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Global Stressors on Water Quality and Quantity
Growing population and
wealth will impact sustain-
ability, technology selection,
and governance strategies
related to water issues.
J U L I E BE T H Z I M M E R M A N YA L E U N I V E R SI T Y
JA M E S R . M I H E L C IC M IC H IG A N T EC H NOLO GIC A L U N I V E R SI T Y
JA M E S SM I T H U N I V E R SI T Y OF V I RGI N I A
This inequality is especially critical for Asia, which has 60% of the world’s population but only 36% of the world’s water. Water quality in terms of pollutant loading also is not distributed equally and is related to the type of use and a country’s level of develop- ment (Figure 2). Developing countries often have less capacity to improve water quality and depend on lower-quality water for a variety of uses, includ- ing drinking water.
To capture an overall picture of a nation’s water use, researchers calculate the national water foot- print; this represents the total volume of freshwater used to produce the goods and services consumed by a population and the impact of globalization by
F or more than a decade, the scientific com- munity as well as nongovernmental orga- nizations have sought to raise an alarm concerning the unsustainable use of the planet’s available water resources (1). Rising
world populations and consumption are inexorably increasing human demand for domestic, industrial, and agricultural water. Population and wealth along with other global stressors will have a direct and significant impact on the sustainability goals, tech- nology selection, and governance strategies that are related to water quality and quantity.
On a global basis, ~70% of freshwater is currently used for crop irrigation, ~20% for industrial purpos- es, and ~10% for domestic purposes (2). However, wa- ter use varies dramatically from one part of the world to another. Egypt, for example, uses 98% of its water for irrigation, leaving only ~27 L/capita-day for do- mestic use. In contrast, the U.S. uses 40% of its water for irrigation, and domestic water use exceeds 410 L/ capita-day. In refugee camps in Africa and Asia, resi- dents may receive only 15 L/capita-day for both con- sumption and hygiene. For comparison, the World Health Organization defines reasonable access as the availability of at least 20 L/capita-day from a source within 1 km of the user’s dwelling (3).
Although the quantity of water used varies by region, water is not distributed equally (Figure 1).
© 20 08 American Chemical Society
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accounting for water across the life cycle of imports and exports (5). Water use is measured in terms of water volumes consumed (evaporated) or pollut- ed per unit of time. In this way, it is similar to the concept of “virtual” water, because it accounts for water use associated with consuming agricultural and industrial imports. For the period of 1997–2001, the global water footprint was 1243 m3/capita-year, 16% of which was the external water footprint (as- sociated with importing goods and services for
consumption; 5). However, some countries have ex- ternal footprints that account for 50–80% of their to- tal footprint, whereas other countries—for example, some countries in Africa—have external footprints near zero (Figure 3).
Given the current state of global water interde- pendence, water will become even more critical and difficult to manage under highly variable future sce- narios that involve numerous interconnected global stressors. In this article, we examine the individu- al and integrated effects of several important stres- sors on the global water resource: population and consumption, demographic and land-use changes, urbanization, and of course climate change, all of which can contribute to changes in quality, quan- tity, and availability of water (7). We then explore the relationships between these stressors and the design, development, and implementation of tech- nology and governance strategies for sustainable water systems in a dynamic world.
Water stressors Increased stresses on the world’s water are affect- ing quality, quantity, and availability. Many studies have predicted that climate change will affect water supply and use (8–10). Water scarcity felt by 20% of the 1 billion people who are estimated to experi- ence shortages by 2025 is projected to be the direct effect of climate change (11). The Intergovernmental Panel on Climate Change has summarized impacts on water resources that are expected by the mid to late 21st century (Table 1; 12).
According to the best estimates associated with climate change, 75% of the earth’s land area will ex- perience increases in runoff compared with 1995
F I G U R E 1
Water for people Global overview of water availability vs population (data from Ref. 4).
F I G U R E 2
Water quality Annual biochemical oxygen demand (BoD) loading in global water- ways by sector as an indicator for water quality for 1995 and esti- mated for 2010 and 2020. the data are compared for organisation for Economic co-operation and Development (oEcD) countries and non- oEcD countries (4).
