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Solar power is the cheapest source of renewable energy available to produce hydrogen.Credit: Timothy Hearsum/Getty Images

To steer economies away from fossil fuels and to cut carbon emissions, hydrogen needs to be produced from low-carbon energy sources, such as wind and solar power. This green hydrogen is particularly attractive to certain big-energy users such as the shipping, aviation and steel industries, which would struggle to run on batteries or plug into renewable energy directly through power grids. And producing hydrogen is straightforward: electricity zapped into water splits the H2O molecules into hydrogen and oxygen, which bubble off as gaseous H2 and O2. The hydrogen can then be piped to where energy is needed.In a world of growing water stress, however, the process’s reliance on water is raising alarm (see ‘Watering the power plant’). Earlier this year, the non-governmental organization Food & Water Watch in Washington DC warned that, by 2050, hydrogen production could gulp down as much water as is used by 34 million US citizens each year. A spate of studies over the past few years provides a more positive picture, however, presenting evidence that scaling up hydrogen production need not threaten water supplies. “Water withdrawals for hydrogen production are negligible compared to total water withdrawals,” says Lorenzo Rosa, an environmental engineer who specializes in links between water, energy and food at the Carnegie Institution for Science in Palo Alto, California.

Source: E. Grubert. Clean. Prod. Lett. 4, 100037 (2023).

Still, Rosa says that water considerations should shape how and where hydrogen is produced, especially because heat and altered precipitation driven by climate change are tightening pressure on water supplies. “Hydrogen’s water consumption is small compared to what’s currently used in fossil-energy conversion and inconsequential compared to agricultural water use,” says Jack Brouwer, director of the Clean Energy Institute at the University of California, Irvine. “But there are serious water availability and delivery challenges at the local and regional levels that will need to be considered.”Brouwer and Rosa say that expanding hydrogen-production technologies and water-treatment options could enable hydrogen producers to tap into a range of non-potable water resources — including seawater — or to slash their water consumption. At the same time, geospatial analyses that maps water and renewable-energy resources against projected hydrogen production and demand can pinpoint where investing in water-saving technology — or deciding to import rather than produce hydrogen — will be key to minimizing tension between the push for hydrogen and the need to preserve water resources.A thirsty processThe water requirements for producing hydrogen by electrolysis begin with a simple calculation: every kilogram of H2 molecules requires 9 litres of H2O. Treatment to purify that water — eliminating minerals that would gum up the works — consumes another 15 litres of water per kilogram of H21.That’s not the end of the story, however. There’s a lot more water use to be counted if the renewable energy that powers the process is included. The operation of solar panels and wind turbines might not consume much water, but manufacturing them does. All told, manufacturing a wind turbine adds 11 litres to green hydrogen’s water footprint. And the manufacture of today’s leading variety of solar power adds a huge 124 litres, mostly from the fabrication of silicon photovoltaic wafers.Still, the water requirements of hydrogen production seem to be manageable. As part of an analysis co-led by Rosa, one scenario for a net-zero global economy in 2050 that consumes 400 megatonnes of green hydrogen per year would use a meagre 0.13% of the world’s available water supply if the energy came entirely from wind power, and about 0.56% if it all came from solar power 2.Countries that already face water scarcity, Rosa and his colleagues argue, could choose to import hydrogen to meet domestic needs rather than ramping up production. In fact, many of the countries that their study identified as land-limited are already working to foster a hydrogen trade, including Japan and parts of Europe. Other regions and nations — including areas of sub-Saharan Africa, South America, Canada and Australia — have sufficient land and water to become major hydrogen exporters, and many are already gearing up to fulfil this role.Multiple factors suggest that the impacts of green hydrogen on regional water could be even more limited than Rosa’s findings suggest. For example, over the past few years analysts and energy planners have lowered their estimates of how much green hydrogen will be produced over the long term. In its September prediction for limiting global temperature rise to 1.5 °C, the International Energy Agency (IEA) projects there will be one-fifth less hydrogen use in 2050 than it had predicted in its 2021 report.Another important consideration when computing hydrogen’s hydrological footprint is the capacity of hydrogen to be a substitute for fossil fuels, which are themselves water-intensive sources of energy. Most studies so far do not account for the effect of such a substitution. Rosa and his colleagues, for example, assume a fivefold reduction in petroleum refining between 2020 and 2050, as the use of gasoline and diesel is phased down. But they do not work in a corresponding reduction in water use, which could further improve hydrogen’s hydrological footprint. And if green hydrogen replaces natural gas, the substantial water consumption by gas producers that use hydraulic fracturing will be reduced. For example, many jurisdictions in the United States and Europe are considering using green hydrogen to fuel gas-fired power plants to cover gaps in supply from solar and wind farms.Love that dirty waterIn many cases, hydrogen producers might be able to avoid adding strain to potable water supplies by tapping polluted or salty water, instead of potable water. Options include municipal waste water, waste water from oil and gas production and even seawater. Water treatment and desalination plants are expensive to build, but the investment is comparatively small relative to the overall cost of hydrogen production.

Jack Brouwer, director of the Clean Energy Institute at the University of California, Irvine, demonstrates the electrolysis stack in a system used to make renewable hydrogen.Credit: Steve Zylius/UCI

