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water scarcity

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• Frontiers - Reducing Water Scarcity by Reducing Food Loss and Waste
• National Center for Biotechnology Information - PubMed Central - The world’s road to water scarcity: shortage and stress in the 20th century and pathways towards sustainability
• Biology LibreTexts - Water Scarcity and Solutions
• Nature - Evaluating the economic impact of water scarcity in a changing world
• UN-Water - Water Scarcity
• World Wildlife Fund - Water Scarcity
• Table Of Contents

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water scarcity , insufficient freshwater resources to meet the human and environmental demands of a given area. Water scarcity is inextricably linked to human rights , and sufficient access to safe drinking water is a priority for global development. However, given the challenges of population growth , profligate use, growing pollution , and changes in weather patterns due to global warming , many countries and major cities worldwide, both wealthy and poor, faced increasing water scarcity in the 21st century.

There are two general types of water scarcity: physical and economic. Physical, or absolute, water scarcity is the result of a region’s demand outpacing the limited water resources found there. According to the Food and Agricultural Organization (FAO) of the United Nations , around 1.2 billion people live in areas of physical scarcity; many of these people live in arid or semi-arid regions. Physical water scarcity can be seasonal; an estimated two-thirds of the world’s population lives in areas subject to seasonal water scarcity at least one month of the year. The number of people affected by physical water scarcity is expected to grow as populations increase and as weather patterns become more unpredictable and extreme.

Economic water scarcity is due to a lack of water infrastructure in general or to the poor management of water resources where infrastructure is in place. The FAO estimates that more than 1.6 billion people face economic water shortage. In areas with economic water scarcity, there usually is sufficient water to meet human and environmental needs, but access is limited. Mismanagement or underdevelopment may mean that accessible water is polluted or unsanitary for human consumption . One of the most significant infrastructure problems is known as “non-revenue water,” in which treated water is never used because it is lost to leaks in the water supply pipes. In the United States , for example, non-revenue water averages around 20 percent; a remarkable loss of potable water. Economic water scarcity can also result from unregulated water use for agriculture or industry , often at the expense of the general population. Finally, major inefficiencies in water use, usually due to the economic undervaluing of water as a finite natural resource, can contribute to water scarcity.

Often, economic water scarcity arises from multiple factors in combination. A classic example of this is Mexico City , home to more than 20 million people in its metropolitan area . Although the city receives abundant rainfall, averaging more than 700 mm (27.5 inches) annually, its centuries of urban development mean that most precipitation is lost as contaminated runoff in the sewer system . In addition, elimination of the wetlands and lakes that once surrounded the city means that very little of this precipitation feeds back into local aquifers . Nearly half of the municipal water supply is taken unsustainably from the aquifer system under the city. Withdrawals so greatly exceed the aquifer’s renewal that some parts of the region sink up to 40 cm (16 inches) every year. In addition, it is estimated that somewhere between 40–70 percent of the city’s water is lost through leaks in pipes that have been damaged by earthquakes , by the sinking of the city, and by old age . Many areas, especially poorer neighborhoods, regularly experience water shortages, and water for residents there is routinely brought in by trucks. The historical and modern mismanagement of surface and ground waters and natural areas, coupled with the complexities of being an old but ever-growing city, have made Mexico City one of the top cities threatened by economic water scarcity in the world. In early 2024, nearly 90% of Mexico City was in severe drought and the possibility of “day zero,” in which the city could run out of water, loomed for the summer months.

In places with low rainfall or limited access to surface water, reliance on aquifers is commonplace. The exploitation of groundwater resources can threaten future water supplies if the rate of withdrawal from the aquifer exceeds the rate of natural recharge. It is estimated that a third of the world’s largest aquifer systems are in distress. In addition, the redirection, overuse, and pollution of rivers and lakes for irrigation , industry, and municipal uses can result in significant environmental harm and the collapse of ecosystems. A classic example of this is the Aral Sea , which was once the world’s fourth largest body of inland water but has shrunk to a fraction of its former size because of the diversion of its inflowing rivers for agricultural irrigation.

As water resources become scarce , there are increasing problems with fair water allocation. Governments may be forced to choose between agricultural, industrial, municipal, or environmental interests, and some groups win at the expense of others. Chronic water scarcity can culminate in forced migration and domestic or regional conflicts, especially in geopolitically fragile areas.

Areas with chronic water scarcity are particularly susceptible to water crises, where water supplies dwindle to critical levels. In 2018, residents of Cape Town , South Africa , were faced with the possibility of “Day Zero,” the day on which municipal taps would run dry, the first potential water crisis of any major city. Thanks to extreme water conservation efforts and the fortuitous arrival of rain, the immediate threat passed without major incident. However, given that humans can survive only a few days without water, a water crisis can rapidly escalate into a complex humanitarian emergency . The 2017 Global Risks Report of the World Economic Forum ranked water crises as the third most important global risk in terms of impact on humanity, following weapons of mass destruction and extreme weather events, though water issues were ranked behind other global risks on subsequent reports. In 2023, the United Nations World Water Development Report conveyed an imminent risk of a global water crisis and urged greater international cooperation..

Addressing water scarcity requires a multidisciplinary approach. Water resources must be managed with the goal of equitably maximizing economic and social welfare without compromising ecosystem functioning. This ideal is sometimes referred to as the “ triple bottom line”: economics, environment , and equity .

A number of environmental, economic, and engineering solutions have been proposed or implemented worldwide. Public education is undoubtedly key for water conservation efforts, and all public and environmental policy must utilize sound science for the implementation of sustainable resource management initiatives .

The preservation and restoration of ecosystems that naturally collect, filter, store, and release water, such as wetlands and forests , is a key strategy in the fight against water scarcity. Freshwater ecosystems also provide a number of other ecosystem services , such as nutrient recycling and flood protection. Only an intact ecosystem can support these ecological processes, which have economic and social value. Natural areas, however, are often not evaluated with their ecological importance in mind and are destroyed or degraded for more immediate economic benefits. Urban planning and sustainable development must prioritize the conservation and restoration of wild lands adjacent to urban areas and properly value the ecosystem services they provide.

A number of studies have shown that higher water prices reduce water waste and pollution and can serve to fund water infrastructure improvements. However, price increases are publicly and politically unpopular in most places, and policy makers must be careful to consider how such increases may affect the poor. A water tax on heavy users could deter wasteful water consumption in industry and agriculture while leaving household water prices unaffected. While consumers would likely experience higher product prices due to the increased costs of production, ideally such a tax would help decouple economic growth from water use. In many places, rebates for the replacement of water-wasteful appliances, such as toilets and shower heads, are a common and cost-effective alternative .

Industrial agriculture is a major consumer of freshwater resources and a major contributor to water pollution from pesticide and fertilizer runoff and animal wastes. Policies that incentivize organic farming and other sustainable farming practices serve to protect water sources from agricultural pollutants. Other agricultural policies could work to incentivize the cultivation of more drought-tolerant crops in areas that experience water stress. For example, environmentalists have long criticized the growing of heavily water-dependent crops such as almonds and alfalfa in California’s semi-arid Central Valley.

A number of water scarcity challenges can be addressed with traditional engineering, often with immediate benefits. One of the most obvious solutions is infrastructure repair. Finding ways to lower installation and maintenance costs, especially in less-developed countries, and designing engineering solutions that benefit the environment and address climate change impacts are challenges in infrastructure repair.

Given that about 70 percent of all freshwater resources are devoted to agriculture, another major solution is the improvement of irrigation technologies. Many agricultural areas rely on simple flooding, or surface irrigation , as the principle means of irrigation. However, flooding often inundates fields with more water than crops require, and significant amounts of water are lost to evaporation or in transportation from its source. Educating farmers about potential water loss from such practices, setting clear water-use reduction targets, and funding irrigation improvements and water-conservation technologies can help reduce wasteful water use in agriculture.

Desalination has been proposed to curb water scarcity problems in areas with access to brackish groundwater or seawater. Indeed, desalted water is already a main source of municipal water supplies in a number of densely populated arid regions, such as Saudi Arabia . However, existing desalination technology requires a substantial amount of energy, usually in the form of fossil fuels , so the process is expensive. For this reason, it is generally used only where sources of fresh water are not economically available. In addition, the amounts of greenhouse gas emissions and brine wastewater generated by desalination plants pose significant environmental challenges.

Wastewater can be a valuable resource in cities or towns where the population is growing and water supplies are limited. In addition to easing the strain on limited freshwater supplies, the reuse of wastewater can improve the quality of streams and lakes by reducing the polluted effluent discharges that they receive. Wastewater may be reclaimed and reused for crop and landscape irrigation, groundwater recharge, or recreational purposes. Reclamation for drinking or household use is technically possible, but this reuse faces significant public resistance. The development of water-recycling plants is increasingly common in cities worldwide. The use of wastewater to fertilize algae or other biofuels has been proposed as a way to efficiently cultivate these water-intensive crops while promoting renewable energy sources. See also wastewater treatment .

Rainwater harvesting for nonpotable functions, such as gardening and washing clothes, can significantly reduce both the demand on public freshwater supplies and the strain on stormwater infrastructure. The savings in demand and supply of potable fresh water can be significant in large cities, and a number of water-stressed municipalities, such as Mexico City , are actively developing rainwater harvesting systems. Many localities encourage and even subsidize rain barrels and other rainwater harvesting systems. In some areas, however, particularly in the western United States, rainwater harvesting is viewed as a water rights issue, and restrictions are placed on such collections. In addition, catchment systems that collect runoff and allow it to percolate into the ground are useful for recharging groundwater.

Press release

Imminent risk of a global water crisis, warns the UN World Water Development Report 2023

Globally, 2 billion people (26% of the population) do not have safe drinking water and 3.6 billion (46%) lack access to safely managed sanitation, according to the report, published by UNESCO on behalf of UN-Water and released today at the UN 2023 Water Conference in New York.

Between two and three billion people experience water shortages for at least one month per year, posing severe risks to livelihoods, notably through food security and access to electricity. The global urban population facing water scarcity is projected to double from 930 million in 2016 to 1.7–2.4 billion people in 2050. The growing incidence of extreme and prolonged droughts is also stressing ecosystems, with dire consequences for both plant and animal species.

