With half the world’s populations now living in urban areas and that number projected to swell to around 6.5 billion by mid-century, shoring up city water supplies represents a significant challenge. Failure to do so could result in widespread shortages.

Delhi, Cape Town and Mexico City have already experienced the implication of dwindling supplies and responded, at least in part, by rationing drinking water. But as our climate continues to change, longer-term management strategies will come into play. These include measures such as saving water, adapting existing infrastructure and making better use of natural sources. How effective can different solutions be?

Encouraging private households to save water

Private households consume a lot of water — with 40% earmarked for showering, 30% for flushing the toilet and 13% for washing clothes. Cooking and drinking, by comparison, account for around just four percent of daily water needs. The average American uses roughly 300 liters a day, and in Germany 125 liters.

New technologies such as efficient washing machines or water-saving toilet cisterns and shower heads, often require under half as much water as conventional ones. Raising awareness of such things can encourage people to consider their own useage.

Cape Town, which has been struggling with water scarcity for years, is focusing on both education and modernization.

The city authorities have introduced a scheme that provides low-income households with free repairs of sanitary facilities, and they have also increased the price of water to encourage people to use less. In 2018, when Cape Town faced a severe drought, the city was able to reduce water consumption to an average of just 50 liters per person.

The Canadian city of Vancouver is also taking financial measures. In dry summer months, water from the city's supplier costs 25% more than in rainy winter months.

Modernizing pipes and infrastructure

Leaky pipes and outdated supply networks can lead to substantial water loss. Across Europe, more than 25% of drinking water is sacrificed to such infrastructure weaknesses, while in some cities around the world, up to 60% of available supplies never reach the tap. Repairing pipes can significantly reduce unnecessary loss.

Until a few decades ago, the densely populated city of Tokyo lost between 15 and  20 % of water to leaky pipes. With more than 27,000 kilometers of pipes constantly being checked and if necessary, replaced, that figure is now at less than three percent.

Planning efficiently when building new pipelines can save a lot of water. Yet many cities have inadequate existing water infrastructure, particularly in informal settlements and rapidly growing slums. Richer districts usually have better supply systems.

Collecting rain and reusing wastewater

When it comes to flushing toilets, running washing machines and feeding industry, there is no need to rely on precious drinking water. So-called gray water, which refers to harvested rain or treated wastewater, offers a sustainable alternative.

In new urban districts in Melbourne (Australia) and Aarhus (Denmark), for example, rainwater is diverted from streets and sidewalks, filtered, and then used in nearby buildings. Collecting rainwater is now mandatory for new buildings in many cities in the US, India, Taiwan, Spain and Turkey.

Other cities, meanwhile, are beginning to reuse wastewater rather than discharging it into rivers. In Beijing, recycled wastewater accounts for a third of total supplies, and is mainly used to clean streets, wash cars, flush toilets and stop plants from going thirsty.

Likewise in Madrid, treated wastewater is used to irrigate city parks, while in Singapore, an additional purification step renders it drinkable.

Soaking up water in a sponge city

While water is scarce in dry periods, a simultaneous increase in heavy rainfall and flooding events can easily push urban sewage systems to overflow. And widespread sealed surfaces, such as roads and pavements, prevent rainwater from naturally draining away.

The Chinese city of Wuhan and the city-state of Singapore responded to this reality by conceiving so-called sponge cities, which are designed to collect and cope with even heavy rain events. Green strips and roofs guide rain to seep into underground storage basins while permeable road surfaces allow water to trickle away. And the additional vegetation helps keep cities cool.

Today, there are more than 60 sponge cities in China alone. And urban centers across the world, such as the Danish capital of Copenhagen,  are being redeveloped along the same lines.

Protecting natural water sources

Above all, cities need to protect and regenerate natural water sources in outlying areas.

Bogota in Colombia, for example, obtains 80 % of its water from the surrounding mountains – a unique ecosystem with moors and lakes. But as farmers overuse this land, water supplies are put at risk. Local authorities are reacting by buying up land and raising awareness of the need to regenerate water sources.

Another solution is to remove plants that are particularly thirsty in areas where water is scarce. In a particular region of Cape Town, for example, many pine and eucalyptus trees have been replaced by the fynbos shrub, which is native to South Africa and needs less to drink.

Edited by: Anke Rasper

This article was originally published in German.

Sources:

"Future global urban water scarcity and potential solutions" in Nature Communications https://www.nature.com/articles/s41467-021-25026-3

World Resources Institute (2023) https://www.wri.org/insights/highest-water-stressed-countries