In 2022, an analysis 3 by a team at Yale University in New Haven, Connecticut, concluded that even to treat seawater — the toughest water source to prepare as input to such a facility — would require only 0.3% of an electrolysis plant’s total energy budget. “The bottom line is that “we do not need to consume freshwater resources used for drinking” to produce hydrogen, says Lea Winter, a chemical and environmental engineer at Yale and the study’s lead author. The International Renewable Energy Agency agrees, concluding in 2020 that: “Even in places with water stress, seawater desalination can be used with limited penalties on cost or efficiency.”In California, electrolyser manufacturer Plug Power in New York plans to build a water-treatment plant and hand it over to the local municipality, in return for a source of water for hydrogen production. Mendota, where the plant is to be built, is currently depleting ground water to meet demand for potable water. City officials say that the new plant will clean up sewage to increase the city’s water supply, so that it can reduce its use of ground water and sell water to Plug Power.Using seawater presents almost limitless potential, but also troubling environmental impacts. Some desalination plants release heated brines laden with treatment chemicals back into the sea; they can also suck in and destroy marine creatures. The most significant ecosystem impact of these plants, according to a 2020 review4, is lethal osmotic shock to marine organisms, including fish, plankton and algae when super-salty brines cause their cells to dehydrate. Most at risk are organisms in semi-closed seas such as the Red Sea, the Mediterranean and the Persian Gulf. Nearly half of the world’s desalination capacity is concentrated in the Persian Gulf.Some observers, however, foresee potential environmental dividends if hydrogen producers tap seawater and waste water. Thomas Adisorn, a political scientist at Germany’s Wuppertal Institute for Climate, Environment and Energy, sees potential for projects such as that of Plug Power to improve the environment by supporting international development. “Putting more effort into using recycled waste water in developing countries that are exporting hydrogen could raise their capacity to build wastewater infrastructure,” says Adisorn, who organized a meeting in 2022 to help officials from water-scarce Jordan who were planning its hydrogen economy.New technology and engineering integrations promise to trim the cost of non-conventional water use even further, while capturing other valuable benefits that pay for the extra water treatment. One active area of research and development, for example, would monetize green hydrogen’s oxygen by-product. Aerobic treatment tanks at wastewater plants rely on pumped air to sustain their waste-eating microbes. According to Brouwer, some large wastewater plants pump in pure oxygen instead of air to spur faster digestion. With an electrolyser they could get that oxygen for free, says Brouwer, rather than operating pricey air-separation units.Researchers with the municipal water authority Sydney Water and the University of Sydney, Australia, estimated in 2022 that integrating electrolysers into wastewater treatment plants could save the city about US$1.5 million per year 5. They calculate that the city’s 13.7 gigalitres per year of unused effluent could yield 0.88 megatonnes of green hydrogen per year — one-tenth of the amount Australia and New Zealand are expected to produce in 2030, according to analysts S&P Global in New York. Sydney Water says that its unpublished research confirms the viability of hydrogen and oxygen production using its treated water, following further purification.Another innovation that could prove a major benefit to using seawater for hydrogen production is the ability to operate electrolysers offshore. Over the past year, teams in China and Europe have deployed platforms combining desalination equipment and electrolysers. The hope is that the floating electrolysis plants — if they can operate reliably amid storms and other assaults to offshore hardware — will cut the cost of offshore wind energy. Shipping hydrogen through pipelines is generally cheaper than moving the equivalent amount of energy through electrical transmission lines, and hydrogen proponents are betting that this rule will hold for passing energy from offshore wind farms back to land.The hydrogen producer Lhyfe’s 1-megawatt pilot platform operated offshore for 5 months this year using desalinated seawater, and a 10-megawatt platform is planned for Belgian waters in 2026. Lhyfe in Nantes, France, wants to mitigate the impact of desalination by eschewing chemical additives in its treatment process, and by diluting brine with extra seawater, says Stéphane Le Berre, Lhyfe’s offshore project manager.Lhyfe is now exploring whether the oxygen from offshore electrolysis could counteract declining levels of dissolved oxygen in the ocean — conditions that are stunting marine ecosystems in some regions. In July, researchers projected that artificial oxygenation from global deployment of offshore wind farms and electrolysers could reduce the volume of severely hypoxic zones by 1.1–2.4%6. But they also reported some counterintuitive regional impacts. For example, their simulation projected that oxygen injection might enlarge an existing hypoxic zone in the Indian Ocean’s Bay of Bengal.Known unknownsTechnological wild cards, meanwhile, could alter water consumption calculations around hydrogen production. In a review, the IEA identified 40 companies that are exploring a potential hydrogen source that might be cleaner than electrolysis: natural pockets of the gas, some of which might be tapped using little water. But, as with injecting oxygen in to the sea, seemingly water-saving technologies could have perverse effects. Accessing ‘geological’ hydrogen might require fracturing of rock layers akin to hydraulic fracturing or ‘fracking’ used to recover oil and gas. And some of the hydrogen prospectors plan to stimulate hydrogen production in situ by injecting water into iron-rich rock formations.Green-energy company Eden GeoPower in Somerville, Massachusetts, plans to test hydrogen stimulation in the peridotite rock formations of water-scarce Oman, in collaboration with the country’s Ministry of Energy and Minerals. The company hopes to increase underground permeability using its water-free electrical fracturing technology. Chief executive Paris Smalls, says “back of the envelope calculations” suggest that net water consumption will be comparable with that of electrolysis per kilogram of hydrogen delivered.Eden GeoPower’s attention to water resources is the exception to the rule among hydrogen producers. Water supply is not mentioned in the IEA report or in a 2022 “critical” review of hydrogen-production technology 7.Given that hydrogen production and water use are inextricably bound, it is unlikely that water supply will continue to be omitted. The inconvenient truth, say both Brouwer and Rosa, is that solar energy is the cheapest source of low-carbon power available to produce hydrogen, but the regions with the best solar resources are also some of the most parched.Brouwer is one of the main team members behind California’s hydrogen-development programme, which picked up $1.2-billion of the $7 billion in US federal funds awarded in October to regional ‘hydrogen hubs’ that link producers and consumers. He says that hydrogen plants can tap into conventional water supplies or clean up waste water, but that reaching net-zero carbon emissions in California will ultimately require a lot more solar panels in the desert.Converting that solar energy to hydrogen will force the state to build more infrastructure, and to make an important choice. As Brouwer puts it: “We’re going to eventually have to figure out whether we want to run big wires from the solar resource to where the water is, or big pipes sending water to the desert.” More

Floods, droughts, pollution, water scarcity and conflict — humanity’s relationship with water is deteriorating, and it is threatening our health and well-being, as well as that of the environment that sustains us. The good news is that a transition from the water policies and technologies of past centuries to more effective and equitable ways of using and preserving this vital resource is not only possible, but under way. The challenge is to accelerate and broaden the transition.Water policies have typically fostered a reliance on centralized, often massive infrastructure, such as big dams for flood and drought protection, and aqueducts and pipelines to move water long distances. Governments have also created narrow institutions focused on water, to the detriment of the interconnected issues of food security, climate, energy and ecosystem health. The key assumption of these ‘hard path’ strategies is that society must find more and more supply to meet what was assumed to be never-ending increases in demand.That focus on supply has brought great benefits to many people, but it has also had unintended and increasingly negative consequences. Among these are the failure to provide safe water and sanitation to all; unsustainable overdraft of ground water to produce the food and fibre that the world’s 8 billion people need; inadequate regulation of water pollutants; massive ecological disruption of aquatic ecosystems; political and violent conflict over water resources; and now, accelerating climate disruption to water systems1.A shift away from the supply-oriented hard path is possible — and necessary. Central to this change will be a transition to a focus on demand, efficiency and reuse, and on protecting and restoring ecosystems harmed by centuries of abuse. Society must move away from thinking about how to take more water from already over-tapped rivers, lakes and aquifers, and instead find ways to do the things we want with less water. These include, water technologies to transform industries and allow people to grow more food; appliances to reduce the amount of water used to flush toilets, and wash clothes and dishes; finding and plugging leaks in water-distribution systems and homes; and collecting, treating and reusing waste water.Remarkably, and unbeknown to most people, the transition to a more efficient and sustainable future is already under way.Singapore and Israel, two highly water-stressed regions, use much less water per person than do other high-income countries, and they recycle, treat and reuse more than 80% of their waste water2. New technologies, including precision irrigation, real-time soil-moisture monitoring and highly localized weather-forecasting models, allow farmers to boost yields and crop quality while cutting water use. Damaging, costly and dangerous dams are being removed, helping to restore rivers and fisheries.

Source: US Geological Survey

In the United States, total water use is decreasing even though the population and the economy are expanding. Water withdrawals are much less today than they were 50 years ago (see ‘A dip in use’) — evidence that an efficiency revolution is under way. And the United States is indeed doing more with less, because during this time, there has been a marked increase in the economic productivity of water use, measured as units of gross domestic product per unit of water used (see ‘Doing more with less’). Similar trends are evident in many other countries.

Source: US Geological Survey/US Department of Commerce.

Overcoming barriersThe challenge is how to accelerate this transition and overcome barriers to more sustainable and equitable water systems. One important obstacle is the lack of adequate financing and investment in expanding, upgrading and maintaining water systems. Others are institutional resistance in the form of weak or misdirected regulations, antiquated water-rights laws, and inadequate training of water managers with outdated ideas and tools. Another is blind adherence by authorities to old-fashioned ideas or simple ignorance about both the risks of the hard path and the potential of alternatives.Funding for the modernization of water systems must be increased. In the United States, President Biden’s Infrastructure Investment and Jobs Act provides US$82.5 billion for water-related programmes, including removing toxic lead pipes and providing water services to long-neglected front-line communities. These communities include those dependent on unregulated rural water systems, farm-worker communities in California’s Central Valley, Indigenous populations and those in low-income urban centres with deteriorating infrastructure. That’s a good start. But more public- and private-investments are needed, especially to provide modern water and sanitation systems globally for those who still lack them, and to improve efficiency and reuse.Regulations have been helpful in setting standards to cut waste and improve water quality, but further standards — and stronger enforcement — are needed to protect against new pollutants. Providing information on how to cut food waste on farms and in food processing, and how to shift diets to less water-intensive food choices can help producers and consumers to reduce their water footprints3. Corporations must expand water stewardship efforts in their operations and supply chains. Water institutions must be reformed and integrated with those that deal with energy and climate challenges. And we must return water to the environment to restore ecological systems that, in turn, protect human health and well-being.In short, the status quo is not acceptable. Efforts must be made at all levels to accelerate the shift from simply supplying more water to meeting human and ecological water needs as carefully and efficiently as possible. No new technologies need to be invented for this to happen, and the economic costs of the transition are much less than the costs of failing to do so. Individuals, communities, corporations and governments all have a part to play. A sustainable water future is possible if we choose the right path. More

A break in the levee holding back the Mokelumne River, California, resulted in the flooding of a farm built on the river’s floodplain.Credit: Erica Gies

