. Seasonal summer monsoonal rains in the Ethiopian Highlands are the source of much of the Nile waters, through the Blue Nile.

Figure 1. Map of part of the Nile River basin showing the course of the White and Blue Nile tributaries with their confluence in Khartoum, Sudan. The Grand Renaissance Dam in Ethiopia is being built near the border of Sudan and Ethiopia on the Blue Nile.

Source: University of Texas Library, Perry-Castañeda Map Collection

Figure 2. Note the Ethiopian highlands that focus high rainfall resulting from the summer monsoons that blow in from the Indian Ocean to the east. It is runoff from these impressive rains that feed the Blue Nile, among other rivers in Ethiopia.

Source: University of Texas Library, Perry-Castañeda Map Collection

Conflicts have arisen, particularly since Ethiopia embarked on dam building. In 2010, six of the nine upstream countries (Ethiopia, Kenya, Uganda, Rwanda, Burundi, and Tanzania) signed a Cooperative Framework Agreement seeking more water shares from the Nile. Egypt and Sudan rejected the agreement because it challenged their historic water allocations but to no avail. A major dam on the Blue Nile, the Grand Renaissance Dam, is under construction near the Ethiopia-Sudan border. As of May 2016, the dam was about 70% complete, with a target date of 2017 to begin producing power (it is worth noting, however, that the original target date was 2015 - political conflict and construction issues have slowed progress on the dam). Sudan and Egypt are, understandably, concerned about what will occur to Nile flows as the reservoir behind this huge dam fills, but Ethiopia is hoping that the water and power supplied by this structure will boost their economy and help other surrounding nations as well. Ethiopia's population in 1950 was about 18.1 million, but by 2023 it had grown to 126.5 million (114 people/km2). In 1950, Egypt had a population of 21.5 million, and in 2023 there are 112.7 million (112 people/km2).One can see that demand for water must be increasing (source: United Nations Department of Economic and Social Affairs [137]), and that Ethiopia's growth has outstripped that of Egypt. However, Ethiopia has some other sources of water (estimated total river runoff at 122 bcm/y and additional large groundwater resources estimated at 6 bcm), whereas Egypt and Sudan must depend primarily on Nile water. However, Ethiopia has no storage capacity, hence the move to build a number of large dams. Will Egypt and Ethiopia go to war over Nile water? (see Analysis: Why Ethiopia and Egypt aren’t fighting a water war [138] for a perspective ).

The India-Bangladesh Ganges River Split

Bangladesh achieved independence from Pakistan in 1971, following a short uprising. Bangladesh occupies the region of the original state of Bengal in India, which first became East Pakistan in 1947. India supported Bangladesh in the conflict with Pakistan.

The Ganges River (Fig. 3) was supposed to be shared in some way between India and Pakistan. It is fed by many tributaries (54) the largest of which is the Brahmaputra River that flows through Bangladesh, but most of the Ganges River Basin is in northeastern India. Summer monsoons deliver nearly 80 percent of annual rainfall for this region resulting in peak river flows from June-September. In an average year, it is estimated that 1200 billion m³ of precipitation falls in the Ganges catchment. Of this, nearly 500 billion m3 moves downriver. Dry season flows are much reduced.

Figure 3. Map showing Bangladesh and the course of the Ganges River. Note Kolkata and the Hooghly River in southern India near the Bay of Bengal. The Farakka Barrage (dam) diverts water from the Ganges, where it would flow into Bangladesh, and redirects it to the Hooghly River, keeping the water in India. This resulted in severe water shortages between 1976 and 1996.

Source: Perry-Castañeda Library Map Collection, University of Texas.

