Global Stressors on Water Quality and Quantity

Growing population and wealth will impact sustain-ability, technology selection, and governance strategies related to water issues.

For more than a decade, the scientific community as well as nongovernmental organizations 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, technology 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 purposes, and ~10% for domestic purposes (2). However, water 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 domestic 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, residents may receive only 15 L/capita-day for both consumption 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). 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 development (Figure 2). Developing countries often have less capacity to improve water quality and depend on lower-quality water for a variety of uses, including drinking water.

To capture an overall picture of a nation’s water use, researchers calculate the national water footprint; this represents the total volume of freshwater used to produce the goods and services consumed by a population and the impact of globalization by accounting for water across the life cycle of imports and exports (5). Water use is measured in terms of water volumes consumed (evaporated) or polluted 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 (associated with importing goods and services for consumption; 5). However, some countries have external footprints that account for 50–80% of their total footprint, whereas other countries—for example, some countries in Africa—have external footprints near zero (Figure 3).

Given the current state of global water interdependence, water will become even more critical and difficult to manage under highly variable future scenarios that involve numerous interconnected global stressors. In this article, we examine the individual and integrated effects of several important stressors 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, quantity, and availability of water (7). We then explore the relationships between these stressors and the design, development, and implementation of technology and governance strategies for sustainable water systems in a dynamic world.

Water stressors

Increased stresses on the world’s water are affecting 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 experience 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 experience increases in runoff compared with 1995 levels, and 25% will experience decreases (8). Higher variability in precipitation and runoff impacts erosion and sedimentation. This particular impact of climate will be exacerbated by changes in land use that result in sedimentation associated with loss of forested riparian cover along streams, filling of wetlands, and agricultural practices. The coastal–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 examples 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 system 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 Figures 4a–4f. Two resulting changes that are occurring globally are also illustrated: the use of water (Figure 4g) and the deterioration of water quality as indicated by nitrogen loading (Figure 4h). Figure 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 affected by these stressors in terms of water quality and quantity are increasing.

One challenge to both the scientific community and governments is that the impacts of global stressors are not independent (Table 2). The link between energy generation from fossil fuels and climate change is a clear example. Previously, we discussed the impacts that climate change, land use, and population have on water quality in terms of sedimentation. Another example would be that as the stress of globalization progresses, the external water 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 consumption (13).

The stressors and impacts of the “hockey-stick world” to come suggest that we need to expand our design considerations in infrastructure systems for water and sanitation, which typically have useful lifetimes meant to last for decades (and often function beyond their designed 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 interconnectedness has led the African Development Bank and other development organizations (14) to agree that integrating adaptation responses into development planning, which includes improvements 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 technologies are available that require no water (15). Sewers 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 effects of climate change? These questions raise the issue of how to best meet basic human needs for water and sanitation, including technology selection 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 without 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 significant advances have helped address water-quality and -quantity issues, many challenges still exist for technology research, development, and implementation. As these next-generation technologies are considered, the selection of those for further development and implementation ought to consider a broadened definition of performance to include improved water quality and quantity as well as energy and materials consumption, ecosystem function at the source and sink, life-cycle impacts, and human-health outcomes.

Table 3 provides a review of several technological opportunities and challenges associated with addressing 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 technology designs and selections, governance strategies should account for the dynamic global condition. These strategies, which will influence human behavior regarding water consumption and use, need to be able to support sustainable water systems under the scenario of multiple stressors and rapid rates of change. Governance strategies can have a significant influence over which technologies are pursued and on the amount of water consumed, recycled, and discharged and by whom.

For example, water-allocation systems are challenged by droughts, which can adversely affect human and natural systems. To address this problem, drought-management mechanisms have been instituted in jurisdictions around the world. Historically, these mechanisms have involved a crisis-management or reactive approach. An important trend during the past decade in places such as the U.S. has been a shift to a more proactive approach, emphasizing drought preparedness and local involvement (26–28). Water managers traditionally have maintained that consumers do not respond to price signals, 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 technologies. In theory, raising prices to bring about water 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 expanded opportunities for governance options to encourage desired behavior in terms of water use; however, it also raises issues about setting the appropriate pricing scheme, because adjustments in cost may mean that certain segments of society or industry are “priced out” of the market, affecting local economic development. It also raises issues of fairness, 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 influence behavior and, subsequently, the human-dominated water stressors (Figure 4) is provided in Table 4. Like technological 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, demographic 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 number 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 engineering program and the School of Forestry and Environmental 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. Mihelcic is a professor of civil and environmental engineering at Michigan Technological University and serves as the director of the master’s international program in engineering. 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 Julie.Zimmerman@yale.edu.

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