BOD loading to waterways by sector
1995 2010 2020 Megatons
0 50 100 150 200
Industry
Household
Agriculture
Industry
Household
Agriculture
N on
-O EC
D O
EC D
4248 ■ environmentAl SCienCe & teChnology / June 15, 2008
15%
North and Central America
South America
Africa
Percentage of the world’s water Percentage of the world’s population
8%
5%
Australia and Oceania
<1% 26% 6%
Europe
8% 13%
11% 13%
Asia
36% 60%
levels, and 25% will experience decreases (8). Higher variability in precipitation and runoff impacts ero- sion and sedimentation. This particular impact of climate will be exacerbated by changes in land use that result in sedimentation as- sociated with loss of forested riparian cover along streams, filling of wetlands, and agri- cultural practices. The coast- al–urban interface is especially vulnerable to the impact of these integrated stressors because the average population density in these fragile areas is twice the global average. These exam- ples demonstrate the complex and unresolved links between individual stressors. Although the environmental and human response to integrated stressors is still largely unknown, the best strategy to achieve sustainable water systems is likely one that considers the stressors as a sys- tem with positive and negative feedback loops, synergies, and interferences.
Figure 4 depicts trends from 1750 to 2000 of some major stressors on water quality and quantity. We note the common shape to all these stressors and resulting impacts—it mirrors the shape of a hockey stick. Stressors are depicted in Fig-
ures 4a–4f. Two resulting changes that are occur- ring globally are also illustrated: the use of water (Figure 4g) and the deterioration of water quality as indicated by nitrogen loading (Figure 4h). Fig-
F I G U R E 3
Water for trade National virtual-water balances related to the international trade of products for 1997–2001. Net exporters are shown in green, and net importers are shown in red (6).
–100 to –50 –50 to –25 –25 to –10 –10 to – 5 –5 to 0 0 to 5 5 to 25 25 to 50 50 to 100 No data
Net virtual-water import (Gm3/yr)
T A B L E 1
Possible impacts of climate change on water resources project- ed for the mid to late 21st century1
Phenomena and direction of trends
Likelihood of future trends2 Major impact(s)
over most land areas, warmer and fewer cold days and nights as well as warmer and more frequent hot days and nights
Virtually certain
Effects on water resources that rely on snowmelt; effects on some water supplies
Warm spells/heat waves; frequen- cy increases over most land areas
Very likely increased water demand; water-quality problems (e.g., algal blooms)
heavy precipitation events; fre- quency increases over most areas
Very likely Adverse effects on quality of surface water and groundwater; contamination of water supply; water scarcity may be relieved
Area affected by drought increases
Likely More widespread water stress
intense tropical cyclone activity increases
Likely power outages causing disrup- tion of public water supply
increased incidence of extreme high sea level (excludes tsunamis)
Likely Decreased freshwater availabili- ty due to saltwater intrusion
1table spM.1 in Ref. 12. 2these projections are for the 21st centur y and are based on the special Repor t on Emissions scenarios.
June 15, 20 0 8 / env ironmentA l SCienC e & t eChnology ■ 4249
ure 4 shows that changes are occurring on a global scale and, more importantly, that the rate of change is increasing. Furthermore, as shown in Figure 5, the spatial scale that will be affected by water-quality and -quantity challenges is also increasing. That is, the impacts of the stressors are increasing, and both
the number of locations and the number of people that will be af- fected by these stressors in terms of water quality and quantity are increasing.
One challenge to both the sci- entific community and govern- ments is that the impacts of global stressors are not independent (Ta- ble 2). The link between energy generation from fossil fuels and climate change is a clear exam- ple. Previously, we discussed the impacts that climate change, land use, and population have on wa- ter quality in terms of sedimen- tation. Another example would be that as the stress of globaliza- tion progresses, the external wa- ter footprints of many countries (the part of the footprint that is served by other countries) will become even more significant because of greater importation of goods and services. Similarly, as societies develop, they tend to increase water and energy con- sumption (13).