There is an urgent need to establish strong international mechanisms to prevent the global water crisis from spiraling out of control. Water is our common future and it is essential to act together to share it equitably and manage it sustainably.

Protecting and preserving this precious resource for future generations depends on partnerships. The smart management and conservation of the world’s water resources means bringing together governments, businesses, scientists, civil society and communities – including indigenous communities – to design and deliver concrete solutions. 

There is much to do and time is not on our side. This report shows our ambition and we must now come together and accelerate action. This is our moment to make a difference.

International cooperation: the key to access to water for all

Nearly every water-related intervention involves some kind of cooperation. Growing crops require shared irrigation systems among farmers. Providing safe and affordable water to cities and rural areas is only possible through a communal management of water-supply and sanitation systems. And cooperation between these urban and rural communities is essential to maintaining both food security and uphold farmer incomes.

Managing rivers and aquifers crossing international borders makes matters all the more complex. While cooperation over transboundary basins and aquifers has been shown to deliver many benefits beyond water security, including opening additional diplomatic channels, only 6 of the world’s 468 internationally shared aquifers are subject to a formal cooperative agreement.

On this World Water Day, the United Nations calls for boosting international cooperation over how water is used and managed. This is the only way to prevent a global water crisis in the coming decades.

Partnerships and people’s participation increase benefits

Environmental services, such as pollution control and biodiversity, are among the shared benefits most often highlighted in the report, along with data/information-sharing and co-financing opportunities. For example, ‘water funds’ are financing schemes that bring together downstream users, like cities, businesses, and utilities, to collectively invest in upstream habitat protection and agricultural land management to improve overall water quality and/or quantity.

Mexico’s Monterrey Water Fund, launched in 2013, has maintained water quality, reduced flooding, improved infiltration and rehabilitated natural habitats through co-financing. The success of similar approaches in Sub-Saharan Africa, including the Tana-Nairobi river watershed, which supplies 95% of the Nairobi’s freshwater and 50% of Kenya’s electricity, illustrate the global potential of such partnerships.

Inclusive stakeholder participation also promotes buy-in and ownership. Involving the end-users in planning and implementing water systems creates services that better match the needs and resources of poor communities, and increases public acceptance and ownership. It also fosters accountability and transparency. In displacement camps in the Gedo region of Somalia, residents elect water committees that operate and maintain the waterpoints that supply tens of thousands of people. Committee members partner with local water authorities of the host communities to share and manage water resources.

The United Nations World Water Development Report is published by UNESCO on behalf of UN-Water and its production is coordinated by the UNESCO World Water Assessment Programme. The report gives insight into the main trends concerning the state, use and management of freshwater and sanitation, based on work by Members and Partners of UN-Water. Launched in conjunction with World Water Day, the report provides decision-makers with knowledge and tools to formulate and implement sustainable water policies. It also offers best practice examples and in-depth analyses to stimulate ideas and actions for better stewardship in the water sector and beyond.

Press contacts

UNESCO : François Wibaux, [email protected] , +33145680746 

UN-Water:  Daniella Bostrom Couffe, [email protected] , +41796609284

UNESCO WWAP:  Simona Gallese, [email protected] , +390755911026

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This article is related to the United Nation’s Sustainable Development Goals .

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Water Stress: A Global Problem That’s Getting Worse

• Water scarcity happens when communities can’t fulfill their water needs, either because supplies are insufficient or infrastructure is inadequate. Today, billions of people face some form of water stress.
• Countries have often cooperated on water management. Still, there are a handful of places where transboundary waters are driving tensions, such as the Nile Basin.
• Climate change will likely exacerbate water stress worldwide, as rising temperatures lead to more unpredictable weather and extreme weather events, including floods and droughts.

Introduction

Billions of people around the world lack adequate access to one of the essential elements of life: clean water. Although governments and aid groups have helped many living in water-stressed regions gain access in recent years, the problem is projected to get worse due to global warming and population growth. Meanwhile, a paucity of international coordination on water security has slowed the search for solutions.

Water stress can differ dramatically from one place to another, in some cases causing wide-reaching damage, including to public health, economic development, and global trade. It can also drive mass migrations and spark conflict. Now, pressure is mounting on countries to implement more sustainable and innovative practices and to improve international cooperation on water management.

What is water stress?

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Water stress or scarcity occurs when demand for safe, usable water in a given area exceeds the supply. On the demand side, the vast majority—roughly 70 percent—of the world’s freshwater is used for agriculture, while the rest is divided between industrial (19 percent) and domestic uses (11 percent), including for drinking. On the supply side, sources include surface waters, such as rivers, lakes, and reservoirs, as well as groundwater, accessed through aquifers.

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But scientists have different ways of defining and measuring water stress, taking into account a variety of factors including seasonal changes, water quality, and accessibility. Meanwhile, measurements of water stress can be imprecise, particularly in the case of groundwater. “Any numbers out there have to be taken with a grain of salt,” says Upmanu Lall , a Columbia University professor and water expert. “None of these definitions are typically accounting for groundwater usage, or groundwater stock.”

What causes water scarcity?

Water scarcity is often divided into two categories: physical scarcity, when there is a shortage of water because of local ecological conditions; and economic scarcity, when there is inadequate water infrastructure.

The two frequently come together to cause water stress. For instance, a stressed area can have both a shortage of rainfall as well as a lack of adequate water storage and sanitation facilities. Experts say that even when there are significant natural causes for a region’s water stress, human factors are often central to the problem, particularly with regard to access to clean water and safe sanitation. Most recently, for example, the war in Ukraine damaged critical infrastructure, leaving six million people with limited or no access to safe water in 2022.

“Almost always the drinking water problem has nothing to do with physical water scarcity,” says Georgetown University’s Mark Giordano , an expert on water management. “It has to do with the scarcity of financial and political wherewithal to put in the infrastructure to get people clean water. It’s separate.”

At the same time, some areas that suffer physical water scarcity have the infrastructure that has allowed life there to thrive, such as in Oman and the southwestern United States.

A variety of authorities, from the national level down to local jurisdictions, govern or otherwise influence the water supply. In the United States, more than half a dozen federal agencies deal with different aspects of water: the Environmental Protection Agency (EPA) enforces regulations on clean water, while the Federal Emergency Management Agency (FEMA) prepares for and responds to water disasters . Similar authorities exist at the state and local levels to protect and oversee the use of water resources, including through zoning and rehabilitation projects.

Which regions are most water-stressed?

The Middle East and North Africa (MENA) is the worst off in terms of physical water stress, according to most experts. MENA receives less rainfall than other regions, and its countries tend to have fast-growing, densely populated urban centers that require more water. But many countries in these regions, especially wealthier ones, still meet their water needs. For example, the United Arab Emirates (UAE) imports nearly all of its food, alleviating the need to use water for agriculture. The UAE and other wealthy MENA countries also rely heavily on the desalination of abundant ocean water, albeit this process is an expensive, energy-intensive one.

Meanwhile, places experiencing significant economic scarcity include Central African countries such as the Democratic Republic of Congo , which receives a lot of rain but lacks proper infrastructure and suffers from high levels of mismanagement.

Even high-income countries experience water stress. Factors including outdated infrastructure and rapid population growth have put tremendous stress on some U.S. water systems , causing crises in cities including Flint, Michigan, and Newark, New Jersey.

How is climate change affecting water stress?

For every 1°C (1.8°F) increase in the global average temperature, UN experts project a 20 percent drop in renewable water resources. Global warming is expected to increase the number of water-stressed areas and heighten water stress in already affected regions. Subtropical areas, such as Australia, the southern United States, and North African countries, are expected to warm and suffer more frequent and longer droughts; however, when rainfall does occur in these regions, it is projected to be more intense. Weather in tropical regions will likewise become more variable, climate scientists say.

Agriculture could become a particular challenge. Farming suffers as rainfall becomes more unpredictable and rising temperatures accelerate the evaporation of water from soil. A more erratic climate is also expected to bring more floods, which can wipe out crops an overwhelm storage systems. Furthermore, rainfall runoff can sweep up sediment that can clog treatment facilities and contaminate other water sources.

In a 2018 report , a panel consisting of many of the world’s top climate researchers showed that limiting global warming to a maximum 1.5°C (2.7°F) above preindustrial levels—the aim of the Paris Agreement on climate—could substantially reduce the likelihood of water stress in some regions, such as the Mediterranean and southern Africa, compared to an unchecked increase in temperature. However, most experts say the Paris accord will not be enough to prevent the most devastating effects of climate change.

What are its impacts on public health and development?

Prolonged water stress can have devastating effects on public health and economic development. More than two billion people worldwide lack access to safe drinking water; and nearly double that number—more than half the world’s population—are without adequate sanitation services . These deprivations can spur the transmission of diseases such as cholera, typhoid, polio, hepatitis A, and diarrhea.

At the same time, because water scarcity makes agriculture much more difficult, it threatens a community’s access to food. Food-insecure communities can face both acute and chronic hunger, where children are more at risk of conditions stemming from malnutrition, such as stunting and wasting, and chronic illnesses due to poor diet, such as diabetes.

Even if a water-stressed community has stable access to potable water, people can travel great lengths or wait in long lines to get it—time that could otherwise be spent at work or at school. Economists note these all combine [PDF] to take a heavy toll on productivity and development.

Living in a Water-Stressed World

A housing development lies on the edge of Cathedral City, a desert resort town in southern California, in April 2015.

Eleven-year-old Chikuru carries water in a plastic jerrican, which weighs about forty pounds when full, to her home in Goma, Democratic Republic of Congo, in September 2019.

The water level at Camlidere Dam in the Turkish capital of Ankara is low due to seasonal drought and high water consumption amid the COVID-19 pandemic, November 2020.

A young boy washes a cooking pot in a pool of rainwater outside a slum where members of the Muhamasheen minority group live in Sanaa, Yemen, July 2020.

Abdel-Shaheed Gerges, a farmer, touches water flowing through a government-developed irrigation channel in Esna, Egypt, in October 2019.

Summer Weeks bathes her two-year-old daughter, Ravynn, outside their home in the Navajo Nation in Arizona, September 2020.

A worker waters turf at a sprawling horse-racing facility in Dubai in March 2021.