India's proposal to construct the Farraka Barrage (a large dam) in West Bengal on the Ganges River right near the border led the two countries to meet over disputed water claims that originated in the 1950s following Bangladesh statehood. There was no resolution to the conflict and the dam was put into place by India without an agreement, with completion in 1975. The dam was constructed to divert some proportion of the Ganges flow into the Hooghly River (during the dry season in order to remove silt that was negatively affecting the port of Calcutta or Kolkata, Fig. 3). Although Bangladesh complained to the United Nations following severe water shortages in 1976, there was no significant resolution until 1996, when India and Bangladesh signed a 30-year treaty that provided for the partitioning of the flow of the Ganges River. The Ganges forms a border between the two countries for part of its course and continues to flow through Bangladesh to the Bay of Bengal. The 1996 treaty guaranteed India a flow of nearly 1000 m³/sec between January 1 and May 31. Much of the time Bangladesh receives less water than allocated by the treaty. India's population in 1950 was 376.3 million while the population of Bangladesh was only 37.9 million. Now India's population is 1.4 billion, (425 people/km2) and the Bangladesh population is 168.7 million (1136 people/km2). Bangladesh has a much greater population density but both countries have a great need for clean water and dependable water supply.

The United States and Mexico—Sharing the Flow?

Most people in the U.S. probably don't think much about what water Mexico takes from the Colorado or the Rio Grande Rivers, which originate in the U.S. and flow along the U.S.-Mexico Border for some distance, and, in the case of the Colorado River, flow through Mexico to the sea (Fig. 4). Like the examples above (the Nile and Ganges Rivers), there are treaties that provide for sharing of the flow of these two North American rivers between the U.S. and Mexico. You have already read about the Colorado River Compact of 1922 (Module 8.1). In effect, the flow of the Colorado River is, on average, significantly less than the total amount apportioned to individual states in the watershed. The 1922 Colorado River Compact was vague about the amount of water that was to be supplied to Mexico. This was rectified in a 1944 Treaty that provided for 1.5 million acre-feet of water per year to flow to Mexico (about 10% of the average Colorado River flow).

SWRO and Energy Costs

Technological advances, coupled with innovative approaches to reduce energy costs (i.e. by using solar, tidal, or ocean thermal energy) have helped to make SWRO a potential solution to water supply or hedge against climate change for large cities like Perth - rather than simply a novelty for wealthy countries. In the 1970s, SWRO costs hovered around $2.50/m³. Currently, costs for the most efficient plants are well below $1/m³, or between ~$1000-2000 per acre-foot (Figures 3 and 4). This is still more expensive than imported surface water or groundwater in most areas (these costs range from $400-1000/acre-foot, depending on location), but in the realm of viability for areas without those sources, or to augment limited supply. The total costs include everything from construction costs for the facility (amortized over its expected lifespan), land access, permitting for discharge and intakes, and operation & maintenance.

Despite its promise, it remains to be seen if SWRO will be a universal or large scale answer to water scarcity. In particular, key challenges include the (still relatively high) costs and associated energy demand; management of the environmental impact associated with intakes and disposal of the brine waste stream; delivery of SWRO water to regions away from the coast; and the up-scaling that would be necessary to meet demand for irrigation or industrial use.

Figure 3. Reductions in energy required for SWRO from 1970-2008, driven largely by advances in membrane technology.
Click to expand to provide more information

Reductions in Energy Required for SWRO (1970-2008)

Year	Power Consumption (kWh/m3)
1970	16
1980	8
1990	5
2000	~3
2004	2
2008	~2

Source: Seawater Desalination: Can it Become a Significant Lever to Reduce Water Shortage? [148] Figure used by permission of the Pacific Institute.

Figure 4. Reductions in the cost of SWRO-generated water since 1982, showing the proportion of cost associated with capital expenditure (i.e. plant construction), maintenance, and energy. Note that the primary reductions in cost have come from increased efficiency
Click for a text description

Reductions in the cost of SWRO-generated water since 1982, showing the proportion of cost associated with capital expenditure

Year	Electric	Maintenance	Capex Charges	Total
1982	0.7	0.2	0.6	1.5
1992	0.6	0.15	0.4	1.15
2002	0.3	0.1	0.3	0.7
2010	0.35	0.1	0.3	0.75

Source: Seawater Desalination: Can it Become a Significant Lever to Reduce Water Shortage? [149]Figure used by permission of the Pacific Institute.