The stressors and impacts of the “hockey-stick world” to come suggest that we need to expand our design consider- ations in infrastructure systems for water and sanitation, which typically have useful lifetimes meant to last for decades (and often function beyond their de- signed lifetime). We also need to acknowledge that the conditions in which the design will function over its life cycle will take place in a world of rapid and increasing change. Recognition of this inter- connectedness has led the Afri- can Development Bank and other development organizations (14) to agree that integrating adaptation responses into development plan- ning, which includes improve- ments in water and sanitation, is an important way to address climate change impacts on the poor. For example, is a sanitary sewer an appropriate technology in a city that will become water- scarce by 2025? Sewers require on average up to 75 L/capita-day, whereas other sanitation technol-
ogies are available that require no water (15). Sew- ers can also distribute nutrients over a wide spatial scale, whereas other sanitation technologies can consolidate nutrients at the community level. And if a sewer project is deemed appropriate today, what should the community do to prepare for future ef-
F I G U R E 4
Global water stressors Global trends over time of stressors (a–f) and the corresponding trends in water quanti- ty and quality (g–h). (a–e, g, and h adapted with permission from Ref. 16; f adapted with permission from Ref. 13.)
Pe op
le (b
ill io
n)
17 50
18 00
18 50
19 00
19 50
20 00
Population (a) (b)
(c) (d)
(e) (f)
(g) (h)
Year
7 6 5 4 3 2 1 0
Pe rc
en ta
ge o
f t ot
al la
nd a
re a
17 50
18 00
18 50
19 00
19 50
20 00
Amount of domesticated land
Year
50
25
0
CO 2
(p pm
v)
17 50
18 00
18 50
19 00
19 50
20 00
CO2
Year
360
340
320
300
280
0
km 3 /
yr
17 50
18 00
18 50
19 00
19 50
20 00
Water use
Year
6000
4000
2000
0
Pe rc
en ta
ge o
f 1 70
0 va
lu e
Q ua
dr ill
io n
B TU
17 50
18 00
18 50
19 00
19 50
20 00
Loss of tropical rain forest and woodland
Year
35 30 25 20 15 10
5 0
N itr
og en
fl ux
(1 01
2 m
ol es
/y r)
17 50
18 00
18 50
19 00
19 50
20 00
Nitrogen flux to coastal zone
Year
10
8
6
4
2
0
17 50
18 50
18 00
19 00
19 50
20 00
Total energy consumption
Year
500
400
300
200
100
0
19 90
In te
rn at
io na
l do
lla rs
(1 01
2 )
17 50
18 00
18 50
19 00
19 50
20 00
Total real GDP
Year
45
30
15
0
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fects of climate change? These questions raise the issue of how to best meet basic human needs for water and sanitation, including technology selec- tion and governance strategies, under increasingly variable and more water-scarce circumstances. They also raise broader questions of why we continue to design solutions that have an extended lifetime with- out considering the dynamic global conditions and the increasing rate of change.
Selecting technology in a dynamic world Science must play a key role in solving present and
future global water problems (20). Although signif- icant advances have helped address water-quality and -quantity issues, many challenges still exist for technology research, development, and implemen- tation. As these next-generation technologies are considered, the selection of those for further de- velopment and implementation ought to consider a broadened definition of performance to include improved water quality and quantity as well as en- ergy and materials consumption, ecosystem func- tion at the source and sink, life-cycle impacts, and human-health outcomes.
T A B L E 2
Current status of several global stressors on water resources and examples of the relation- ships between individual stressors Stressor Current situation (13) Link to other stressors
Energy and climate
More than 82% of global ener- gy demand was met by fossil fuels in 2004.
co2 levels have risen to 380 ppm from preindustrial levels of 280 ppm.
the global impacts on the water cycle from human development stressors of changes in land cover, urbanization, and water-resources development may surpass impacts from recent or anticipated climate change (17).
the stressor will result in greater variability in precipitation, and runoff will af- fect erosion and sedimentation, which is already impacted by land use.
population current population is 6.7 billion with average annual grow th rate of 1.4%.
of the world’s 33 megacities, 21 are located in coastal areas.
During the next century, most of the 3 billion people added will live in urban areas.
Large segments of the population are still impacted by water-related pathogens; this results in 64.4 million disability-adjusted life years (also known as DALYs).
Just achieving the Millennium Development Goal for reducing hunger will re- quire a doubling in agricultural water use by 2050. climate-induced variability in precipitation and runoff will impact farmers dependent on rain-fed agricul- ture to a greater extent.
climate change affects the burden of disease due to malnutrition, diarrhea, and vector-borne diseases significantly more than it affects the risk associat- ed with flooding or thermal extremes (18).