A woman collects water from a well dug in the Black Umfolozi Riverbed, which is dry due to drought, outside of Durban, South Africa, in January 2016.

The shadow of a girl who fled Raqqa is cast on the wall of a water spigot at a camp for internally displaced people in Syria, August 2017.

Kevin Dudley carries his daughter, Katelyn, and bottles of water to his apartment amid weeks-long water outages across Jackson, Mississippi, in March 2021.

A woman uses swamp water to wash clothes in northern Jakarta, Indonesia, in March 2018.

The COVID-19 pandemic heightened the need for safe water access. Handwashing is one of the most effective tools for combating the coronavirus, but health experts noted that three in ten individuals —2.3 billion people globally—could not wash their hands at home at the pandemic’s onset.

How has water factored into international relations?

Many freshwater sources transcend international borders, and, for the most part, national governments have been able to manage these resources cooperatively. Roughly three hundred international water agreements have been signed since 1948. Finland and Russia, for example, have long cooperated on water-management challenges, including floods, fisheries, and pollution. Water-sharing agreements have even persisted through cross-border conflicts about other issues, as has been the case with South Asia’s Indus River and the Jordan River in the Middle East.

However, there are a handful of hot spots where transboundary waters are a source of tension, either because there is no agreement in place or an existing water regime is disputed. One of these is the Nile Basin, where the White and Blue Nile Rivers flow from lakes in East Africa northward to the Mediterranean Sea. Egypt claims the rights to most of the Nile’s water based on several treaties, the first dating back to the colonial era; but other riparian states say they are not bound to the accords because they were never party to them. The dispute has flared in recent years after Ethiopia began construction of a massive hydroelectric dam that Egypt says drastically cuts its share of water.

Transboundary water disputes can also fuel intrastate conflict; some observers note this has increased in recent years , particularly in the hot spots where there are fears of cross-border conflict. For example, a new hydropower project could benefit elites but do little to improve the well-being of the communities who rely on those resources.

Moreover, water stress can affect global flows of goods and people. For instance, wildfires and drought in 2010 wiped out Russian crops, which resulted in a spike in commodities prices and food riots in Egypt and Tunisia at the start of the Arab uprisings. Climate stress is also pushing some to migrate across borders. The United Nations predicts that without interventions in climate change, water scarcity in arid and semi-arid regions will displace hundreds of millions of people by 2030.

What are international organizations and governments doing to alleviate water stress?

There has been some international mobilization around water security. Ensuring the availability and sustainable management of water and sanitation for all is one of the UN Sustainable Development Goals (SDGs) , a sweeping fifteen-year development agenda adopted by member states in 2015. Smart water management is also vital to many of the other SDGs, such as eliminating hunger and ensuring good health and well-being. And while the Paris Agreement on climate does not refer to water explicitly, the United Nations calls [PDF] water management an “essential component of nearly all the mitigation and adaptation strategies.” The organization warns of the increasing vulnerability of conventional water infrastructure, and points to many climate-focused alternatives, such as coastal reservoirs and solar-powered water systems.

However, there is no global framework for addressing water stress, like there is for fighting climate change or preserving biodiversity . The most recent UN summit on water, held in March 2023, was the first such conference since 1977 and didn’t aim to produce an international framework. It instead created a UN envoy on water and saw hundreds of governments, nonprofits, and businesses sign on to a voluntary Water Action Agenda, which analysts called an important but insufficient step compared to a binding agreement among world governments.

Some governments and partner organizations have made progress in increasing access to water services: Between 2000 and 2017, the number of people using safely managed drinking water and safely managed sanitation services rose by 10 percent and 17 percent, respectively. In 2022, the Joe Biden administration announced an action plan to elevate global water security as a critical component of its efforts to achieve U.S. foreign policy objectives. But the pace of climate change and the COVID-19 pandemic have presented new challenges. Now, many countries say they are unlikely to implement integrated water management systems by 2030, the target date for fulfilling the SDGs. 

Still, some governments are taking ambitious and creative steps to improve their water security that could serve as models for others:

Green infrastructure . Peruvian law mandates that water utilities reinvest a portion of their profits into green infrastructure (the use of plant, soil, and other natural systems to manage stormwater), and Canada and the United States have provided tens of millions of dollars in recent years to support Peru’s efforts [PDF]. Vietnam has taken similar steps to integrate natural and more traditional built water infrastructure.

Wastewater recycling . More and more cities around the globe are recycling sewage water into drinking water, something Namibia’s desert capital has been doing for decades. Facilities in countries including China and the United States turn byproducts from wastewater treatment into fertilizer.

Smarter agriculture . Innovations in areas such as artificial intelligence and genome editing are also driving progress. China has become a world leader in bioengineering crops to make them more productive and resilient.

Recommended Resources

The Wilson Center’s Lauren Risi writes that water wars between countries have not come to pass, but subnational conflicts over the resource are already taking a toll.

CFR’s Why It Matters podcast talks to Georgetown University’s Mark Giordano and the Global Water Policy Project’s Sandra Postel about water scarcity .

The World Economic Forum describes the growing water crisis in the Horn of Africa, while National Geographic looks at how the prolonged drought is pushing wildlife closer to towns.

The World Resources Institute’s Aqueduct maps the areas facing extremely high water stress.

The United Nations shares facts about water and its role in all aspects of life.

BuzzFeed News interviews residents of Jackson, Mississippi , who lost access to safe water after freezing temperatures wreaked havoc on the city’s decaying infrastructure.

• Sustainable Development Goals (UN)

Emily Lieberman contributed to this Backgrounder. Michael Bricknell and Will Merrow helped create the graphics.

• What are its impacts on health and development?
• What is being done to alleviate water stress?

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Water – at the center of the climate crisis

Water and climate change are inextricably linked. Climate change affects the world’s water in complex ways. From unpredictable rainfall patterns to shrinking ice sheets, rising sea levels, floods and droughts – most impacts of climate change come down to water water ( UN Water ).

Climate change is exacerbating both water scarcity and water-related hazards (such as floods and droughts), as rising temperatures disrupt precipitation patterns and the entire water cycle ( UNICEF ).

Get more facts on climate and water below.  

Water scarcity  

• About two billion people worldwide don’t have access to safe drinking water today ( SDG Report 2022 ), and roughly half of the world’s population is experiencing severe water scarcity for at least part of the year ( IPCC ) . These numbers are expected to increase, exacerbated by climate change and population growth ( WMO ).  
• Only 0.5 per cent of water on Earth is useable and available freshwater – and climate change is dangerously affecting that supply. Over the past twenty years, terrestrial water storage – including soil moisture, snow and ice – has dropped at a rate of 1 cm per year, with major ramifications for water security ( WMO ).  
• Water supplies stored in glaciers and snow cover are projected to further decline over the course of the century, thus reducing water availability during warm and dry periods in regions supplied by melt water from major mountain ranges, where more than one-sixth of the world’s population currently live ( IPCC ).  
• Sea-level rise is projected to extend salinization of groundwater, decreasing freshwater availability for humans and ecosystems in coastal areas ( IPCC ).  
• Limiting global warming to 1.5°C compared to 2°C would approximately halve the proportion of the world population expected to suffer water scarcity, although there is considerable variability between regions ( IPCC ).  
• Water quality is also affected by climate change, as higher water temperatures and more frequent floods and droughts are projected to exacerbate many forms of water pollution – from sediments to pathogens and pesticides ( IPCC ).  
• Climate change, population growth and increasing water scarcity will put pressure on food supply ( IPCC ) as most of the freshwater used, about 70 per cent on average, is used for agriculture (it takes between 2000 and 5000 liters of water to produce a person’s daily food) ( FAO ).

Water-related hazards  

• Climate change has made extreme weather events such as floods and droughts more likely and more severe ( IPCC ).  
• Rising global temperatures increase the moisture the atmosphere can hold, resulting in more storms and heavy rains, but paradoxically also more intense dry spells as more water evaporates from the land and global weather patterns change. ( World Bank )  
• Drought and flood risks, and associated societal damages, are projected to further increase with every degree of global warming ( IPCC ).  
• The frequency of heavy precipitation events will very likely increase over most areas during the 21st century, with more rain-generated floods. At the same time, the proportion of land in extreme drought at any one time is also projected to increase ( IPCC ).  
• Water-related disasters have dominated the list of disasters over the past 50 years and account for 70 per cent of all deaths related to natural disasters ( World Bank ).  
• Since 2000, flood-related disasters have risen by 134 per cent compared with the two previous decades. Most of the flood-related deaths and economic losses were recorded in Asia ( WMO ). The number and duration of droughts also increased by 29 per cent over this same period. Most drought-related deaths occurred in Africa ( WMO ).

Water solutions  

• Healthy aquatic ecosystems and improved water management can lower greenhouse gas emissions and provide protection against climate hazards ( Water and Climate Coalition ).  
• Wetlands such as mangroves, seagrasses, marshes and swamps are highly effective carbon sinks that absorb and store CO2, helping to reduce greenhouse gas emissions ( UNEP ).  
• Wetlands also serve as a buffer against extreme weather events ( UNEP ). They provide a natural shield against storm surges and absorb excess water and precipitation. Through the plants and microorganisms that they house, wetlands also provide water storage and purification.  
• Early warning systems for floods, droughts and other water-related hazards provide a more than tenfold return on investment and can significantly reduce disaster risk: a 24-hour warning of a coming storm can cut the ensuing damage by 30 per cent ( WMO ).  
• Water supply and sanitation systems that can withstand climate change could save the lives of more than 360,000 infants every year ( New Climate Economy report ).  
• Climate-smart agriculture using drip irrigation and other means of using water more efficiently can help reduce demand on freshwater supplies ( UNEP ).

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Water and Climate Change

Climate change is primarily a water crisis. We feel its impacts through worsening floods, rising sea levels, shrinking ice fields, wildfires and droughts.

However, water can fight climate change. Sustainable water management is central to building the resilience of societies and ecosystems and to reducing carbon emissions. Everyone has a role to play – actions at the individual and household levels are vital.

The issue explained

Water and climate change are inextricably linked. Extreme weather events are making water more scarce, more unpredictable, more polluted or all three. These impacts throughout the water cycle threaten sustainable development, biodiversity, and people’s access to water and sanitation. 

