In an ancient wood in Hampshire, a county in southern England, construction workers are felling trees and clearing stumps. Over the workers’ shoulders, ecologists check to make sure that no bats or bird’s nests are being disturbed. They are building a road that will eventually lead to 160 hectares of grassland where Portsmouth Water, the utility company that manages the water supply here, is going to build a reservoir.

The reservoir will sit in a clay valley, and so its water will naturally be sealed from the surrounding woodland. Portsmouth Water expects to fill it using nearby springs by 2029. If all goes to plan, the reservoir will then supply up to 21 million liters of water a day to around 160,000 people in the southeast of England.

That may sound like a lot, but 160,000 people is not very many in the grand scheme of things—especially on an island that, like many parts of the world this year, has been experiencing water shortages. The United Kingdom was hit by extreme heat this summer and has been contending with its worst drought in almost 50 years. Farmers have been banned from drawing river water, and residents from using hoses to water their gardens, wash their cars, or fill up pools. With more heatwaves and droughts probable in the future, it’s a sign that the UK is going to need greater supplies of water. And yet, this planned reservoir will be the first to be built in southern England since the 1970s. Constructing new ones might seem a straightforward solution at a time when more water is needed—but the reality is more complicated.

It’s not that water companies in the UK haven’t had other projects in the works. But it takes around 10 years from the decision to build a new reservoir to being able to use the water. When it was initially planned in the late 1960s, Kielder Water reservoir in the northeast of England was designed to provide water for the steel and chemical industries in the area. By its inauguration though, in 1982, so much time had passed that these industries had shut down. Branded a white elephant when it opened, today thousands of tourists flock to Northumberland each year to see the UK’s largest artificial lake.

And construction can cost hundreds of millions: Portsmouth Water’s new reservoir, despite its small size, will cost over £120 million ($140 million) to build. Two new reservoirs being built by Anglian Water in the east of England are projected to cost £3.3 billion ($3.79 billion) in total, and won’t actually supply water until 2035 in a best-case scenario.

“There’s a reluctance in companies, or even the Environment Agency, to authorize construction of a new reservoir unless it is really proven,” says Chris Binnie, an independent consultant who advises government agencies and companies on water resource development in the UK.

Another reason why no reservoirs have been built recently, according to Binnie, is that water use has become more efficient in recent decades due to the privatization of the sector. Since the widespread introduction of water meters in households, consumption has gone down considerably. Some British water companies even sold off reservoirs to land developers because they were no longer able to use water from them.

Yet this overlooks the fact that water reservoirs serve multiple purposes. In addition to supplying water to households and factories, reservoirs can also be used to channel water through turbines in a dam to generate clean electricity. They also drain water collected during the rainy season onto dry agricultural land. “With temperature warming, farmers will need to irrigate much more in the future than they have in the past,” says Binnie. “So I can see quite a lot more reservoirs being built for agricultural purposes, storing irrigation water.” The National Infrastructure Commission, which advises the UK government, said back in 2018 that England alone would need an extra 4 billion liters of water by 2050 to avoid extreme droughts—a third of which should come from new reservoirs and transfer pipes.

There’s just one major problem: space. Regions like the southeast of England need more water than Scotland, for example, but there is little suitable land on which large reserves could be stored. The rest of Europe is also densely populated and heavily industrialized. “This disfavors reservoirs, simply because of the competition for space,” says Wouter Buytaert, professor in hydrology and water resources at Imperial College London. For hydropower reservoirs the demands are different—most dams are built in hilly, upstream parts of river basins, so that captured water flows with great force to a lower-lying power station—but they still need space too.

Buytaert sees more potential for capturing water in new reservoirs in places like the Andes in South America or the Rocky Mountains in North America, but their water cannot simply be imported to Europe. “Water as a substance is very heavy, very difficult to transport”, he says. “So for water supply, you’re limited to a certain geographical area, typically a few 100 kilometers from where the demand is.”

China, though, has an ambitious technical solution that would defy this wisdom. To bring water from its typically wetter south to its drought-stricken north, it is building a $8.9 billion underwater tunnel to ferry water from the Three Gorges Dam—the world’s largest power plant—into the Han River before it flows north to Beijing through a 1,400-kilometer open channel.

Even where there is space to create new reservoirs, their construction doesn’t always come without controversy. With the global boom in hydropower, more and more dams are being built on land that was set aside for protection. In 2020, researchers overlaid hydropower dams that are either under construction or planned for the next two decades onto maps of protected areas, and found that 509 dams around the world—14 percent of all those in the works—would be built in protected areas. And the impacts of dams and reservoirs go beyond the space needed to build them. They impede the movement of fish and other wildlife, and prevent rivers from flowing and transporting nutrient-rich sediments to downstream habitats.

Yet when comparing the global maps, the researchers also noted that two-thirds of the large dams already in existence were established before the areas they’re located in became protected, suggesting that the dams played a role in determining areas worth protecting. The reservoirs created by dams also often become tourism attractions, the authors write.

And despite seeming pretty innocuous, dams and reservoirs are also not particularly climate friendly: Sediments brought in by rivers and streams accumulate in reservoirs, where microbes decompose organic matter and produce greenhouse gases in the process. CO2 that bubbles out of a reservoir because of carbon being deposited there would probably be emitted as CO2 somewhere else farther downstream if the reservoir didn’t exist, explains Tonya DelSontro, an aquatic biochemist from the University of Waterloo in Canada. “Methane, however, is a different story,” she says. It’s 25 times more effective than CO2 at trapping heat in the atmosphere, and is produced in sediments and waters without oxygen, which occurs on the bottom of reservoirs, especially in warm seasons. “These are ideal conditions for forming methane, and hence reservoirs are often methane hotspots,” says DelSontro.

Due to a lack of data and inconsistent measurement methods, it’s been difficult to determine exactly what the CO2, methane, and other greenhouse gas emissions are from reservoirs, but a study involving DelSontro estimated a yearly release of 0.8 gigatons of CO2 equivalent. That’s more than what Germany, Canada, or Saudi Arabia each produce annually. More recent research suggests emissions from reservoirs could be even higher.

Buytaert says that there are nature-based solutions that, while not directly eliminating the need for reservoirs, could be used to keep more water in the landscape, and so lessen the impact of drought and reduce the volume of water storage that would otherwise be required. He gives the example of wetlands, which naturally store water and also purify it by removing sediments, excess nutrients, and chemicals that run off land. They can also be home to animals and plants at risk of extinction. Such natural solutions are “probably the most intelligent way to go about water management,” he says.

Back in Hampshire, Portsmouth Water has recognized this. It plans to create an area of wetland along the northern edge of the reservoir, along with facilities for bird watching. Like so many other reservoirs around the world, it’s hoped that it won’t be just a source of water, but one of tourism too.