When water inundated parts of New York City in September 2023, 28 people had to be rescued from their cars and basement apartments. Thankfully, no one died this time. In 2021, flooding in New York killed 11 people. Neighbourhoods in the city also flooded in 2020 and 2022, and it’s not just New York. Floods are becoming increasingly frequent and severe globally, as are droughts. Steve Bowen, chief science officer for reinsurance firm Gallagher Re in London, described the most recent New York floods on X (formerly Twitter) as “the latest example of ageing infrastructure built for a climate that no longer exists”. Such sentiments are common, and frequently followed by calls for more infrastructure: bigger levees and seawalls, larger pipes and stormwater tanks, and more dams, aqueducts and desalination plants.But human-built infrastructure and land-development practices that leave little space for water are actually a big part of the problem. Eric Sanderson, a conservation ecologist and author of Mannahatta: A Natural History of New York City (2009), called this out pithily in a series of posts on X. He captioned a video of water pouring into a subway stop: “Under former salt marsh” and one of a flooded area in Brooklyn, “Former bog”.Engineered structures intended to control water, urban sprawl and industrial forestry and agriculture have drastically altered the natural water cycle, contributing to both increased flooding and water scarcity. Society has dammed and diverted two-thirds of the world’s large rivers, drained as much of 87% of global wetlands and degraded 75% of Earth’s land area. “We need to let nature play its original function,” says Adnan Rajib, an engineer and director of the H2I lab at the University of Texas at Arlington. “Water doesn’t have anywhere to go.”Climate change is also a factor in today’s water extremes, scientists agree, but blunting the impact of floods and droughts will take more than reducing carbon emissions. Decision makers must also change how they manage land and water. “The climate crisis is real,” says Kris Johnson, a conservation biologist and director of agriculture for The Nature Conservancy, an environmental organization in Minneapolis, Minnesota. “But the biodiversity and water crises are also real and interdependent.” Engineered solutions to water problems — such as levees or dams — typically overlook the complex relationships between water, rocks, soil, plants, animals and atmosphere. Failing to account for that complexity often damages the natural systems that support life and the water cycle, contributing to increased flooding and water scarcity.

Most of the traditional eris tanks in Chennai, India, are now only associated with temples.Credit: Erica Gies

Around the world, scientists, farmers, urban planners, landscape architects, and water utilities and flood managers are taking heed and restoring wetlands, floodplains and forests that development has disrupted. Their efforts, which return space to water where it naturally slows or stalls on land, are unique to each place’s geology, ecology and culture. ‘Slow water’ projects work with natural systems rather than trying to control them and they are socially just. They are distributed across the landscape, not centralized, and make the most of local water.Slow it downFloodplains are one phase of slow water that are prone to human disruption. They hold and release water, redistribute sediment and generate food for aquatic life. But around the world, engineers and farmers have built levees along rivers, cutting them off from their floodplains. “Everyone is doing research on how floods impact humans,” says Rajib. However, he adds, “it’s also the humans that are causing the floods”.

Eris tanks were traditionally used to slow the flow of water from the mountains.Credit: Erica Gies

In a study published in July1, Rajib and his colleagues found that, from 1992 to 2019, humans have encroached on 600,000 square kilometres of floodplains — an area about the size of Ukraine. In taking space from water, such development causes rivers to rise and places people living nearby at higher risk of flooding.Reducing that risk requires the engineered infrastructure installed by humans to be altered or undone. Hydrologist Nicholas Pinter at the University of California, Davis, studies how some communities reduce their risk. Along smaller rivers in the Sacramento Valley, California, non-governmental environmental organizations have returned floodplain space to rivers. Pinter says that during the numerous atmospheric river storms of winter 2022–23, “the only portion where the levees broke were where they didn’t set them back”.Sprawling citiesMany cities around the world are built on floodplains, covered-over streams and filled-in wetlands. Urban areas have doubled since 1992, exacerbating flood risk — for every 1% increase in paved area, annual flood magnitude in nearby rivers increases by 3.3% from run-off. When cities flood, municipal leaders attempt to disperse the water as fast as possible, rather than retain it for dry seasons. Then when water is in short supply, they drill deeper wells to reach ground water, bring in distant water through aqueducts or desalinate seawater to meet the needs of the community.In the wake of increasingly frequent and expensive disasters, some cities are changing tack and making places for water to soak into the ground again. These include stormwater ditches lined with native plants, permeable pavement, green roofs, planted medians, tree wells, and parks on reclaimed industrial areas in river floodplains. These strategies go by different names: low-impact design in the United States, for example, and sponge cities in China — where creating them is a national policy.Chennai, India, is one place that is returning space to water. Nearly every summer, the city runs out of water. The painful irony, however, is that the annual monsoon brings 1.5 times the water that Chennai’s residents use each year. Flooding is also frequent, and starts soon after even moderate rains. The city’s area is now nine times larger than it was in 1980, hemming in three rivers, as well as covering over backwaters, coastal estuaries, mangrove forests and ancient human-built lakes. In 2015, a disastrous flood killed at least 470 people and pushed city managers to alter their course.Today, Chennai deploys slow-water techniques across the city, including protecting remaining wetlands, restoring them where possible and reviving the region’s 2,000-year-old water infiltration system that was once used to provide water year-round. Made up of structures called eris, the system ran from the mountains down to the Bay of Bengal. Eris (Tamil for tanks) are open on the higher side to catch water flowing downhill and closed by an earthen wall on the lower side. A divet in the wall on the low side allows water to flow downhill to the next eri. By slowing the flow of water, the eris reduce flooding, prevent soil erosion and give water time to seep underground — where it is filtered and kept within reach of wells.Despite being impressed by the estimated 53,000 or so eris across southern India, the British introduced centralized management in the nineteenth century, destroying the communal system by which local people maintained their eris and shared water resources. The eris that remain in cities are often connected with temples. Chennai is home to 54 temple eris, and water managers are restoring pathways for storm water to flow to them — and to link them with remaining and restored natural water bodies. The managers expect this to reduce both flooding and scarcity by absorbing and storing local rain.The eris system is unique to southern India. But as Yu Kongjian, co-founder of landscape architecture firm Turenscape, Beijing, and proponent of sponge cities in China, says in the Chinese edition of Water Always Wins (2023), “Each nation has a ‘slow water’ cultural heritage.” Part of the solution in a given place is to include the strategies of earlier inhabitants to work with natural systems to manage water.Forests and farmsSlow-water approaches can also reduce fire severity. Canadian wildfires burnt almost 19 million hectares during the summer of 2023, choking cities across North America. Climate change and misguided policies of stamping out all fires have played a large part in extreme blazes, but commercial forestry shares some of the blame because of how it alters the natural water cycle.

A levee was broken for the Cosumnes River near Sacramento, California, to allow water from the river access to part of the floodplain.Credit: Erica Gies