Flooding and rising sea levels can contaminate land and water resources with saltwater or faecal matter, and cause damage to water and sanitation infrastructure, such as waterpoints, wells, toilets and wastewater treatment facilities. 

Glaciers, ice caps and snow fields are rapidly disappearing. Meltwater feeds many of the great river systems. Volatility in the cryosphere can affect the regulation of freshwater resources for vast numbers of people in lowland areas.

Droughts and wildfires are destabilizing communities and triggering civil unrest and migration in many areas. Destruction of vegetation and tree cover exacerbates soil erosion and reduces groundwater recharge, increasing water scarcity and food insecurity.

Growing demand for water increases the need for energy-intensive water pumping , transportation, and treatment, and has contributed to the degradation of critical water-dependent carbon sinks such as peatlands. Water-intensive agriculture for food production, particularly meat, and for growing crops used as biofuels, can further exacerbate water scarcity.

The way forward

Climate policymakers must put water at the heart of action plans . Sustainable water management helps society adapt to climate change by building resilience, protecting health and saving lives. It also mitigates climate change itself by protecting ecosystems and reducing carbon emissions from water and sanitation transportation and treatment.

Politicians must cooperate across national borders to balance the water needs of communities, industry, agriculture and ecosystems.

Innovative financing for water resource management wil l be needed to help attract investment, create jobs, and support governments in fulfilling their water and climate goals.

Sustainable, affordable and scalable water solutions include:

• Improving carbon storage. Peatlands store at least twice as much carbon as all of Earth’s forests. Mangrove soils can sequester up to three or four times more carbon than terrestrial soils. Protecting and expanding these types of environments can have a major impact on climate change.
• Protecting natural buffers. Coastal mangroves and wetlands are effective and inexpensive natural barriers to flooding, extreme weather events and erosion, as the vegetation helps regulate water flow and binds the soil in flood plains, river banks and coastlines.
• Harvesting rainwater. Rainwater capture is particularly useful in regions with uneven rainfall distribution to build resilience to shocks and ensure supplies for dry periods. Techniques include rooftop capture for small-scale use and surface dams to slow run-off to reduce soil erosion and increase aquifer recharge.
• Adopting climate-smart agriculture. Using conservation techniques to improve organic matter to increase soil moisture retention; drip irrigation; reducing post-harvest losses and food waste; and, transforming waste into a source of nutrients or biofuels/biogas.
• Reusing wastewater. Unconventional water resources, such as regulated treated wastewater, can be used for irrigation and industrial and municipal purposes. Safely managed wastewater is an affordable and sustainable source of water, energy, nutrients and other recoverable materials.
• Harnessing groundwater. In many places, groundwater is over-used and polluted; in other places, it is an unknown quantity. Exploring, protecting and sustainably using groundwater is central to adapting to climate change and meeting the needs of a growing population.

Facts and Figures

• Only 0.5% of water on Earth is useable and available freshwater – and climate change is dangerously affecting that supply. Over the past 20 years, terrestrial water storage – including soil moisture, snow and ice – has dropped at a rate of 1 cm per year, with major ramifications for water security. ( WMO, 2021 )  
• By 2050, the number of people at risk of floods will increase from its current level of 1.2 billion to 1.6 billion. In the early to mid-2010s, 1.9 billion people, or 27% of the global population, lived in potential severely water-scarce areas. In 2050, this number will increase to 2.7 to 3.2 billion people. ( United Nations, 2020 )  
• Over a fifth of the world’s basins have recently experienced either rapid increases in their surface water area indicative of flooding, a growth in reservoirs and newly inundated land; or rapid declines in surface water area indicating drying up of lakes, reservoirs, wetlands, floodplains and seasonal water bodies. ( UN-Water, 2021 )  
• The ambition of new climate change mitigation pledges for 2030 need to be four times higher to limit global warming to 2°C and seven times higher to get on track to limit global warming to 1.5°C. ( UNEP, 2021 )   
• The current Arctic sea-ice cover (both annual and late summer) is at its lowest level since at least 1850 and is projected to reach practically ice-free conditions at its summer minimum at least once before 2050. ( IPCC, 2021 )   
• The world will not achieve sustainable water management until 2049. 40% of countries still have limited capacity to balance competing demands across sectors and cope with increasing pressures, including from climate change. ( UN-Water, 2024 ) 

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• Published: 26 March 2021

Evaluating the economic impact of water scarcity in a changing world

• Flannery Dolan   ORCID: orcid.org/0000-0001-8916-3903 1 ,
• Jonathan Lamontagne   ORCID: orcid.org/0000-0003-3976-1678 1 ,
• Robert Link 2 ,
• Mohamad Hejazi 3 , 4 ,
• Patrick Reed   ORCID: orcid.org/0000-0002-7963-6102 5 &
• Jae Edmonds   ORCID: orcid.org/0000-0002-3210-9209 3  

Nature Communications volume  12 , Article number:  1915 ( 2021 ) Cite this article

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• Climate and Earth system modelling
• Environmental economics

Water scarcity is dynamic and complex, emerging from the combined influences of climate change, basin-level water resources, and managed systems’ adaptive capacities. Beyond geophysical stressors and responses, it is critical to also consider how multi-sector, multi-scale economic teleconnections mitigate or exacerbate water shortages. Here, we contribute a global-to-basin-scale exploratory analysis of potential water scarcity impacts by linking a global human-Earth system model, a global hydrologic model, and a metric for the loss of economic surplus due to resource shortages. We find that, dependent on scenario assumptions, major hydrologic basins can experience strongly positive or strongly negative economic impacts due to global trade dynamics and market adaptations to regional scarcity. In many cases, market adaptation profoundly magnifies economic uncertainty relative to hydrologic uncertainty. Our analysis finds that impactful scenarios are often combinations of standard scenarios, showcasing that planners cannot presume drivers of uncertainty in complex adaptive systems.

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Introduction.

Global water scarcity is a leading challenge for continued human development and achievement of the Sustainable Development Goals 1 , 2 . While water scarcity is often understood as a local river basin problem, its drivers are often global in nature 3 . For instance, agricultural commodities (the primary source of global water consumption 4 ), are often traded and consumed outside the regions they are produced 5 . These economic trade connections mean that global changes in consumption result in impacts on local water systems 6 . Likewise, local water system shocks can also propagate globally 7 , 8 . Water is a critical input to other sectors, such as energy, transportation, and manufacturing 9 , 10 , so that changes in the regional water supply or sectoral demand can propagate across sectors and scales. Continued population growth, climate change, and globalization ensure that these multi-region, multi-sector dynamics will become increasingly important to our understanding of water scarcity drivers and impacts 11 .

Quantifying water scarcity and its impacts are active and growing research areas 12 . Early and influential work in the area largely focused on supply-oriented metrics of scarcity: per-capita water availability 13 , the fraction of available water being used 14 , and more sophisticated measures that account for a region’s ability to leverage available water given its infrastructure and institutional constraints 15 . Recent work proposes indicators such as water quality 16 , green water availability 17 , and environmental flow requirements 18 that focus on specific facets of water scarcity. Qin et al. incorporate the flexibility of current modes of consumption to identify regions where adaptation to scarcity may be relatively difficult 19 . Other recent work focuses on the water footprint of economic activity 20 , 21 making it possible to identify the economic drivers of scarcity (through virtual water trade) 6 , 8 . Yet knowledge gaps remain concerning how the economic costs of future water scarcity will propagate between sectors and regions as society adapts to scarcity, and how the cost of this adaptation depends on uncertainties in the projections of future conditions.

From the economic perspective, water scarcity impacts arise when the difficulty of obtaining water forces a change in consumption. For instance, abundant snowmelt may be of little use to would-be farmers if barriers (cost, institutional, etc.) prevent them from utilizing it. They will be forced to go elsewhere for water or engage in other activities, and this bears an economic cost that is not reflected in conventional water scarcity metrics. When water becomes a binding constraint, societies adapt through trade and shifting patterns of production, and the cost of that adaptation is tied to the difficulty of adopting needed changes. Changing annual cropping patterns to conserve water is easier and will impact an economy less than shuttering thermal power generation during prolonged drought 19 . In a globalized economy, the impact of such adaptation cannot be assessed in a single basin or sector in isolation, as hydrologic changes in one region reverberate across sectors around the world 3 , 22 . Indeed, reductions in water supply in one region may increase demands for water in another, simultaneously inducing both physical scarcity and economic benefit in ways that are difficult to anticipate ex ante 23 . Our primary research question is how these dynamics will impact society in the future, and how both the magnitude and direction of those impacts depend on future deeply uncertain conditions 24 .

To address this question, we deploy a coupled global hydrologic-economic model with basin-level hydrologic and economic resolution 25 to compute the loss (or gain) of economic surplus due to that scarcity in each basin across a range of deeply uncertain futures. Here “economic surplus” refers to the difference between the value that consumers place on a good and the producers’ cost of providing that good 26 . The surplus is a measure of the value-added, or societal welfare gained, due to some economic activity. The change in economic surplus is an appealing metric because it captures how the impact of resource scarcity propagates across sectors and regions that depend on that resource. Change in surplus has been used in past studies to assess the impacts of water policies and to understand how to efficiently allocate water in arid regions 27 , 28 , though it has not typically been used to analyze the impact of water scarcity itself. One exception is a study by Berritella et al., who used the loss in equivalent variation, another welfare metric, to measure the effects of restricting the use of groundwater 29 . On a broader scale, our analysis tracks the impacts of scarcity in hundreds of basins across thousands of scenarios, revealing important global drivers of local impacts that are often missed when the spatial and sectoral scope is defined too narrowly.

Global water scarcity studies depend on long-term projections of climate, population growth, technology change, and other factors that are deeply uncertain, meaning that neither the appropriate distribution nor the correct systems model is agreed upon 24 , 30 . Complicating matters, the coupled human-earth system is complex, exhibiting nonlinearities and emergent properties that make it difficult to anticipate important drivers in the scenario selection process. In such a case, focusing on a few scenarios, as is common in water scarcity studies, risks missing key drivers and their interactions 31 . In contrast, recent studies advocate exploratory modeling 32 to identify important global change scenarios 33 , 34 . In that approach, the uncertainty space is searched broadly and coupled-systems models are used to test the implications of different assumptions on salient measures of impact across a scenario ensemble 35 . Exploratory modeling is especially important in long-term water scarcity studies, where we show that meaningful scenarios vary widely from basin-to-basin, highlighting the inadequacy of relying on a few global narrative scenarios.