Land use Approximately 50% of precip- itation recharges to ground- water in a natural system, whereas only 15% recharg- es in a highly urbanized envi- ronment.
human changes in land cover increase runoff and sediment loading.
current methods of urbanization result in loss of forested riparian cover along streams and increased flooding, sedimentation, dredging and filling of wet- lands, and eutrophication.
Urbanization only half of the increase of the 100 largest urban areas in the U.s. from 1970 to 1990 was be- cause of population grow th.
Average population density in coastal areas is twice the global average, and the biodiversity of aquatic ecosystems continues to decline.
Nitrogen loading is directly related to the loss of wetlands within an urban wa- tershed.
Urbanization leads to heat islands that influence energy use and climate.
Economic grow th
By 2025, water withdrawals are predicted to increase from current levels by 50% in de- veloping countries and 18% in developed countries.
impact includes water used for energy production and increased use in both the nonconsumptive domestic sector and the consumptive industrial sector.
one-half of the jobs worldwide are associated with water-dependent resourc- es, such as fisheries, forests, and agriculture (19).
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Table 3 provides a review of several technological opportunities and challenges associated with ad- dressing the impacts of the global water stressors presented in Figure 4. Design methodologies and assessments that will adapt to fast rates of change in stressors and will integrate current and emerging global stressors need to be developed.
Developing governance strategies for a dynamic world As with technolog y designs and selections, gover- nance strategies should account for the dynamic global condition. These strategies, which will inf lu- ence human behavior regarding water consump- tion and use, need to be able to support sustainable water systems under the scenario of multiple stres- sors and rapid rates of change. Governance strat- egies can have a significant inf luence over which technologies are pursued and on the amount of water consumed, recycled, and discharged and by whom.
For example, water-allocation systems are chal- lenged by droughts, which can adversely affect hu- man and natural systems. To address this problem, drought-management mechanisms have been insti- tuted in jurisdictions around the world. Historically, these mechanisms have involved a crisis-manage- ment or reactive approach. An important trend dur- ing the past decade in places such as the U.S. has been a shift to a more proactive approach, empha- sizing drought preparedness and local involvement (26–28). Water managers traditionally have main- tained that consumers do not respond to price sig- nals, so demand management has occurred most frequently through restrictions on specific water uses (i.e., banning car washing and lawn irrigation) and requirements for the adoption of specific tech- nologies. In theory, raising prices to bring about wa- ter conservation is less costly than implementing a command-and-control approach, even if the prices in question are inefficient (29).
This example demonstrates the potential for ex- panded opportunities for governance options to encourage desired behavior in terms of water use; however, it also raises issues about setting the ap- propriate pricing scheme, because adjustments in cost may mean that certain segments of society or in- dustry are “priced out” of the market, affecting local economic development. It also raises issues of fair- ness, because much of the global population is not currently served by adequate water sanitation and will need access to clean water to improve hygiene. Again, it is imperative to include considerations of sustainability outcomes and dynamic conditions when establishing long-term policies, that is, water allocation strategies, that can significantly influence water quality and quantity.
A review of several governance opportunities and challenges that can inf luence behavior and, subsequently, the human-dominated water stres- sors (Figure 4) is provided in Table 4. Like techno- logical solutions, governance strategies established to address human behavior and water quality and quantity must also consider how the incentivized actions will relate to desired behavior under future conditions.
Conclusions One challenge to solving global water problems is the large number and integrated impact of global stressors such as population and consumption, de- mographic and land-use changes, urbanization, and climate change. Another challenge is the rapid rate of change projected for all of these stressors and their resulting impacts, which are expected to accelerate even further over the next century. The large num- ber of stressors, their unknown interrelations, and the observed rapid change must all be considered when selecting and adapting new technology and governance structures for sustainable water and sanitation systems.
Julie Beth Zimmerman is an assistant professor of green engineering jointly appointed to the environmental en gineering program and the School of Forestry and Envi
4252 ■ environmentAl SCienCe & teChnology / June 15, 2008
F I G U R E 5
Freshwater concerns Freshwater stress by country (top map) in 1995 and (bot tom map) projected for 2025 (13).
