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.