Tree roots create pathways for water to move underground, storing rainfall locally. The ground water that trees transpire into the air forms clouds and, along with evaporation from soil, becomes the source of 10–80% of rain over continents, depending on location2. Losing forests can, therefore, increase run-off and decrease rainfall.Francina Dominguez, a hydroclimatologist at the University of Illinois Urbana–Champaign, has found another way that deforestation reduces rain. The surface roughness of mixed-species forests makes them better than tree plantations or crops at slowing wind, and thereby makes it more likely that vapour will condense into rain3.Natural forests are much more efficient at regulating water and climate than are commercially logged forests. A mature native forest transpires more water than younger tree plantations, and it contains understorey plants, rich soil and decomposing wood that create a spongy, moist environment. Clear-cutting desiccates this system. Anastassia Makarieva, an atmospheric physicist at the Technical University of Munich in Germany, says that for greater water-cycle stability, remaining old growth should be conserved and some altered areas restored. This should start at the edge of wetter areas, she says, to cumulatively increase water-vapour density and restart the local rain cycle.Replacing perennial vegetation, either forests or grasslands, with annual crops also reduces the amount of evaporation and transpiration, says Dominguez. Agriculture changes the water cycle in more obvious ways too, such as accounting for 70% of water use, and, in wetter places, such as the US Midwest, through draining of swamps by farmers to create crop land.Other standard agricultural practices tend to work against a sustainable water cycle. The higher the percentage of organic matter in soil, the more water it can hold, and the better it can absorb flood water and retain the water until plants need it, reducing the need for irrigation. But ploughing dries out and compacts soil, and pesticide treatments kill animals that help to keep water and biochemical cycles healthy.Returning some marginal cropland — land with degraded agricultural value — to wetland or grassland “could actually reduce the flood peak for the system overall”, says Johnson, who is a co-author on Rajib’s floodplain encroachment study. That wouldn’t have to mean a reduction in the quantity of food produced. Globally, people are pulling back from marginal farmland, leaving that land available for restoration. Some agricultural lands that flood routinely should be fallowed, says Johnson, rather than insured against flood damage. “We want to make sure that we’re not incentivizing behaviour that is likely to fail.” In places such as California and the Netherlands, some floodplains have been partially returned to rivers as relief valves for high flows. Farmers who grow on the land are compensated when they lose their crops.In drought-prone areas, agricultural and urban expansion, and unsustainable groundwater pumping are exacerbating water scarcity, says Johnson. Shifting thirsty crops away from water-stressed places makes sense, he says. California, for example, has introduced a funded programme that could take as much as 400,000 hectares out of use by 2040, because agriculture in the area has expanded beyond what the available water can support.A draining experienceSome water stress is caused by what biologist and hydrologist Brock Dolman calls the “age of drainage”. According to Dolman, who is co-founder of the non-profit organization Occidental Arts & Ecology Center in California, European settlers and their descendants dried out land by killing beavers that created wetlands across 10% of North America, overgrazing animals they brought with them, and overpumping ground water so that plant roots could no longer reach it. But various efforts are starting to turn that around, including supporting the recovery of beaver populations.

Compared with Rock Creek (left), which has no natural infrastructure, the slow-water approach at Turkey creek (right) extends water availability into the dry season.Credit: Laura Norman

Where beavers aren’t present, land managers are also attempting to slow water in degraded streams. When Valer Clark and Josiah Austin moved to their ranch south of Tucson, Arizona, in the 1980s, they found a land denuded of trees and grazed to the bone. Monsoon rains roared through stream channels, called washes, eroding them. The water then quickly disappeared. Clark and Austin hand-built small rock dams in the headwaters of the often-dry Turkey Creek, following local Indigenous methods. Within a few monsoon seasons, the structures caught sediment, held water and became a series of wetland sponges that seeped water year-round. Downstream landowners were worried that Clark and Austin were holding onto ‘their’ water.But physical scientist Laura Norman at the US Geological Survey in Tucson found that this was not the case. Intrigued by the transformation, she compared Turkey Creek with neighbouring Rock Creek. She found that the rock dams slowed flash floods and extended water availability into the dry season. And most surprisingly, the structures actually increased the stream’s flow by 28%4. That’s because, in Rock Creek, some of the water flowing over the bare bedrock evaporates immediately, she explains. By contrast, the water-slowing approach taken at Turkey Creek allows the water to sink underground. The US Forest Service and the state of Arizona are now authorizing the building of these structures on their land.A growing body of evidence is showing that floods and droughts are caused, in part, by people’s land-use choices. And researchers are documenting the multiple benefits of restoring slow-water systems in cities, forests, agricultural lands and grasslands. Bringing the natural water cycle back into balance, researchers say, will require a decentralized mindset, with a focus on developing thousands of small projects throughout water’s path. “When you look at one particular storage space for water in one particular location, maybe that is insignificant,” says Rajib. “But when you look at their connectivity across the basin, continent or the world, the cumulative impact is substantial.” More

Illustration: Sam Falconer

In late summer, Death Valley National Park earns its name. The heat in this region of California and Nevada is relentless. Record temperatures are set, and the air is often bone dry. The 22 August 2022 was no exception, with an average temperature during daytime of 51.6 °C and humidity of just 14% in the location aptly known as Furnace Creek.Despite the heat and aridity, there was a slow but steady drip of water into the collection vial of Omar Yaghi’s device, an assembly of components loosely resembling a telescope. By the end of the day, this system had collected only a few millilitres of water — barely enough for a refreshing sip. But these results, published in July1, nevertheless represent a landmark in the field of atmospheric water harvesting (AWH).Given the extremity of the testing conditions, the results suggest that the key ingredient in this device — a water-absorbing compound called MOF-303 — has the potential to deliver life-sustaining volumes of clean water to regions that currently struggle to access it. “The vision there is to have something like a village-scale device,” says Yaghi, a chemist at the University of California, Berkeley. “If you’ve got a tonne of MOF-303, you could deliver about 500 litres of water a day, every day for five to six years.”By current estimates, roughly two billion people lack access to clean drinking water. Desalinated seawater can meet some of this need, but the technology required remains costly and is limited to communities with coastal access. This explains the growing enthusiasm for alternative solutions that extract clean water from the air. The US Geological Survey estimates that Earth’s atmosphere contains nearly 13,000 cubic kilometres of water — more than six times the volume of the world’s rivers. “You cannot deplete it — it’s always replenished by natural evaporation from a larger water body,” says Tian Li, a materials scientist at Purdue University in West Lafayette, Indiana. And although many of the most promising AWH technologies are still at the stage of lab demonstrations or proof-of-concept devices, the field is quickly building momentum towards real-world systems that produce plentiful amounts of water at low cost.Searching for suitable sorbentsThere are already several commercially available AWH systems. In mountainous, foggy regions, it is possible to literally cast a net to collect water from ever-shifting cloud masses. Such installations are producing water from the air in South America, India and parts of Africa, according to Thomas Schutzius, a mechanical engineer at the University of California, Berkeley. There are also systems for collecting the water that accumulates overnight as dew. But both fog and dew harvesting are limited to high-humidity areas. And for dew, only modest volumes of water can be produced even under optimal conditions.

Dry (left) super-moisture-absorbent gels swell as they absorb atmospheric water (right).Credit: Guihua Yu, University of Texas at Austin

Systems that condense water from ambient air offer a more generally useful solution. Several companies have already developed electrically powered ‘active’ AWH machines for this purpose. In most cases, these use fans to draw warm, moisture-bearing air into an apparatus that directly cools the air and collects the resulting water condensate; in some cases, this water is also subject to filtration and additional treatment. These systems can produce considerable volumes. The Maximus system from the firm SkyH2O in Irvine, California, for example, can produce more than 10,000 litres of purified water per day. But this system is complex and massive — weighing around 13 tonnes — and requires continuous external power to run. It is also priced at a costly US$395,000. Such systems could be a solution in wealthy water-deprived regions — the southwestern United States, for example, or Saudi Arabia — but they are a non-starter in locations with limited budgets or unreliable electrical infrastructure.The need for more affordable options has spurred interest in ‘passive’ AWH systems that use moisture-hungry sorbent compounds to collect water. The small amounts of power that such systems require could, ideally, be supplied by the Sun. Typically, these sorbents are exposed to the air overnight, when temperatures are cooler and moisture is more abundant. They collect the airborne moisture as liquid in a process known as adsorption. When day breaks, the sorbents are transferred to a device that uses solar energy to drive the release of water. This water is then condensed and collected. These passive systems are tricky, however, because they require sorbents that bind water strongly — but not so strongly that they refuse to yield their bounty without a fight. “That’s an energy penalty that you need to pay,” says Guihua Yu, a materials scientist at the University of Texas at Austin.The field got a big boost in 2017 when Yaghi, along with engineer Evelyn Wang at the Massachusetts Institute of Technology in Cambridge and their colleagues, described a solar-powered system that could extract nearly 3 litres of water per day per kilogram of sorbent — an unprecedented feat at the time2. “I was inspired by that paper,” says Peng Wang, an environmental scientist at Sun Yat-sen University in Guangzhou, China. “This is how I got into this field.”The leap in performance was thanks to the use of a different kind of sorbent — a metal–organic framework or MOF. These porous compounds, developed in Yaghi’s lab, offer a vast surface area for water to bind to, and can be readily chemically modified to further enhance their capacity and water affinity. “It takes up water even at as little as 5% relative humidity,” says Yaghi about his current sorbent of choice, MOF-303. Equally important is that little heat is needed to drive the water back out, with temperatures of 40–45 °C typically proving sufficient. Moreover, Yaghi says, MOFs remain stable throughout years of continuous use.Other promising sorbents are also emerging. Polymers known as hydrogels are a low-cost and highly customizable class of materials that can potentially achieve even greater capacity for moisture capture than MOFs. This is especially true if these gels are loaded with water-absorbing salts such as lithium chloride. Hydrogel-based AWH systems are not yet as efficient as their MOF-based counterparts at capturing and releasing water — particularly under ultra-dry conditions — but they are steadily improving. In September, Yu’s team described a microgel formulation that offers a much larger water-binding surface area than other hydrogel designs, and incorporates a heat-sensitive component to induce water release at lower temperatures3. This allows water to be cleared from the gel in about 20–30 minutes — three to four times faster than previous iterations of his team’s hydrogel-based system, Yu says. This is still about ten times slower than the release from MOF-303, however.Even simpler materials are also being explored. Li and her colleagues have been developing specialized fabrics based on cellulose, a plant-derived fibrous molecule that can absorb water4. In addition to being abundant and inexpensive, says Li, cellulose “has the nanoscale features already there without you doing anything”. Her group is exploring ways to extend the capabilities of cellulose. Impregnating the fabric with lithium salts, for example, has been shown to boost its water-harvesting capacity by more than five-fold relative to the salt-free version5.But cellulose-based systems yield a substantial amount of water only when the relative humidity is at least 60%. By comparison, the MOF-303-based system operates effectively at relative humidity of 20% or less, as shown in the Death Valley field test. And Yu’s microgels could achieve reasonably fast uptake of meaningful volumes of water at 30% relative humidity — although, of course, the water yield will always be lower in such conditions owing to the limited moisture available.Preparing for the harvestA good sorbent is only a starting point. Wang says that most passive AWH systems that have been described so far have the capacity for only one round of water absorption and release every 24 hours. This single-cycle operation can squander the potential output of a material that saturates quickly.