People suffering from water stress and scarcity
Water withdrawal as percentage of total available
Population projections 1995
Stress Scarcity
7 6 5 4 3 2 1 0
2050
Low
Medium
High>40
20–40
10–20
<10
People (billion)
T A B L E 3
Opportunities in technology and engineering and associated challenges Opportunity Current status Challenge(s)
Desalination More than 15,000 desalination plants operate in >125 countries, with a total daily capacity of 32.4 million m3 of water (21).
significant energy demands are required (e.g., a reverse-osmosis plant needs 1.5 –2.5 kWh of electricity to produce 1 m3 of water).
Emissions from brine effluent
Large global population located in water-stressed areas cannot afford the technology.
Long-distance water transfer
Water needs of southern california re- quire 770 mi of pipes and canals, 34 reservoirs, 20 pumping plants, 3 pump- generating plants, and 5 hydroelectric power plants.
Embodied energy and materials associated with transferring wa- ter and wastewater long distances are large over the life cycle.
Water transfer can impact hydrology of local ecosystems at both the source and sink.
Design of distributed wastewater treatment systems that con- serve water resources, nutrients, and biogas potential within the watershed (22)
Water reclamation and reuse
Water-reuse capacity is expected to increase globally from 19.4 to 54.5 mil- lion m3/day by 2015 (23).
Matching demand and supply as they relate to quality of treated water for a particular use and geographical location
Water efficiency/ substitution in en- gineering design
industrial and agricultural sectors rep- resent >80% of global water consump- tion (24).
Large water footprints in agriculture are caused by inappropriate water pricing, subsidies, inefficient technology, and the lack of water-saving measures (5).
Decreasing embodied (virtual) water associated with process or product through disruptive or “leapfrog” innovations
Appropriate water technologies
point-of-use treatment technologies provide a barrier to pathogen exposure and are a possible solution when cen- tralized systems are ineffective or in- appropriate (25).
health improvement studies have largely ignored nondiarrhea water-borne diseases (e.g., typhoid, hepatitis A) and water- washed diseases (e.g., trachoma).
Does not provide as many nonhealth benefits compared with source improvements, and noneconomic factors (e.g., compatibil- ity, complexity) need to be integrated with adoption.
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ronmental Studies at Yale University, and she serves as the associate director for research at the Center for Green Chemistry and Green Engineering at Yale. James R. Mi helcic is a professor of civil and environmental engineer ing at Michigan Technological University and serves as the director of the master’s international program in en gineering. James Smith is a professor of environmental and water resources engineering in the department of civil and environmental engineering at the University of Virginia. Address correspondence about this article to Zimmerman at [email protected].
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T A B L E 4
Examples of governance and economic issues associated with managing water
Governance strategy Opportunities and challenges
technology incentives
interventions by governments through funding, purchase commitments, regulatory driv- ers, and voluntary programs can influence the success or failure of a given technology. Decisions to support certain technologies should consider performance under current and future conditions that integrate multiple stressors and acknowledge the rapid changes in stressors.
Water valuation population and other stressors will increase demand for water, which is currently underval- ued relative to other scarce natural resources. increases in population and wealth will shif t demand among agricultural, industrial, and domestic sectors in many parts of the world; this will affect land use, energy demand, and climate change and, in turn, will impact water quantity, quality, and availability. of ten much of the value of water is unaccounted for in the price (30).
Full-cost water assessments require that the value be related to economic, environmental, and social (including health) benefits. By using the full cost of water to influence prices, the market can potentially contribute to sustainable water systems through innovations in effi- ciency, water substitution, and conservation to avoid higher costs. this will reduce the im- pacts of individual and combined stressors, thereby reducing demand.
Gray water regulations
Establishing dual water-quality standards (drinking water vs service water) can significant- ly enhance the adoption rate of water-recovery and -reuse systems. Because of the grow- ing drivers for conservation, an opportunity exists to quantify the risks and benefits of water-reuse systems, define the appropriate scale, and develop regulations to support im- plementation. Management of gray water provides an opportunity to bet ter manage nu- trients primarily found in black water and to integrate the gray water resource with local agricultural activities. this can mitigate several stressors, including those related to pro- viding food for a growing population through agriculture and land-use changes, by reduc- ing the demand for freshwater and the energy related to water treatment through recycling and reuse.
4254 ■ environmentAl SCienCe & teChnology / June 15, 2008