By analyzing a large ensemble of global hydro-economic futures, we arrive at three key insights. First, basin-level water scarcity may be economically beneficial or detrimental depending on a basin’s future adaptive capacity and comparative advantage, but that advantage is highly path-dependent on which deeply uncertain factors emerge as the basin-specific drivers of consequential outcomes. For instance, in the Lower Colorado Basin, the worst economic outcomes arise from limited groundwater availability and high population growth, but that high population growth can also prove beneficial under some climatic scenarios. In contrast, the future economic outcomes in the Indus Basin depend largely on global land-use policies intended to disincentive land-use change in the developing world. Our second insight is that those land-use policies often incentivize unsustainable water consumption. In the case of the Indus Basin, limiting agricultural extensification results in intensification through increased irrigation that leads to unsustainable overdraft of groundwater, with similar dynamics playing out elsewhere. Third, our results show that the nonlinear nature of water demand can substantially amplify underlying climate uncertainty, so that small changes in runoff result in large swings in economic impact. This is pronounced in water-scarce basins (like the Colorado) under high-demand scenarios. Collectively, these insights suggest that understanding and accounting for the adaptive nature of global water demand is crucial for determining basin-level water scarcity’s path-dependent and deeply uncertain impacts.

Global-to-basin impacts

We calculate both physical water scarcity (Fig.  1B ) and its economic impact (Fig.  1C ) over the 21st century for 235 river basins for each of the 3000 global change scenarios, simulated using the Global Change Analysis Model (GCAM) integrated assessment model 36 . With the effects of inter-basin trade, hydrologic basins may experience highly positive or highly negative economic impact due to water scarcity (Fig.  1A ). Here, economic impact is defined as the difference in total surplus in water markets (Supplementary Fig.  1 ) between a control scenario with unlimited water and an experimental scenario with limited water supply (Supplementary Fig.  2 ). Water scarcity usually induces negative economic impact (loss of surplus), although positive economic impact from global water scarcity can arise if a basin holds a comparative advantage over others. With this comparative advantage, a basin can become a virtual water exporter through inter-basin trade 37 , meaning it will export water-embedded goods to other regions. Though some basins experience positive impact more often than others (across the scenario ensemble), all basins experience both negative and positive impacts in some scenarios (Supplementary Table  1 ): no basin has a universally positive or negative outlook. As may be expected, the basins with the highest number of positive impact scenarios are those that are relatively water-rich by conventional measures (Fig.  1B ), for example, the Orinoco River in northern South America (Fig.  1A ).

The scatter plot in panel A shows the two metrics in panels B and C plotted against each other in four basins. Each point represents the maximum absolute value of that metric over time in each scenario. The map in panel B shows WTA in each water basin while the map in panel C shows the log-modulus of economic impact. Both maps plot the maximum absolute value of the metric over time and the median across all scenarios. The correspondence between the two metrics is not perfect. Some water-scarce basins have more capacity to handle water scarcity and thus are not as impacted economically as others.

We measure physical water scarcity using the Withdrawal-To-Availability ratio (WTA) which is computed by dividing water withdrawals by renewable supply. The correspondence between the WTA and the economic impact metric is not perfect (Fig.  1B and C ). In some scenarios (for instance, those with restricted reservoir storage), basins with high physical scarcity have a small negative or even positive economic impact, and in others, basins with low physical scarcity have a negative economic impact (Fig.  1A ). This highlights the importance of capturing the interdependencies between physical and economic factors that affect the welfare of a basin.

Several basins show high variance in economic impacts, including the Indus River Basin, the Arabian Peninsula, and the Lower Colorado River Basin (Supplementary Fig.  4 ). In addition to variance in economic impacts, those basins exhibit a wide range of physical water scarcity, are geographically diverse and are of geopolitical importance. The Orinoco Basin is also highlighted as an example of a basin that is not physically water-scarce and experiences slightly positive economic impact in most scenarios (Fig.  1A and B ). Such water-rich basins are particularly well-positioned to produce more water-intensive products to offset lost production in water-scarce basins (Supplementary Fig.  3 ), though the stylized water markets as represented in GCAM (and indeed in all other global hydro-economic models) may overstate these benefits for some basins compared to real-world conditions. The market representation assumes that all agents have an equal opportunity to acquire water and that water is allocated in the most economically efficient manner (except for agricultural subsidies 38 ). In reality, water rights frameworks and barriers to trade may block potential users from putting the water to more economically beneficial use.

The distributions of the plotted scenarios in the four selected basins (Fig.  1A ) give some indication of the relationship between water scarcity and economic impact in each basin. The bi-modal spread of the scenario points (Fig.  1A ) shows that higher physical water scarcity can be associated with both highly positive and severely negative economic impacts. When the distributions are wide and shallow (e.g., the Indus Basin in Fig.  1A ), smaller changes in physical scarcity lead to much higher changes in economic impact compared with other basins (Table  1 ). This occurs if the basin cannot easily supplement renewable supply with other water sources and the price of water rises precipitously. Shifts in demand subject to these high prices lead to large swings in economic impact.

The direction of shifts in demand depends on a basin’s comparative advantage (or disadvantage) due to the scenario assumptions and how these assumptions affect other basins around the world. As evidenced by the positive scenarios in water-scarce basins in Fig.  1 , this comparative advantage can arise from mechanisms other than abundant water supply (e.g., higher agricultural productivity, different dietary or technological preferences, or a lower population). The equilibrium demand over the renewable supply (the WTA) could be the same in two scenarios with very different economic impacts depending on if the scenario assumptions enable a basin’s comparative advantage in one but are detrimental in another (Fig.  1A ). Influential factors that determine economic impact are basin-specific (examples given in the next section). The changes in demand and resulting impacts due to these factors underscore the importance of projecting basin-level scarcity in a global context that allows for market adaptation.

Climate system uncertainty amplification

The market response to water scarcity within a hydrologic basin usually amplifies the uncertainty in hydro-climatic projections (Fig.  2 , Supplementary Fig.  5 ), leading to higher changes in economic impact. Analysis of the scenario ensemble revealed that differences in Earth System Model (ESM) forcing often determines the sign of impact (SA Figs.  6 – 9 ). The ESMs contribute precipitation and temperature projections to the hydrologic model used by GCAM, generating water runoff estimates (see “Methods” section). Surface water supply fluctuations heavily affect changes in economic surplus within these hydrologic basins. Other important factors include reservoir expansion (in Arabia and the Orinoco), land-use scenario (in the Indus and Orinoco), and agricultural productivity (in Arabia, the Indus, and the Orinoco) (Supplementary Figs.  6 – 9 ).

Uncertainty over time plots of the four chosen basins. Values are taken relative to the 2015 baseline. Uncertainty prior to 2015 is illustrative only. The scenario group shown in A – D has the lowest mean climate-induced economic impact uncertainty over time out of the 600 groups. The scenario group shown in E – H has the highest mean climate-induced economic impact uncertainty over time. In most scenarios, runoff uncertainty is amplified by the human system, leading to higher uncertainty in economic impact.

Climate uncertainty is one dimension over which decision-makers have very little control (as opposed to socioeconomic trajectories, agricultural advancements, reservoir storage, etc.). To isolate the uncertainty in the economic impact due to this fundamental climate uncertainty, 600 groups of five scenarios were created by holding all factors constant, except the ESM (of which 5 were considered). The difference between the maximum impact in this group of five and the minimum is one measure of climate-induced impact uncertainty. This uncertainty is plotted in blue in Fig.  2 compared to the runoff uncertainty in red. We find that the economic impact uncertainty is usually higher than the runoff uncertainty (Supplementary Fig.  5 ). Here, runoff uncertainty is the difference between the maximum runoff and the minimum runoff in the set of five scenarios. Peaks and troughs in Fig.  2 correspond to slight deviations in climate forcing in the ESMs. This in turn leads to differences in the runoff, which changes the unit costs of water, causing market adaptations and thus amplifying the economic surplus change (Supplementary Fig.  10 ).

High economic impact uncertainty relative to runoff uncertainty indicates that the market is very sensitive to changes in water supply. In high-demand scenarios (e.g., those with a high population and high food demand), the price of water steeply rises when shifting toward nontraditional water sources such as non-renewable reserves and desalination (Supplementary Fig.  11A ). When this occurs, deviations in supply lead to highly nonlinear impacts (Fig.  2E–H ). Vulnerable basins in these high-demand scenarios see steep and rapid declines in economic impact (Fig.  2E, H ). Scenarios where the economic impact continues dropping through the end of the century are of particular concern and suggest that a basin no longer has the economic capacity to stabilize these negative impacts. We will henceforth call this loss in capacity an ‘economic tipping point’.

Importantly, the conditions that lead to tipping points can vary substantially across basins. For instance, in the Arabian Peninsula, tipping point conditions include low groundwater availability and pricing carbon emissions from all sectors (see “Methods” section). Even with ample groundwater supply, tipping points can occur with high population and low GDP (SSP 3 socioeconomic assumptions) in addition to pricing carbon emissions from all sectors. In some scenarios, we can see that the Arabian Peninsula experiences a positive impact mid-century by relying on relatively inexpensive water resources. After these resources run out subject to the constraints, the economic impact becomes more negative until the end of the century (Fig.  2A ) and the basin utilizes an increasing amount of desalinated water (Supplementary Fig.  11B ). The lack of perfect foresight within GCAM helps explain this short-term thinking, though historically the area has withdrawn groundwater at unsustainable rates 39 .

Meanwhile, the Lower Colorado River Basin experiences an economic tipping point when there is low groundwater availability, low agricultural productivity (SSP 3 agriculture and land use assumptions), and high wealth socioeconomic trajectories (SSP 5 socioeconomics). The uncertainty in economic impact in the Lower Colorado Basin is the highest out of all of the highlighted basins (Fig.  2C ) and is one of the basins with the highest uncertainty in economic impact in the world.