A water harvester containing MOF-303 can collect water from desert air with high efficiency and without power.Credit: Yaghi Laboratory, UC Berkeley

To address this, many researchers are using batch-process systems, which require swapping the sorbent beds between an air-exposed state for water absorption and an enclosed state for Sun-assisted water release. Most of these are active systems that require external sources of electrical power. That’s not necessarily a deal breaker, however — such systems could prove cost-effective. “If you just have a battery that can open a door and close it, you can triple your delivery because now you can do more than one cycle a day,” says Yaghi. In a 2019 study, his group demonstrated a compact device6 that used batteries to power multiple cycles of atmospheric water collection throughout the day. These batteries could be fully recharged by solar power during daylight hours, allowing the system to function off grid.Cost is a crucial consideration, especially given that passive AWH will — at least initially — be targeted at resource-limited populations. Fortunately, many of the sorbents now under development should be affordable. Yaghi says that MOF manufacture is already being done at an industrial scale, and that the cost is largely determined by the metal involved. For MOF-303, that means aluminium, which he says costs just $1–2 per kilogram. Some hydrogel polymers can be expensive to produce, but others can be made more cheaply. Yu’s team is even exploring whether hydrogel ingredients can be directly extracted from biomass. The opportunity for low-cost production from easily accessible materials is a key asset of Li’s cellulose fabrics. Her group is working on deploying its system in coastal communities in Senegal where fresh water is scarce. “The burden of getting fresh drinkable water there falls onto the teenage girls,” she says. “We’re trying to educate the girls, and developed a curriculum so that they can build a set-up themselves with locally available cellulose sources.”Li’s system simply requires a textile drape that can be wrung out by hand. Other sorbent-based systems depend on more sophisticated apparatus for the harvesting process — but even those do not need to be expensive. For example, Wang recalls a prototype hydrogel-sorbent-based device that he developed about five years ago7. Apart from the sorbent itself, Wang says, all the materials for the system were purchased from a local supermarket. For just $3.20, Wang and his colleagues estimated that they could construct a device that would supply roughly 3 litres — the minimum amount of water needed daily by a typical adult.Of course, there is also the issue of ensuring that the water pulled from the air is free of dangerous substances. Yaghi says that his experiences in field testing in US deserts have been reassuring. “We tested the water for metal and organics, and it was like the purest water you could find,” he says. But this is not a certainty in every environment, particularly near sources of industrial pollution. Careful assessments will be needed to ensure that collected water is separated efficiently from contaminants.Pollution has been a particular concern when harvesting fog, Schutzius says. In August, his group described a fog-harvesting net enhanced with a titanium dioxide coating, which efficiently breaks down organic pollutants such as diesel after being activated by ultraviolet light from the Sun8. He thinks that researchers should take similar considerations into account for other domains of AWH. “The whole point of adsorption is you can concentrate a lot of stuff that’s otherwise dilute,” he says.Opening the tapSome passive AWH systems are already moving into commercial development. Yaghi’s lab, for example, has spun off a start-up firm in Irvine, California, called Atoco, which aims to roll out first-generation MOF-based harvesters in the next year or so. Different water-harvesting technologies will find different applications. The robust performance of MOFs in extremely arid conditions will make them a versatile choice, whereas systems based on cellulose or hydrogels might be restricted to more humid environments.

A fog-collector park (left) in the mountains of Morocco traps water vapour on nets (right).Credit: aqualonis.com

These technologies are unlikely to fully replace existing systems such as seawater desalination, which has a proven track record of high-volume water production. But AWH could greatly reduce dependency on centralized water processing, making it accessible at the village or even single-household scale. Yaghi sees a future in which any house with electricity could reliably address its drinking-water needs with an appliance roughly the size of a microwave oven.And there are abundant opportunities beyond simply producing drinking water. For example, Wang’s group has described a harvesting system that piggybacks on existing photovoltaic solar panels, using the waste heat and energy from these panels to power water production9; the resulting water helps to cool the panels and therefore improves their efficiency. Similar approaches have been described for managing — and exploiting — waste heat in industrial settings. AWH also has agricultural applications; Yu’s group, for example, is working on using hydrogel-based materials to produce self-watering soils that directly draw moisture from the air10.It is indisputable that, as the ongoing climate catastrophe worsens, society will need to leverage every solution at its disposal to meet the planet’s water needs. “I worked in Saudi Arabia, and people there say water security is national security — that’s 100% true,” says Wang. “It’s getting more serious, and we need to do things more effectively.” More

Floods are becoming more intense and frequent around the world.Credit: Thianchai Sitthikongsak/Getty Images

In the past six months alone, floods have hit countries including Italy, Libya and the United States. Beyond the immediate tragedy of lost lives and livelihoods, floods also have longer-term effects, such as causing damage to urban infrastructure and agriculture. In this webcast, a panel of researchers discuss how to improve resilience to floods and the need for expertise from fields such as climate modelling, urban studies and behavioural science. More

Many people had to be evacuated following flooding caused by the destruction of the Kakhovka dam in Ukraine.Credit: ZUMA Press, Inc./Alamy Stock Photo

Oleksandra Shumilova was hundreds of kilometres away from her native Ukraine when, in early 2022, Russian troops invaded and destroyed a water pipeline near her hometown of Mykolaiv. For 24 days, the taps ran dry. When the water finally returned it was contaminated with salt and harmful chemical deposits that rendered it unsafe for drinking.The incident struck a chord with Shumilova, a freshwater ecologist at the Leibniz Institute of Freshwater Ecology and Inland Fisheries in Berlin — on multiple levels. Not only was Mykolaiv her hometown, but she studies the floodplains of a river in Italy that still bear the scars of the First World War.Pockmarked by craters from decades-old aerial bombardments, the area around the Italian river is scattered with unexploded military ordnance. When discovered, these munitions must undergo controlled detonations, creating blasts that reverberate in Shumilova’s mind during her fieldwork.“Remnants of war can stay in nature for a long time,” Shumilova says. Now, she fears, history is repeating itself in the Russia–Ukraine war.Shumilova took it upon herself to chronicle what is happening in Ukraine. “This research is very personal,” she says. She began poring over government records and media reports, meticulously searching for instances of damage to water resources and infrastructure, resulting from military actions. She and her collaborators cross-checked their findings with information from a diverse array of Ukrainian, Russian and international sources.

