The Water Footprint Concept and Water's Grand Environmental Challenges

Widespread water scarcity, water pollution, and depletion of freshwater resources are among the grand environmental challenges of the 21st century related to water. Central to these challenges is the fact that humanity uses too much water. But what are we using all that water for? The water footprint concept can help answer this question, and more. Addressing the relation between human freshwater consumption and water's grand environmental challenges, the water footprint concept resonates with stakeholders within and beyond the walls of science. This Primer describes the basics of the water footprint concept, how it works, and why it came about. Drawing from recent studies in the new research field of Water Footprint Assessment, it highlights some intriguing applications and delves into what is next on the exciting interdisciplinary research agenda. Widespread water scarcity, water pollution, and depletion of freshwater resources are among the grand environmental challenges of the 21st century related to water. Central to these challenges is the fact that humanity uses too much water. But what are we using all that water for? The water footprint concept can help answer this question, and more. Addressing the relation between human freshwater consumption and water's grand environmental challenges, the water footprint concept resonates with stakeholders within and beyond the walls of science. This Primer describes the basics of the water footprint concept, how it works, and why it came about. Drawing from recent studies in the new research field of Water Footprint Assessment, it highlights some intriguing applications and delves into what is next on the exciting interdisciplinary research agenda. Freshwater is a finite and vulnerable resource, essential to sustain life, development, and the environment. However, despite its readily acknowledged importance, the way humanity has managed—and continues to manage—its precious water resources has led to a number of grand environmental challenges related to water. Numerous river basins worldwide are facing water scarcity. Many water bodies are polluted with all sorts of substances, and stocks of both surface water and groundwater are depleted in many places around the world. As a consequence, ecosystems and soils have degraded, sometimes beyond repair. Species that depend on these water resources are losing their habitat and are going extinct at alarmingly high rates. Finally, vulnerability of water systems to (climate) shocks has increased dramatically. The main drivers for the overuse and pollution of water in rivers, lakes, and groundwater bodies are population growth and economic development. More people means more consumption of goods and services that require water for their production, and wealthier people typically consume more goods and services per person. Specifically, when affluence rises, people tend to shift toward diets that contain more animal products. Omnivorous diets are generally more water intensive to produce than vegetarian diets. Climate change is also affecting the use of water resources, as warmer temperatures, erratic rainfall, and extreme weather events raise demand for water by farmers, industries, households, and power producers. While both science and policy discussions on these water challenges are often dominated by concerns over the role and impact of climate change, it is important to note that our current water crises are best explained by growing populations and consumption of water-intensive goods and services. Even if we manage to reduce or prevent additional negative effects of climate change from happening, humanity's unquenchable thirst for water will continue to rise and exceed environmentally sustainable thresholds. The grand challenges around water transcend the environmental domain into the societal and economic realms. Competition over (access to) limitedly available water resources among various users has been linked to inequality and marginalized user groups, conflict—sometimes even violent conflict— and migration. If businesses and farmers cannot meet their demand for water to produce their goods or crops, deepening insecurity of food and energy is looming. The World Bank, among others, repeatedly warns of significant stalling of economic development because of shortages of (clean) freshwater, and the World Economic Forum lists water crises consistently as one of the largest global risks in terms of impact. Although there is much more to say about the causes and consequences of these water crises, it is clear that humanity is using and polluting too much water for its various activities in many places around the world. This water use comes at the cost of nature and communities, and cannot be sustained into the future. Solving water's grand challenges thus calls for a considerable bridling of our water consumption. A question that naturally arises, then, is what are we using all that water for? This is where the water footprint concept comes in. At its base, the water footprint (WF) is a multidimensional indicator of volumetric water use and pollution. Whereas traditional water use indicators such as abstraction or withdrawals typically report (gross) volumes taken from a water body, the WF indicates (net) water consumption, which it explicitly links to a beneficiary human activity (e.g., growing a potato or washing a car). Consumption in WF terms refers to water that is “lost” from the system, and that therefore cannot be used for other purposes at that particular time at that particular location. In other words, a WF indicates water appropriation in both a time- and location-specific manner. Next, the WF includes a connotation to the source of the water, as represented by its green, blue, and gray color components. The green WF refers to water from rainfall and melted snow that is stored in the root zone of the soil that evaporates back into the atmosphere. The green WF is particularly relevant for agricultural and forestry products because of the evaporation of water by plants and trees. The blue WF refers to water that has been sourced from surface water or groundwater that is either evaporated or incorporated into a product. Water consumed by irrigated agriculture, industry, and households is generally blue water. The gray WF refers to the amount of water that is needed to assimilate pollutants associated with a particular activity to meet local water-quality standards. A WF thus measures both the appropriation of freshwater as a natural resource (via the green and blue WF) and as an agent to assimilate waste (via the gray WF). In this way it unites water quantity and quality concerns in one indicator. Finally, WFs are calculated at the base unit of a process or activity. These process WFs can be summed to a product, company, sector, or consumer level (Figure 1). In such aggregated representations, the WF considers both direct and indirect water use, meaning that it accounts for water consumed and polluted along each step of the value chain. WF accounts have been investigated for a wide variety of processes, products, and sectors. Let me give a somewhat random yet telling anthology. We now know that it takes 1,200 L to produce a pizza Margherita (on average, summing water use over all ingredients of the pizza), 3,200 L for a pair of cotton jeans (summing water use and pollution from growing the cotton to dying and sewing the fabric), and 14,600 L for a gigajoule of energy generated by hydropower. The average person on Earth needs 3,800 L per day to support his or her lifestyle, most of which is indirect use needed to produce our food. A vegetarian diet is up to 40% less water intensive than that of an omnivore. Agriculture accounts for 92% of humanity's WF, while the remainder is roughly equally split between industry and household use. Of the nearly 10,000 billion cubic meters per year that humanity consumes across all sectors, 74% is green, 11% blue, and 15% gray water. Insightful as these WF accounts are, from the onset the WF concept was designed to encompass more than “just” an indicator of water consumption. Both the multidimensional character described above and the broader framework shown in Figure 1 emerged for good reasons. First, there was the insight that water is not only a local but also a global resource. Through trade in products in international markets, water is virtually traded too. It is “embedded” in the product. Via trade, buyers or consumers of products in effect make use of water resources elsewhere. What is more, allocation of water resources by local authorities is increasingly driven by the dynamics of the global economy. Acknowledging this global dimension to water opened up a niche for studying water (footprints) in relation to trade and globalization. Despite the global dimension to water, however, the impacts of water use and pollution remain largely local. Since local freshwater renewal rates are limited, the second important notion is that the volumetric WFs of human activities need to be studied in the context of local geographical boundaries or ecological limits. This contextualization of WFs in appropriate (local) settings helps answer the “so what?” question of a large or small volumetric WF. Comparing volumetric WFs with local water availability levels reveals the pressure placed by these WFs on local water systems. Doing so makes WF accounts more meaningful and actionable toward solving the grand challenges related to water. The third driver that was instrumental in the development of the broader WF concept came from outside the walls of science. As multinational companies in particular learned about the concept, they wanted to assess their WF from a production perspective (recall Figure 1). This led to the intensive study of WFs across supply chains of products and—perhaps more importantly—to the development of the Global Water Footprint Assessment Standard (Hoekstra et al., 2011Hoekstra A.Y. Chapagain A.K. Aldaya M.M. Mekonnen M.M. The Water Footprint Assessment Manual: Setting the Global Standard. Earthscan, 2011Google Scholar). In this widely adopted method, four clear steps help practitioners and academics alike to systematically: (1) scope their water footprint assessment; (2) make volumetric WF accounts; (3) place these accounts in their broader and local sustainability contexts; and (4) propose (policy-relevant) response options. Over the decade that passed since the publication of this standard, the WF concept has transformed into a new field of interdisciplinary scientific discourse called Water Footprint Assessment. The next section explores some recent applications that were undertaken in this budding field. The first application of the WF concept is in answering the question