Importantly, the factors that cause economic tipping points in these basins are not the same, nor do they always follow a well-defined global narrative such as the canonical SSPs. Table  2 shows the basins with the most highly negative impact values out of all the time periods in every scenario. Most of these scenarios contain a mixture of SSP elements (e.g., SSP 5 socioeconomics and SSP 4 agriculture in the Sabarmati). There are noticeable trends in the factors, for instance, high wealth socioeconomic trajectories (SSP 5) and the Universal Carbon Tax often lead to tipping points. However, the factors are not all the same in each basin (e.g., in the Ganges-Brahmaputra).

Mitigation-scarcity trade-offs

Pricing carbon emissions from the land-use sector often contributes to an economic tipping point because basins respond by intensifying agricultural land and increasing irrigation, thus exacerbating scarcity. When food demand increases, GCAM responds either by expanding agricultural land or intensifying existing agricultural land. With no price put on land-use change emissions (under the Fossil Fuel and Industrial Carbon Tax, or FFICT) it is more cost-effective to expand. Indeed, we find that scenarios with the FFICT use more agricultural land than the Universal Carbon Tax (UCT) scenarios (Fig.  3A ). Conversely, the carbon prices under the UCT disincentivize expansion and therefore prompt intensification. Carbon prices are derived from the continued ambition scenario of the Nationally Determined Contributions in a future with medium challenges to adaptation and mitigation 40 (see “Methods” section).

Density plots depicting the difference in tax regimes. The plot in A depicts the sum of global cropland over time under the two carbon tax regimes. The density plot in B shows water withdrawals in the Arabian Peninsula in FFICT (orange) and UCT (cyan) scenarios. The density plot in C depicts the shadow price of water in the Indus River basin in the two tax cases. Values in B and C are averaged over time. Total agricultural land increases under the FFICT while water price and water withdrawals increase under the UCT.

When intensification occurs, yields are increased by irrigating crops more instead of relying on rainwater. The intensity of agricultural land management also increases. These changes prompt greater water withdrawals (Fig.  3B ). The shift from rainwater toward irrigated water also increases the price of water in the UCT scenarios (Fig.  3C ). These results are especially significant in basins sensitive to land-use change. A previous study found that the FFICT prompts greater water withdrawals 41 . However, the study used a previous version of GCAM that did not have intensification options and assumed unlimited water. In that version, water use was proportional to land use. Therefore, when the UCT disincentivized expansion, water use was also limited. When extensification-intensification dynamics are considered, we find a substitution between water use and agricultural expansion. This finding emphasizes the importance of considering all trade-offs in mitigation policy options.

In this study, we use an economic surplus metric in order to quantify the economic impacts of water scarcity and the uncertainty of this impact due to different factors (i.e., population, agricultural productivity, etc.). Theoretically, basins would withdraw less when exposed to a limited supply of water and thus experience a negative economic impact, yet we find some basins capitalize on their water resources and become virtual water exporters in the face of global water scarcity. This dynamic would not be captured by looking at physical water scarcity metrics alone, nor by assessing economic impact at the basin-scale.

These variable responses to water scarcity are sometimes due to highly uncertain and largely uncontrollable factors such as the climate system. When normalized by a 2015 baseline, we find that uncertainty of economic impact due to Earth System Model forcing alone is often several thousand times higher than the uncertainty in the forcing itself (Fig.  2 ). Across the sampled states of the world, we find that slight deviations in precipitation drivers are almost always amplified as they propagate through markets. Since we have little control over uncertainty in the climate system, basin economies that are sensitive to fluctuations in hydro-climactic forcings will need especially robust water resource management frameworks in the future. Further, basins with the highest amount of impact variability due to climate uncertainty are often in politically unstable regions such as the Middle East. Thus, there is an even greater need to manage water resources in the most efficient way possible in the face of extreme uncertainty of economic impacts due to climate in these basins. Planners must also be aware of factors (e.g., population growth or carbon pricing regimes) that lead to economic tipping points in unstable basins.

Under the assumption that food production will always meet demand, implementing a Universal Carbon Tax prompts the intensification of agricultural land due to the increased cost of converting land for agricultural use. The intensification is enabled by increased irrigation and greater water withdrawals (Fig.  3 ). Thus, the effects of pricing carbon in a land-use policy on land intensification-extensification dynamics need to be taken into account in basins exhibiting high levels of water stress.

We find that most scenarios of interest (i.e., those that resulted in extremely high or low economic impact) are composed of a mix of SSP dimensions. This demonstrates the importance of using a scenario discovery framework in the context of a highly uncertain problem such as modeling water resources and the drawbacks of focusing on a limited set of narratives. In addition, the dimensions of high importance in certain basins are of less importance in others. Indeed, every dimension varied in this study was the most influential factor in determining the economic impact of water scarcity in at least one basin (Supplementary Fig.  12 ). Scenario discovery addresses this by identifying the most critical scenario components to the specific analysis context. There is no reason to expect universal shared scenarios will capture key challenges in each basin (or indeed in any), and it is very difficult to anticipate what combinations of factors present challenges in every basin before doing extensive exploration. Scenario discovery is a promising approach to identify relevant scenarios to inform water scarcity analyses. In addition, while this work assessed the economic impact in water markets alone, future work could make use of a Computable General Equilibrium model where the interactions between all markets would be accounted for (see “Methods” section). Indeed, we hope this work provides the basis for similar analyses across a range of hydro-economic models to ascertain the sensitivity of our results to model structure. Confidence in our metric depends on the fidelity of the selected hydro-economic model, so future work would benefit from expanded data collection of socio-technological drivers of regional and sectoral water consumption to improve those underlying models. This study’s use of a coupled partial equilibrium-hydrologic model to perform an extensive uncertainty analysis is novel to the integrated assessment modeling literature and enables the discovery of important multi-scale dynamics such as a basin’s wide range of adaptive responses to water scarcity.

Human-earth system model

Multiple factors affect water demand including population, wealth abundance and distribution, agricultural technology and practices, technological improvements, and carbon and land-use policy. These factors all interact with each other and with the climate system. It is therefore necessary to use a model that includes detailed representations of these systems and the interdependent endogenous choices that shape them. To this end, we have used a partial equilibrium model in order to represent the affected systems with as much detail as possible.

This study makes use of the Global Change Analysis Model (GCAM), a human-Earth system model that has been used by numerous agencies to make informed policy decisions 36 . GCAM is a complex model that decomposes the world into 32 geopolitical regions, 384 land-use regions, and 235 water basins 36 . GCAM includes coupled representations of the Earth’s climate, economic, hydrologic, land-use, and energy systems. These systems are expressed in varying degrees of detail. Population and GDP growth are represented as simple exogenous model inputs. Energy and land-use systems are represented in more detail, with shares of supplies and technologies competing using a logit model 36 . Renewable technologies within the model become more efficient over time and therefore some processes such as solar energy production become more competitive. Nonrenewable resources such as oil and fossil groundwater are modeled with graded supply curves and become more expensive as the levels are used up over time. Shares of energy production technologies may change based on different policy choices. For example, a carbon tax may increase the feasibility of using renewable energy sources. These policies may also impact the shares of land uses (e.g., the carbon tax may prompt afforestation).

Water demand and supply

GCAM allows users to specify water constraints and to link water supply to Xanthos, an extensible hydrologic model 42 . Previous versions of GCAM have introduced the water system but have limited its capabilities to computing water demands. The current system calculates both supply and demand and balances the two quantities by solving for an equilibrium regional shadow price for water 38 , 43 , 44 . Water demand in GCAM is modeled through six sectors: irrigation, livestock, municipal, manufacturing, primary energy, and electricity generation 25 . Irrigation demand is based on biophysical water demand estimates for twelve crop classes 25 . Water demand for irrigation is determined by deducting green water (i.e., water available for use by plants) on irrigated areas and green water on rain-fed areas from total water consumption. Livestock water demand is computed using the consumptive rates for six livestock types (cattle, buffalo, sheep, goats, pigs, and poultry) and estimates of livestock density in 2000 25 . Water withdrawals for electricity generation are related to the amount of electricity generated in each region. Once-through cooling systems compete with evaporative cooling systems with the latter becoming more prevalent over time 25 . Water use in the primary energy sector (i.e., the water used to extract natural resources) is calculated using estimates of energy production in each region along with water use coefficients. Municipal water demand is modeled using population, GDP, and assumptions about technological efficiencies 36 , 41 . Finally, manufacturing water demand is the total industrial water withdrawals less the energy-sector water withdrawals 25 . Consumption is calculated using exogenous consumption to withdrawal ratios for industrial manufacturing 25 .

Water supply in GCAM is modeled using three sources: surface water and renewable groundwater, nonrenewable groundwater, and desalinated seawater. Similar to technology use within GCAM, these sources of water compete using a logit structure based on price. Surface water is typically used first in larger quantities than its competing sources as it is the cheapest source of water. The upper limit of surface water in a basin is taken to be the mean average flow modeled using Xanthos, which calculates water supply at a monthly time step using evapotranspiration, water balance, and routing modules 42 . Accessible water 38 is assumed to be the volume of runoff available even in dry years in addition to reservoir storage capacity (after removing environmental flow requirements). The estimates of accessible water and basin runoff are used as inputs in GCAM. After the renewable water supply is fully consumed, GCAM will either use desalinated water or nonrenewable groundwater depending on the relative shares computed in the price-based logit structure 38 . Nonrenewable groundwater increases in price as more of the resource is consumed. The groundwater supply curves account for geophysical characteristics such as aquifer thickness and porosity, as well as economic factors such as the cost of installing and operating the well. As the price of extraction rises, desalination becomes more competitive, resulting in wider use of desalinated water 44 .