Parts of Kherson, Ukraine, were flooded after the Kakhovka dam was destroyed during the Russia–Ukraine war.Credit: Roman Pilipey/Getty Images

Focusing on just the first three months of the conflict, a detailed record of 64 events emerged, each affecting crucial water facilities and often triggering a chain of events1. Missiles striking hydropower dams, for example, might result in power outages that render water-pumping stations and wastewater treatment facilities inoperable — with devastating consequences. Millions of people can be left without access to clean water, agricultural fields might have insufficient irrigation, and the unchecked flow of untreated sewage and contaminated ground water from industrial mines can pollute nearby river basins, causing significant harm to both people and the environment.These incidents highlight just one aspect of the intricate interplay between water and armed conflicts. Water resources can be casualties of violence, but disputes over water control can also act as triggers for unrest — for example, when two communities clash over access to a single water source. Furthermore, water is frequently weaponized, as has happened in Gaza, when Israel responded to the deadly attacks by Hamas on 7 October by restricting access to fresh water and cutting off fuel shipments needed to run desalination and water-treatment plants for local production of potable water.Historical records show that conflicts over water access stretch back millennia, with water systems often being targeted or manipulated as strategic assets on the battlefield. But this water–war nexus seems to be intensifying. Driven by escalating tensions and intensified hostilities in places such as the Middle East, Ukraine, southern Asia and the Sahel region of sub-Saharan Africa, water resources have increasingly become targets or triggers of violence over the past decade, notes environmental scientist Morgan Shimabuku.“What we do see right now is a really large increase in the total number of water-related conflicts around the globe,” says Shimabuku who is at the non-profit organization the Pacific Institute in Oakland, California, and helped to develop an online database of water-related clashes called the Water Conflict Chronology. There were more than 200 incidents in 2022 alone, and with the ongoing conflicts in Ukraine and Gaza, 2023 is projected to surpass that figure (see ‘Warring over water’).

Source: Adapted from O. Shumilova et al. Nature Sustain. 6, 578–586 (2023)

Add in the mounting stresses on water systems from population growth and climate change, and the potential for violence and instability is only expected to grow. Shimabuku thinks that there is an urgent need for comprehensive and cooperative efforts to safeguard water resources and to promote peace, but she also sees reasons to be hopeful.For example, many previously conflicting factions are embracing collaborative water-sharing treaties, which can de-escalate hostilities. “We have a lot more tools in our toolbox now for addressing these challenges,” Shimabuku points out.Troubled watersIn 1995, the then-vice-president of the World Bank, Ismail Serageldin, made a dire warning for the years ahead: “Many of the wars in this century were about oil, but wars of the next century will be over water.”

People queue to refill drinking water in Mykolaiv, Ukraine, after Russian forces cut off the pipeline used to supply water.Credit: AP Photo/Emilio Morenatti/Alamy Stock Photo

It’s an oft-repeated idea that persists to this day, cited frequently in discussions about water scarcity and conflict. But according to Jampel Dell’Angelo, who studies governance and disputes over freshwater resources at the Vrije University Amsterdam Institute for Environmental Studies in the Netherlands, “this is a myth”. At least when thinking about conflicts between nations.In a globalized economy of countries interconnected by trade, including that of agricultural commodities that can be considered a form of virtual water transfer, tensions over cross-border issues of water scarcity are often diffused by forms of interstate cooperation. This typically prevents water from serving as a trigger for major wars, but it doesn’t stop localized violence from arising in water-strapped nations.In Iraq, for example, large-scale protests over tainted water supplies have been met with strong police crackdowns. Frequent droughts in Somalia have prompted herders to abandon their pastoral lifestyles and join militant groups instead. And neighbouring states in India clash every year over their allocated river waters, with demonstrations that often turn violent.What’s more, the very conditions that promote interstate water peace can actually exacerbate water-related instability at the local level. This is particularly evident in water-scarce low-income countries, which frequently export their limited natural resources, amplifying their local water challenges and potentially fuelling smaller-scale conflicts. “There’s a shift in the burden of water scarcity on the heads of people who already suffer from the lack of water,” Dell’Angelo says.Further compounding the problem, foreign agribusiness investors have increasingly acquired vast land holdings in low- and middle-income countries, often at the expense of local communities and appropriating valuable water resources in the process. Termed land or water grabbing, this phenomenon gained momentum after the economic and food crises of 2008 — and, according to Dell’Angelo, the ongoing war in Ukraine is likely to further intensify this trend, owing to reductions in Ukrainian grain exports and heightened concerns about food security. “This will put additional pressure on land and water,” Dell’Angelo says, “with escalating consequences.”One priority of water-security researchers is to prevent similar crises in the future. “We want to put points on a map and show people where water conflict is happening so that we can better apply solutions,” says Samantha Kuzma, a data scientist at the World Resources Institute, an environmental policy and research group in Washington DC.Those efforts begin with prediction. To identify problem hotspots, Kuzma and her colleagues developed an artificial intelligence algorithm called the Global Early Warning Tool. This tool considers a wide range of environmental, economic and social factors. By comparing past trends to present data, it flags regions where water-related issues such as flooding, pollution and scarcity could spell trouble2.Kuzma hopes that the tool will help to inspire localized conflict-mitigation measures when and where they are needed. This could prove instrumental in drought-affected areas such as Ethiopia, where restricted access to potable water might escalate tensions between government forces and armed factions, or in water-scarce regions of Syria that are already marred by conflicts between rival militia groups.Although implementation can be challenging, Kuzma emphasizes that the timely identification of emerging conflicts can streamline the adoption of collective land-use strategies that will ultimately ease tensions. “If we can see water as the security risk that it is — and have more investments going into securing the resource, and managing it sustainably — we should see real impacts on the ground,” she says.Shifting baselinesThe world’s rapidly changing climate could intensify the need for those kinds of water-security intervention. As a team led by Solomon Hsiang, now chief environmental economist at the White House Office of Science and Technology Policy in Washington DC, showed in an influential 2013 analysis, extreme rainfall conditions can be causally linked to an increased risk of violence and civil war3.

Displaced people in Gaza fetch drinking water outside a school.Credit: MAHMUD HAMS/AFP via Getty Images