I started with: what is humanity using its precious water resources for? This analysis, called WF accounting, is what made the concept resonate with both the general public at large and a widening scientific community in particular. Many studies explored WFs of processes and commodities, ranging from local empirical case studies to high-resolution modeling at the global level. These studies showed how explicitly linking water use to human activities helps us understand the amounts of water that are being allocated—often implicitly—to the production of food, feed, fuel, and fibers. As the WF concept provides a systematic language to express units of water (cubic meters) in units of food (tons of produce) or units of energy (joules or calories), for example, it enables energy researchers designing carbon neural energy mixes to assess the water cost of that energy mix. Hydropower, it turned out, is associated with a large blue WF, and bioenergy has a large green WF that moreover may compete with food production. The separate treatment of green and blue sourcing of water emphasized the importance of green water in food production. Green water is often taken for granted or overlooked in traditional agricultural water-management studies that typically deal with blue (irrigation) water only. At the same time, combining water quantity (green and blue WFs) and quality (gray WFs) concerns laid bare inevitable trade-offs between the two WF components. For example, in boosting crop yield by adding fertilizers the green-blue WF per unit of crop may be reduced, while at the same time the gray WF may increase because of the leaching out of excessive fertilizers to water bodies. Spatial variations in WFs also facilitated analyses of efficient water use. Even for similar soils, climatic conditions, and farming practices, studies found major differences in the amount of water that is needed to produce a unit of crop from one place to the other. Reducing these inefficiencies has a substantial water-saving potential that can directly contribute toward solving water's grand challenges. I explained earlier that in order to interpret volumetric WFs in a meaningful way, they have to be contextualized in an appropriate (local) setting. Comprising the sustainability assessment of a Water Footprint Assessment according to the Global Standard, local WFs can be compared with local sustainable water availability levels. From hydrological studies we learn how much water is available when and where; from ecologists we hear how much needs to be reserved for nature. What remains can then be sustainably appropriated by humans. If WFs exceed these maximum sustainable water availability levels, water scarcity results. Likewise, if more pollutants (e.g., fertilizers, pesticides, pharmaceuticals) are added to a water body than it can assimilate, water pollution results. Numerous local and global studies mapped periods and places where green, blue, or gray WFs exceed such local ecological thresholds. Worldwide, it is now estimated that over four billion people live in regions that face blue water scarcity at least one month of the year. Over half of the green water resources are overexploited, and most river basins worldwide are polluted by fertilizer leaching. Because the water consumption underlying such sustainability assessments is linked to processes and products, the WF concept is instrumental in identifying causes and contributions of specific human activities toward these detrimental water challenges. It informs producers, consumers, and policy makers alike where to effectively and practically target reduction efforts. Other applications of the WF concept are presented in studies that focus on the global dimension of water use. To calculate indirect WFs of products, consumers, and other aggregate WF levels (recall Figure 1), supply chains and trade flows have to be unearthed. Tracing these globally interwoven trade chains lays bare intriguing links between producers and consumers. Figure 2, for example, shows the major virtual water flows between countries as a result of trade in agricultural and industrial commodities. Green-colored countries export more water in virtual form than they import. Yellow and red countries, to the contrary, depend on foreign water resources to meet their needs. Such virtual water-flow analyses reveal interdependencies between regions and countries, again linked to (trade in) specific commodities. It illustrates how water has become a truly geopolitical resource. In green-colored countries, the amount of water consumed to make products that are exported is larger than the amount needed to produce products that are imported, making them net virtual water importers. The opposite goes for yellow- and red-colored countries, which are net virtual water exporters. Reproduced from Hoekstra and Mekonnen, 2012Hoekstra A.Y. Mekonnen M.M. The water footprint of humanity.Proc. Natl. Acad. Sci. U S A. 2012; 109: 3232-3237Crossref PubMed Scopus (1100) Google Scholar. Recognizing these global and political aspects, governments (particularly of dryer countries such as in the Middle East and North Africa) utilize these virtual water studies to explore their sometimes unavoidable dependency on (possibly unstable) trade partners for certain key commodities, and to inform their national food security strategies. 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