Basin-specific water policies are not represented within GCAM or indeed any global model. The level of detail needed to represent existing water markets and policies exceeds the capabilities of a global model. GCAM does, however, enforce a subsidy on water for agricultural sectors 36 . Imposing this subsidy in GCAM’s water markets allows water to be allocated first to agricultural producers. This behavior mimics the effect of traditional water rights in that senior rights are usually given to agricultural producers. The water markets within GCAM operate by generating a “shadow” price of water, which reflects the economic value of the last unit of water in terms of the water’s contribution to production. When water supply becomes a binding constraint in a particular water basin, the shadow price of water rises because users cannot use more water than there is in the basin. This forces a reduction in the production of the goods and services that rely on water as an input. Clearly, this approach is a simplification, but it marks an improvement over what is most often done where the implications of water scarcity are ignored (i.e., direct and indirect feedbacks associated with unsatisfied water demands are not captured, and analyses are limited to how water scarcity may increase or decrease in the future without a mechanism for dynamic adaptation measures).

We compute the difference in total economic surplus in these simplified water markets (i.e., the sum of producer surplus and consumer surplus) between a control scenario with no water constraints and its paired limited water scenario (see next section).

Capturing economic impact in the entire economy would require a general equilibrium model. However, general equilibrium models necessarily lose some detail in sectoral resolution so that they can capture market interactions. Water is a non-substitutable input to most markets in the human system and so most market interactions will be represented by the changes in water markets when conditions are perturbed. The surplus change in the water markets includes both direct effects (e.g., restricted supply) and indirect effects (e.g., demand shifts in adjacent markets). There may be economic effects not captured by looking at water markets alone, which could be investigated in future work that employs a computable general equilibrium model. Numerous previous studies have assessed economic impact in water markets using both types of models 45 .

Scenario design

We utilize a scenario discovery approach 35 to study the uncertainty in physical water scarcity and its economic impacts. Using this approach, scenarios are generated using all possible combinations of discrete levels of uncertain factors. All scenarios are weighted equally during scenario exploration so as not to presume the likelihood of outcomes a priori. Doing so may leave the system vulnerable to unanticipated events. In addition, in complex adaptive systems such as the human-Earth system, the main drivers of an outcome of interest may be non-intuitive and context-specific 34 . The traditional “predict-then-act” approaches 46 to planning implies a more complete understanding of the system and of future circumstances than is often the case, which can, in turn, lead to myopic decisions 35 . Alternatively, scenario discovery gives equal weight to all possible future system trajectories (i.e., population, wealth, energy prices) and finds the most influential factors driving outcomes of interest-based on the results of all scenarios. Planners can then make robust management decisions based on the influential factors and their uncertainties as opposed to designing based on a few future projections.

In this study, we use scenario discovery to determine the relative influence of seven dimensions in driving highly consequential economic outcomes due to water scarcity (Supplementary Fig.  2 ). These factors include socioeconomic conditions ( S ), agricultural yield assumptions ( G ), groundwater supply ( W ) and reservoir storage ( R ) levels, climate trajectories ( E ), and land-use scenarios ( T ). All factors are represented in Eq. ( 1 ) and are discussed in more detail below. Every scenario n is composed of a distinct combination of the levels of each factor.

Settings for the first three dimensions are taken from GCAM’s implementation of the Shared Socioeconomic Pathways (SSPs) 47 , 48 , 49 . The SSPs are based on plausible but distinct narratives that envision how the century will unfold 47 . The five SSPs correspond to the four combinations of high and low challenges to adaptation and mitigation of climate change with a fifth narrative that lies in the middle of the adaptation-mitigation challenge plane. The implementations of the SSPs within GCAM are made up of factors including population and GDP, agricultural yields, carbon sequestration implementation, renewable energy use, fossil fuel extraction cost, and energy demand 48 . This study included the population/GDP component and the agricultural component of the SSPs. The remaining components of the SSP framework were linked to either the population/GDP or agricultural component. For instance, in one scenario, SSP 3 fossil fuel extraction costs and renewable energy assumptions would be present with SSP 3 socioeconomics and SSP 5 agriculture assumptions; the converse scenario of this dimension would include SSP 5 fossil fuel extraction costs, renewable energy assumptions, and agriculture yields and SSP 3 socioeconomics. This switch ( L ) represents the third dimension of the design. Previous work found the socioeconomic and agricultural and land use elements of the SSPs had the most profound impact on water use 34 , thus we linked the other elements to ensure the scenario design emphasized potentially impactful factors.

The next three dimensions relate directly to the water supply. Groundwater availability is constrained at different levels (5%, 25%, and 40% of the physical water availability) that reflect the economic feasibility of extracting groundwater using the methodology within Turner et al. (2019a) 50 . We also vary reservoir storage estimates using two extremes following the methodology in Turner et al. (2019b) 44 . A restricted scenario indicates that reservoir storage remains constant from the present to the end of the century while an expanded scenario expresses a linear increase from current levels to maximum storage capacity (meaning all accessible water is stored) at the end of the century 44 . The Earth System Model forcing trajectories used as input to Xanthos were also varied between GFDL, MIROC, IPSL, HadGEM2, and NorESM 51 , 52 , 53 , 54 , 55 .

The final dimension corresponds to land-use scenarios formed by mitigation policies. The first, a Universal Carbon Tax (UCT) scenario, imposes a carbon tax on all sectors of the economy including emissions from land-use change. This scenario has many different land-use implications than the alternative scenario that employs the Fossil Fuel and Industrial Carbon Tax (FFICT) which does not price changes in land use (e.g., preserving grasslands and forests rather than expanding agriculture). To construct these scenarios, we use a carbon price trajectory that approximates the continued ambition scenario of the Nationally Determined Contributions (NDCs) as implemented in Fawcett (2015) and revised in Cui et al. (2018) 40 , 43 . This scenario assumes that countries continue decarbonization at the same rate as was necessary to meet the NDCs by 2030. The price of carbon at the reference scenario (SSP 2) for the continued ambition trajectory was used globally for all scenarios. The price begins at $21/ton of CO2 and increases to $233/ton of CO2 by the end of the century. These carbon prices are applied to all sectors (under the UCT) or to every sector but land-use change (under the FFICT).

In total, all unique combinations of the levels of these dimensions (i.e., the size of the set in Eq.  1 ) yield 3000 scenarios. Of the 3000, the total surplus could be calculated for 2876 scenarios without integration errors. Importantly, using a single carbon price trajectory while varying other socioeconomic and climatological factors yields a spread of emission trajectories. This will produce inconsistencies in a given scenario to the extent inputs depend on exogenous forcing trajectories. In this study, this is most important to the generation of hydrologic realizations (to compute available renewable water), where the Xanthos model was forced using several downscaled ESM simulations of RCP 4.5 even though the actual forcing trajectories varied across scenarios. Since climate change will impact the water cycle 56 , the amount of renewable water would also be different in each scenario had Xanthos been run endogenously. However, the magnitude of this difference is highly uncertain, as climate models have been found to cause as much or more uncertainty in hydrologic realizations as the RCPs themselves 57 . Thus, it is not clear that imposing an emissions cap to ensure consistency in forcing would better characterize hydrologic uncertainty. Future studies, for instance, those focused on the cost of meeting mitigation targets, may instead choose to vary prices rather than emissions, but this is beyond the scope of this work.

In addition to the dimensional components of the design, we added further inputs to reflect the recent advances of GCAM. Agricultural yield inputs based on Earth System Model, Representative Concentration Pathway (RCP) 58 and SSP were included, as well as hydropower inputs based on SSP, RCP, and ESM, and technological water demand estimates based on SSP 49 , 59 .

Water scarcity metrics

Many different metrics for measuring water scarcity have been proposed 56 , 60 , 61 . The most commonly used metrics typically compute physical water scarcity and exclude the socioeconomic information necessary to understand adaptive capacity. For example, the Water-To-Availability ratio (WTA) is computed as water withdrawals over renewable water supply 14 , 25 . Several holistic metrics exist that include socioeconomic information such as the Human Development Index 62 , though these metrics face the challenge of subjectively determining how to weight socioeconomic indicators relative to one another 60 .

This study examines water scarcity vulnerability using a metric that accounts for the economic impact of water scarcity within a hydrologic basin. We use the change in economic surplus in water markets between a basin with unlimited water and one with physical constraints on the water supply to calculate this economic impact. This difference consists of the direct impacts of changes in the water supply, as well as the indirect impacts from markets that rely on water. From this point on, we will refer to the surplus change in water markets as simply the surplus change, or economic impact.

Change in economic surplus has been used in many disciplines since its inception 63 . It has been used to assess the impact of climate change on agriculture 64 , 65 , as well as potential infrastructure projects 66 and adaptation policies 67 , 68 . Its continued use is due in part to its ease of implementation, its theoretical simplicity, and its ability to capture changes across sectors. These qualities are highly beneficial in a water scarcity metric. Computing the loss of surplus due to some factor requires a counterfactual scenario in which that factor is absent. This presents a problem when applying this type of metric to any historical data, including water scarcity: water scarcity has always been present. Even a synthetic history with unlimited water would be inadequate as all other historical values depend on historical water scarcity levels. Still, our metric has significant advantages over conventional physical water metrics that lack information about the ability of the basin to respond to water stress.

Here economic impact is defined as:

where T represents the total economic surplus (Supplementary Fig.  1 ). In this study, the economic impact is reported using its log-modulus and has units billions of 2020 US dollars:

Thus, an impact value of −2 would correspond to a loss of 100 billion 1975 US dollars or 2.3% of US GDP in 2018 after adjusting for inflation 69 .

Sign changes in economic impact correspond to shifts in water demand in a basin between unlimited and limited water scenarios. If the total surplus gained from increased withdrawals exceeds the consumer surplus lost by low-demand consumers when water limitations are imposed, basins experience a positive impact. This counter-intuitive case could result when basins become virtual water exporters when global physical constraints are imposed. With water constraints in place, such regions now have a comparative advantage in producing water-intensive goods (notably agricultural products, see Supplementary Fig.  3 ); therefore, they capture greater market share in the water-constrained scenarios. The increased production of these goods translates into a positive shift in demand in water markets. The additional economic activity also increases the value of water as consumers’ willingness to pay for goods increases. This additional economic activity manifests as a larger economic surplus, which translates to a more positive impact. The magnitude of the metric gives an indication of the difficulty of overcoming water scarcity within a basin since the economic impact depends on the value put on water. Higher values of water correspond to higher magnitudes of economic impact.