Such was the case in Syria, where discontent over the government’s handling of a prolonged drought period contributed to uprisings in 2011. This spiralled into a devastating internal war that persists to this day.Yet, perhaps counter-intuitively, as climate extremes become less anomalous and people become accustomed to more frequent water disasters, it’s possible that the number of water-related conflicts will decline, although the severity of such clashes could intensify.That’s what Marc Müller, a water engineer at the Swiss Federal Institute for Aquatic Science and Technology in Dübendorf, and his colleagues found when they modelled the relationship between climate variability and conflict. Because the reference point for ‘normal’ levels of water availability changes, they concluded, so will the threshold at which people are willing to fight4.Water-related conflicts could therefore become fewer and further between. But when they do arise over ever-shrinking water supplies, Müller warns, the hostilities are likely to escalate quickly. “Conflicts will be less often, but worse,” he says.Such predictions offer little consolation to the people of Gaza, who continue to endure shortages of clean water and a sanitation crisis that is facilitating the spread of waterborne diseases.Water challenges are nothing new in the region. Even before the current conflict, extreme overuse of groundwater resources, a lack of large-scale desalination plants and the discharge of mostly untreated sewage all contributed to water scarcities in Gaza. But the situation is now much worse — and with the world’s attention focused on the water crisis, some researchers are hopeful that more will be done to address the problem once the war ends.“There’s going to be a lot of rebuilding necessary after all the destruction,” says David Katz, a water-policy researcher at the University of Haifa, Israel, “and maybe investing in water infrastructure will be something that can galvanize the global community.”As Katz points out, water cooperation helped to pave the way for a peace agreement between Israel and Jordan nearly 30 years ago, and he’s hopeful that the same might be possible in any future détente between the Israeli and Palestinian governments. “Political change opens up new opportunities,” he says. “It could conceivably happen.”Meanwhile, in Ukraine, the dire impact of Russia’s assaults on the nation’s water infrastructure is consistently and relentlessly worsening. This is starkly evident in the catastrophic destruction of the Kakhovka dam and hydroelectric power plant on 6 June 2023. The massive concrete structure crumbled. Trillions of litres of water gushed out, deluging areas downstream. At least 50 people died and thousands lost their homes.In the intervening months, the extent of the damage to communities, agriculture and the environment has become apparent. The Black Sea, into which the flood waters and accompanying sediment poured, became polluted, with a potentially devastating impact on the local marine ecosystem. Vast tracts of flooded land are no longer amenable to farming, crippling the Ukrainian economy, and areas that remain arable will probably have reduced yields without the reservoir there to provide a steady stream of water for irrigation.This could spell trouble for global food supplies, given the dependence by many low-income countries on Ukrainian grain. “It will touch millions of people, not only those living in Ukraine,” says Viktor Vyshnevskyi, a hydrologist at the National Aviation University in Kyiv, who has studied the myriad water-related repercussions of the dam explosion5.This human-caused flood was not without precedent. During the Second World War, in August 1941, the retreating Soviet army destroyed a dam at the northern end of the same reservoir where the Kakhovka dam was later constructed. The goal was to slow the advance of German forces. But the obliteration of the dam resulted in massive downstream flooding that reportedly claimed the lives of thousands of people. The occupying German forces partially restored the hydrotechnical structure, before they too demolished it as the tide of the war shifted back in favour of the Soviets.With history as a sobering backdrop — in Ukraine and beyond — the persistent use of water as both a casualty and a weapon of war remains a looming threat in a world grappling with the chaos of climate change. However, by shedding light on this issue, water scientists aspire to catalyse efforts that can avert future conflicts and ensure the protection of this invaluable resource for generations to come.“We want to provide a resource for other researchers, policy makers and those who can enact solutions around how to prevent violent conflict related to water,” says Shimabuku.“Could there possibly be water-resource management approaches or governance structures that can be put in place to reduce the potential harm that these conflicts drive?” she asks. In a world in which water is both a source of life and strife, the choice between conflict and cooperation will determine our shared future. More

Old Delhi’s Red Fort, a nearly 400-year-old structure where India’s flag is hoisted every Independence Day, is separated from the Yamuna River by a 55-kilometre ring road that circles the capital. I drive past it on this road, built on an old floodplain, on my way to work at Ashoka University in the neighbouring state of Haryana, a further 30 kilometres or so upstream. The river is hidden from view in places but it can be seen farther along, narrow and darkened by pollution for most of the year.In July 2023, flood water from heavy monsoon rains entered the Yamuna from Haryana, flooding parts of Delhi. TV images of the river lapping at the walls of the Red Fort recalled nineteenth-century paintings of the old course of the river.
Nature Spotlight: India
The Yamuna forms from the melting glaciers of the lower Himalayas, running for more than 1,370 kilometres until it merges with the Ganga in the city of Prayagraj in Uttar Pradesh. Delhi lies along its banks for just 20 kilometres, less than 2% of the river’s total length. But in that short stretch, the city belches out around 80% of the pollution found in the Yamuna. In some seasons, a foamy mixture of sewage and industrial waste coats the river surface in parts of Delhi. Newspapers carry pictures of Hindu devotees offering morning prayers while standing knee-deep in this toxic foam.Much of Haryana’s ground water is used to cultivate rice. Farmers pump it up using electricity that the state government heavily subsidizes to encourage agriculture. Without economic incentives to use this electricity sustainably, groundwater levels declined precipitously from the 1990s into the early 2000s. To combat this, legislation was introduced in 2009 to restrict the sowing of rice to mid-June onwards, timed to begin after the start of the monsoon season. Previously, rice planting had started as early as May.This means that the rice-harvesting season ends even closer to the wheat planting season, which begins in November. Farmers need to get rid of their rice-crop residue as soon after harvesting as possible to clear space for the wheat.Burning the crop residue is the cheapest solution. But in the early winter months, smoke and dust released from a combination of residue burning and other sources is concentrated by frequent changes in temperature and stagnant winds, and shrouds northern India. Schools are closed, construction work is halted and flights are grounded because of the poor visibility. Hospitals fill with people complaining of breathing difficulties. Each resident of Delhi is thought to lose nearly 12 years of their life to air pollution.As the world’s climate changes, extreme events such as the rains that led to the Delhi floods in July will become more common. The southern Indian state of Kerala witnessed a devastating flood in 2018, the worst in almost a century of recorded history. More than one million people were evacuated to higher ground. A study that year showed that about 60% of the coast of Kerala was eroding, and areas where good fishing could be found were shifting, affecting the lives of one million fishers and their families.Two large Indian cities, Chennai, on the Bay of Bengal, and Kolkata, on the banks of the Hooghly River, are expected to be at significant risk from sea-level rise in the next few decades. Along with the resulting intrusion of salt water into groundwater systems, this will increase climate-change-induced migration. The Sundarbans, an ecologically sensitive wetland system on the Bay of Bengal that contains the largest mangrove forest in the world, has already seen substantial climate-change-driven migration into the nearby city of Kolkata.The Namami Gange Programme is one of many started since the mid-1980s to clean up India’s iconic holy river, the Ganga. Around US$3 billion for this has been set aside or spent since 2014, largely on new sewage-treatment plants.But pollution levels remain stubbornly high, the result of lax enforcement of control measures and unregulated river-front development. More data are needed, and more transparency on the causes, so citizens can hold civic bodies and elected representatives responsible.Science can help too. The Ministry of Earth Sciences is funding several initiatives aimed at improving India’s ability to model the monsoon. This will help provide actionable advance warnings of extreme rain events. More research on the geomorphology of coastal India, on near-shore ocean current patterns and on the impact of sea walls could aid the design of interventions to slow coastal erosion.Fresh water is a finite and vulnerable resource that should be managed in a participatory fashion. Citizen groups in the south-Indian city of Bengaluru are helping to renew urban water bodies, reconfiguring them as centres around which communities can coalesce. In arid areas, such as the deserts of Rajasthan and Gujarat, old methods of rainwater harvesting are being revived, helped by traditional community knowledge.Guaranteeing a minimum amount of fresh water for each individual, free of charge, will ensure broader societal equity. Beyond that limit, water usage should be priced, creating an economic incentive for its sustainable use. Such a policy was introduced nearly a decade ago by the Delhi government, but continued groundwater extraction using illegal borewells has partly neutralized this positive step.Ensuring the sustainable use of water intersects climate change, agriculture, politics, pollution, migration and much more. Well-intentioned policy measures, such as subsidizing electricity for agriculture, can have unexpected consequences for water and its sustainable use.Placing water at the centre of our thinking about sustainability can help avoid such pitfalls. We need more conversations about water. More

Groundwater pumped on to Dennis Hutson’s farm in Allensworth, California, had dangerously high levels of arsenic. New technology tested on the farm reduced the levels to drinking-water standard.Credit: Adam Lau/UC Berkeley

In 2022, Ashok Gadgil conducted the first field trial of a water-treatment system for the 600 or so residents of Allensworth, California, who have been battling arsenic contamination for some time. The system is a more efficient iteration of technology that Gadgil and his team installed in India in 2016 to provide rural and marginalized communities with access to safe drinking water at low cost1. Like many small rural communities, Allensworth — a historically Black town with a majority Latinx population today — has no access to high-quality surface-water treatment facilities that are common in urban areas. Instead, these communities often use wells, which are at high risk of contamination with arsenic and other toxic substances.It’s an urgent issue in California: roughly 300,000 of the state’s 39 million residents drink from wells in the state’s rural areas and are therefore exposed to high levels of arsenic in the ground water. “Safe drinking water, which is a big chunk of our work, is all related to rural areas,” says Gadgil, who worked as an environmental engineer for 35 years at the state’s Lawrence Berkeley National Laboratory, and in July assumed an emeritus role at the University of California (UC), Berkeley. His team plans to return to Allensworth next year to continue testing the system, which makes use of technology, developed by a team that included Gadgil’s colleague Siva Bandaru, that uses hydrogen peroxide to rapidly oxidize iron so that it binds to arsenic and removes it from water.California, like many US states, has a mounting number of rural concerns, including not just safe water supplies, but also wildfire preparedness, air quality and equitable access to health care. Researchers in the UC system have key roles in monitoring and addressing such concerns. In addition to the ten academic campuses spread across the state, the UC system has a network of Agriculture and Natural Resources extension centres that serve as bridges to rural areas. Roughly 270 researchers are employed by these centres to support all 58 state counties through collaboration with other UC teams, as well as working with other public and private organizations. And even though funds for these centres have been waning over the past two decades, they are currently allocated around US$100 million of the UC system’s US$47-billion budget for 2022–23 to rebuild their dwindling workforces.
Millions of jobs in food production are disappearing — a change in mindset would help to keep them
It’s a welcome boost for researchers who face unique challenges in rural settings, where the remoteness of the communities can be a barrier to participation and engagement and cultural differences and concerns about confidentiality can cause some residents to be wary of academics. The most successful collaborations between urban-based researchers and rural communities have been based on a commitment to building trust, bridging political divides and delivering meaningful results, say researchers with experience in these settings.A growing number of scientists are pursuing community-based research partnerships2. A common motivation, says Gadgil, is a sense of urgency in addressing environmental-justice issues, especially in historically excluded communities. Researchers at public universities, such as those in the UC system, often have a sense of responsibility to give back to the citizens that fund their mission, he says, adding that some of the most satisfying projects use innovative technologies to find solutions to these communities’ problems. “Scientists are always looking for new frontiers,” says Gadgil.The regional contribution of the UC system is also apparent from Nature Index data on science cities (see ‘Deep connections’ and ‘Slipping back’).Centring community voicesAnn Cheney, a medical anthropologist at UC Riverside, conducts health research in partnership with communities living near the Salton Sea, California’s largest saline lake, roughly 50 kilometres north of the Mexican border. Over the past few decades, the lake has been shrinking owing to drought and reduced run-off from agriculture, which has increasingly exposed the lake bed. This has become a growing source of dust in a region that already has higher than average rates of childhood asthma compared with the rest of the state3.In 2017, Cheney and her colleagues worked with researchers at the US Border Health Commission to identify the health needs of Mexican immigrants in rural southern California. Childhood health was at the top of the list. Since then, she has partnered with local groups to further analyse health impacts, particularly in undocumented immigrants. A major challenge, she says, is that many of these communities have grown resentful of urban academics who — backed by hefty grants — swoop in to collect their data but then fail to return with solutions. “My fear, and what I’ve already seen happen, is the continued exploitation of these incredibly vulnerable communities,” Cheney says.

The largest minority population in California’s rural areas is Latinx people. Cheney, who has become fluent in Spanish so that she can engage with community members more effectively, says that the small number of Latinx staff at academic institutions in California, relative to the general population, makes it difficult to involve local voices in research. It can be time-consuming to partner with community members in identifying research needs, incorporating local knowledge and implementing research findings, and it comes at a cost, says Cheney, given that academic career advancement is typically based on publications.William Porter, who works with Cheney at UC Riverside, says that the Salton Sea’s growing dust problem was too much for him to ignore. An atmospheric scientist, Porter had originally arrived at UC Riverside in 2017 to focus, in part, on wildfire and coastal air quality. But he soon felt an urgent pull. “It’s right here in my backyard,” he says.Over the past few years, Porter has tailored research programmes to track dust storms and their acute health effects. He’s also launched a project to help the region’s residents to build their own indoor air purifiers using box fans and air filters. Scientists are uniquely positioned to address the consequences of policy indecision when it comes to complex environmental crises, he says, and he sees the Salton Sea’s environmental decline as a pressing opportunity to build research capacity around climate-related problems that are becoming increasingly common worldwide. What’s clear, says Porter, is that the days of the ‘ivory tower’ model of research dissemination are over; academics can no longer just publish a paper and assume the results will be delivered to communities. “That is a broken model,” he says. “The way you conduct community research nowadays is as part of a team with communities.”As California’s rural areas experience wildfires, flash floods, heatwaves and drought, researchers are asking what they can do to help, says Diana Moanga, who manages the Spatial Analysis Center at Stanford University in California. Throughout her career in California, Moanga has interviewed ranchers affected by dwindling water supplies, helped to create a risk index of mobile homes that are vulnerable to wildfires and assessed the potential for solar projects on tribal lands. Notably, she says, funders want to ensure that research results reach communities. Federal agencies such as the National Oceanic and Atmospheric Administration and the US National Science Foundation will not fund projects that do not set out clear community impacts — as well as a clear plan to disseminate the results back to community members.
Water crisis: how local technologies can help solve a global problem
Gadgil agrees that rural-community projects must avoid top-down approaches to be successful. In fact, his team always asks community members how else they would like to use scientists’ skills. Most often, the researchers provide teaching modules on making drinking water safe. They have even helped rural students with their science-fair projects. In his experience, if locals aren’t engaged, “as soon as you leave, the intervention will die”. It’s important to set appropriate expectations when embarking on a project, he says, by explaining how the science works, and emphasizing that a few failures might occur before the problem is solved.Better yet, says Susana Matias, a researcher who sits at the nexus between UC Berkeley, the Agriculture and Natural Resources network and community members, is sharing the leadership and decision-making. When analysing data from the state’s largest farmworker population health surveys4, for instance, Matias found increased risk of chronic health conditions, notably obesity, compared with elsewhere in the country.For example, in one project, Matias and her team partnered with a berry grower, who allowed them to run a health-promotion programme focusing on diet and exercise with farmworkers in their fields in California5. Although the programme did not measurably improve health outcomes, the researchers think it was highly valuable in engaging the farmworker community. “Roughly half of these populations are potentially undocumented immigrants, so it is key to have community-based partners to help build trust and access,” says Matias, who emphasizes that placing value on community members’ time and expertise is an important way to build mutual respect and workforce capacity.The value of rural dataInvolvement by rural residents is particularly important in medical research. People living in rural areas are more likely to die from heart disease, cancer, chronic respiratory disease and stroke than are urban dwellers, according to the US Centers for Disease Control and Prevention, but securing their participation in trials and surveys can be major challenge. Participants typically have to contribute their time during business hours, which might not be feasible for those in full-time employment, especially when they live far from urban centres. “Rural participants are often excluded from clinical research,” says epidemiologist Jennifer Radin.

By encouraging participants to engage with studies remotely, Radin and her colleagues at Scripps Research in La Jolla, California, and Johns Hopkins University School of Medicine in Baltimore, have been able to gather high-quality rural-health data. From 2017 to 2019, for example, they ran POWERMOM, a nationwide pregnancy health study in which participants could share data on blood pressure, weight-gain and other factors throughout their pregnancies, through a smartphone app6. Of the 3,612 participants who were initially recruited, 16% had rural addresses — a high percentage compared with previous studies, says Radin. “Doing a completely remote clinical trial allowed us to include everyone,” she says.In 2020, Radin launched another study, called DETECT, which used wearable devices to predict outbreaks of COVID-19 in the United States. The team also conducted outreach in far-flung locations, such as rural Alaska and Guam, to ensure that diverse populations were represented. The initial results, published last year, showed that symptom and sensor data collected by smartwatches and activity trackers could improve surveillance of COVID-197. “These digital tools can be rolled out anywhere, at any time — not necessarily only when it’s convenient for the researchers, but when it’s convenient for the participant to share their data and experience,” says Radin.Perhaps the greatest health impact, however, comes from the UC Programs in Medical Education (PRIME), which aim to address California’s looming physician shortage, especially in rural areas, where only 9% of the state’s doctors practise. The programme has resulted in more than 750 medical-school graduates since 2004, and roughly 43% of those graduated from programmes based in or serving rural areas.It’s projects such as this, which centre societal needs and priorities, that Gadgil hopes will attract greater funding and resources in future, alongside curiosity-driven science. “Research that benefits the communities around us is receiving much more focused attention,” he says. More