To uncover the most influential factors that lead a basin to experience positive versus negative impact, we used the Classification and Regression Trees (CART) algorithm 70 . The CART algorithm has been found useful in determining important factors and scenarios of interest in previous studies 34 , 35 . CART operates by performing binary splits of the data to create the purest possible subgroups. In this study, we use CART to identify the factors that lead to the worst-case scenarios with respect to the economic impact metric. Examining this continuous metric necessitates the use of the regression approach of CART. The regression approach uses an Analysis of Variance (ANOVA) method to discover the purest subgroups. Splits work to maximize the variance between groups and minimize variance within groups.

Data availability

Requests for raw data should be made to [email protected]. Processed data is available at https://doi.org/10.5281/zenodo.4470017 71 .

Code availability

Code to generate the main text figures and calculate economic impact can be found at https://doi.org/10.5281/zenodo.4470017 71 .

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Acknowledgements

This research was supported by the U.S. Department of Energy, Office of Science, as part of research in MultiSector Dynamics, Earth and Environmental System Modeling Program. The authors would also like to acknowledge Sean Turner, Chris Vernon, and Abigail Snyder for their help at the beginning of the project.

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Department of Civil and Environmental Engineering, Tufts University, Medford, MA, USA

Flannery Dolan & Jonathan Lamontagne

Department of Medicine, University of Virginia School of Medicine, Charlottesville, VA, USA

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Mohamad Hejazi & Jae Edmonds

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F.D. ran the model, processed the output, and analyzed the data. J.L. provided guidance throughout the process and proposed the initial experimental design. R.L. provided the necessary computational capabilities to output the economic impact metric. M.H. and P.R. helped propose the initial narratives of the paper. J.E. proposed the economic impact metric. All authors wrote the manuscript.

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Water Scarcity: Classification, Measurement and Management

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Definitions

Water scarcity has many definitions but they have one thing in common, which is an excess of water demand over the available supply (Damkjaer and Taylor 2017 ). The United Nations Water succinctly define it as the point at which the aggregate impact of all users impinges on the supply or quality of water under prevailing institutional arrangements to the extent that the demand by all sectors, including the environment, cannot be satisfied fully, a relative concept that can occur at any level of supply or demand (United Nations Water 2006 ). The Food and Agriculture Organization of the United Nations (FAO) lauded this definition recognizing that water scarcity can occur at any level of supply and demand, that it has various causes, and that it is capable of being remedied or alleviated to a certain extent (FAO 2012 ). Water scarcity means there is not enough water to meet the needs of everyone, including environmental flows (Water and Development Research Group 2020a ). In this...

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WATER SCARCITY

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WATER SCARCITY. Water stress and Water scarcity. occur when the demand for water exceeds the available amount during a certain period or when poor quality restricts its use. The UN states by 2025, 1.8 billion people will be living in countries or regions with absolute water scarcity,.

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Water stress and Water scarcity • occur when the demand for water exceeds the available amount during a certain period or when poor quality restricts its use.

The UN states by 2025, 1.8 billion people will be living in countries or regions with absolute water scarcity, Two-thirds of the world population could be under stress conditions

"water stress" is when annual water supplies drop below 1,700 cubic meters per person per year, according to the Falkenmark Water Stress Indicator

PHYSICAL SCARCITYWhen consumption exceeds 60% of the usable supply Physical access to water is limited. It is when the demand outstrips the lands ability to provide the needed water. For the most part, dry parts of the world or arid regions are most often associated with physical scarcity.

ECONOMIC SCARCITYWhere a country has sufficient water resources but additional storage and transport facilities are required making it expensive • When a population does not have the necessary monetary means to utilize an adequate source of water. It is about a unequal distribution of resources for many reasons, including political and ethnic conflict.

Task • Decide if the following supply issues are environmentally or human induced.

SUPPLY ISSUES

Inaccessible or Out of Reach India gets 90% of its rainfall during the monsoon season and at other times rainfall over much of the country is very low. Sub-Saharan Countries rely on large and expensive water development projects.

No infrastructure The addition of water to areas where it is insufficient for adequate crop growth. – types can range from total flooding such as in paddy fields to drip irrigation where precise measurements are distributed to each plant.

Rain: About three-quarters of the annual rainfall occurs in areas containing less than one-third of the world’s population, whereas two-thirds of the world’s population live in the areas receiving only one-quarter of the world’s annual rainfall

Climate scientists say that the greenhouse effect is accelerating the global water cycle. "It's like the rich get richer scenario where the wet places will get wetter and the dry places will get a lot drier because the conveyor belt is speeding up between those two places.” Source ABC Science: http://www.abc.net.au/science/articles/2012/04/27/3488816.htm Evapo-transpiration: The water supply depends on several factors in the water cycle including evaporation and transpiration rates.

Fossilized Aquifers are rocks that hold water. They provide an important store of water that regulates the hydrological cycle and maintains river flow. Their resources are being depleted.

Wetlands (land with soil that is permanently flooded) During the past centaury half of the worlds natural wetlands have dried up.

Ground water pollution has been on the increase and can happen naturally such as naturally occurring arsenic in groundwater pumped up through tube wells in Bangladesh. After realising that 85 million of the country’s 125 million population will be affected, Bangladesh has sunk millions of wells in the last 30 years in order to provide a supply of drinking water free from the bacterial contamination of surface water. Pollution

Water is one of the most sensitive and unsolvable problems in the Middle East, Africa and South Asia. It is likely that disputes over scarce water resources will fuel and increase armed conflicts. DEMAND FOR WATER

POPULATION INCREASE Population, urbanization and industrialization have increased the use of water in these sectors. As world population and industry has increased the use of water has acceleratedand this is projected to continue. By 2025 global availibility of fresh water may drop to by 25% of the 2000 figure.

Agriculture Agriculture is the largest user consuming almost two-thirds of all water drawn from rivers, lakes and ground water. Scince 1960, water use for crop irrigation has risen from 60-70%. Industry uses about 20% of available water, and the municipal sector uses 10% Exam question: Describe two pollutants which are causes of unsafe drinking water (4 marks)

Current Water Stress

Water-Stress Effects • Amount • Timing • seasonality • drought • flood • Quality • - salinity • nutrients • temperature • toxins • pathogens • sediments

Exam question: • ‘The factors affecting patterns and trends in physical water scarcity are environmental rather than human.’ Discuss this statement.

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The risk of global water scarcity is greater when accounting for the origin of rain.

Stockholm University

Fernando Jaramillo, associate professor in physical geography at Stockholm University, and responsible for the study.

Credit: Anette Gärdeklint Sylla/Stockholm University

Securing the world's water supply is one of the greatest challenges of our time. Research at Stockholm University is now presenting an alternative method for quantifying the global risk of water scarcity. Results indicate higher risks to water supply than previously expected if accounting for the environmental conditions and governability where rain is produced.

The common idea of global water supply is rain falling on the earth's surface and then stored in aquifers, lakes, and rivers. This idea is usually used to assess water security and the risk of water scarcity. However, a new study published in Nature Water shows how the water risks are dependent on governance and environmental conditions present upwind, which means the areas where the moisture for rain comes from.

“Water supply really originates beforehand, with moisture evaporated from land or in the ocean traveling in the atmosphere before falling as rain. This upwind moisture is commonly overlooked when assessing water availability,” says Fernando Jaramillo, associate professor in physical geography at Stockholm University and responsible for the study.

When a lake or river is shared between different countries or authorities, assessments and regulations mainly apply an upstream perspective, considering conditions in the direction upriver from the water body. Instead, an upwind perspective considers the area where evaporated water is transported before ending up as rain. The area is known as a precipitationshed and can cover large areas of the earth’s surface.

“For instance, in tropical South America, most of the Amazon basin is downstream of the Andes mountain range, whereas large areas of the Andes are in themselves downwind of the Amazon rainforest and depending on it, which makes these two regions dependent on each other for water supply,” says Fernando Jaramillo.

The study examined 379 hydrological basins worldwide, revealing that risks to water security are significantly higher when considering the upwind origin of water.

“With this approach, we see that 32,900 km3/year of water requirements worldwide face very high risk, a near 50 percent increase, compared to the 20,500 km3/year resulting from the more traditional upstream focus,“ says José Posada, former doctoral student at Stockholm University and main author of the study.

Political control can have major consequences

Since a large amount of water is evaporated from plants, changes in land use can affect downwind water availability. If deforestation and agricultural development are predominant in upwind areas, the amount of moisture vegetation provides may decrease, reducing rainfall downwind and increasing the risk to water security.

“For coastal countries such as the Philippines, most of the rain comes from the sea, which means that land-use changes pose very little risk to water security. Rainfall in inland countries such as Niger, on the other hand, comes mainly from moisture that evaporates in neighboring countries such as Nigeria and Ghana . This puts many land-locked countries at high risk regarding how water security is affected by changes in land use,“ says Fernando Jaramillo.

In other words, political factors such as environmental management and regulations in areas where moisture first evaporates can affect water safety in completely different areas.

“For instance, the Congo River basin, heavily reliant on moisture from neighboring countries with low environmental performance and governance according to global indicators, faces considerable risks due to potential deforestation and unregulated land use changes in neighboring areas,“ says Lan Wang-Erlandsson, researcher at the Stockholm Resilience Centre at Stockholm University and co-author of the study.

Environmental regulation requires an upwind perspective.

The study reveals why the lack of governability and environmental performance in a country upwind may be relevant to the water supply of a country downwind. It stresses the codependence between upstream/downwind and downstream/upwind countries.

"It is not possible to ignore the interdependence between countries. In the end, all water is connected, so we should not only mind how we manage our water resources within a region or country but also how our neighboring countries do,” says Lan Wang-Erlandsson.

"We hope that the findings of this study can help identify where and to whom cooperation strategies and efforts can be directed to mitigate the causes of water-related tensions, including atmospheric water flows in transboundary decision-making and water governance frameworks. We stress the need for international cooperation to effectively manage upwind moisture sources,“ concludes Fernando Jaramillo.

Find the study "Upwind moisture supply increases risk to water security" in Nature Water DOI: 10.1038/s44221-024-00291-w

Nature Water

10.1038/s44221-024-00291-w

Method of Research

Computational simulation/modeling

Subject of Research

Not applicable

Article Title

Upwind moisture supply increases risk to water security

Article Publication Date

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