This commentary was originally published by the Italian Institute for International Political Studies on May 16, 2024.
Cities are thirsty places. By their very nature, cities concentrate the water needs of large populations into small areas. The economic development that typically accompanies urbanization further boosts per capita water use, as higher incomes enable new domestic water demands from washing machines, dishwashers, and other household requirements. Yet dense urban areas are largely unable to fulfill their sizable water needs from supplies within the city itself. To meet the collective water claims of thousands or millions of people, cities depend upon substantial infrastructure for capturing, storing, treating, and delivering water services to their inhabitants, extending the reach of urban demands on external resources, often far beyond municipal boundaries.
Increasing water needs and escalating environmental pressures are imposing mounting strains on urban water systems and infrastructure. The share of global population living in cities is projected to climb from 56 percent currently to 68 percent in 2050, adding another 2.2 billion urban residents, mostly in Africa and Asia. The World Bank calculates urban water demand will rise accordingly, ballooning from 15–20 percent of total global water use today to 30 percent by mid-century. At the same time, continuing climate change will impact cities’ water demands and water sources alike. Warmer temperatures will generally heighten urban water needs, especially for household use and cooling. Shifting precipitation patterns threaten to modify the river flows and the recharge of groundwater aquifers that ensure urban water supplies. And extreme events will strike with greater frequency and severity, exposing cities to stronger downpours, higher floods, and deeper droughts.
Since the start of the twenty-first century, more than 80 large metropolitan areas around the globe have already experienced severe water shortages. Urban centers from Cape Town to Chennai and São Paulo to Sydney have faced the looming specter of “day zero” when water taps would run dry. In 2016, 933 million people, one-third of all urban residents, lived in water-scarce regions where demand surpassed available supplies. By 2050, half of the world’s large cities with over 1 million inhabitants will be located in water-scarce areas, and the total urban population coping with water scarcity could reach 1.7–2.4 billion people.
Confronted with growing demands and increasingly variable supplies, cities worldwide will have to develop sustainable strategies to strengthen the resilience of urban water systems. Enhancing storage capacities will be key to tackling the water security challenges of “too little, too much, too dirty.” Storage infrastructure—including dams, reservoirs, and tanks—enables water managers to collect and store water supplies to maintain availability and access for consumers when the underlying resources fluctuate, such as with changes in rainfall and river flows. Similarly, “post-consumer” graywater and sewage networks safeguard water quality, permitting the collection and treatment of municipal wastewater before returning it to the environment.
As climate impacts intensify, effective water storage systems will provide crucial defenses from extreme events. Waters held in reservoirs, for example, constitute a critical buffer against worsening dry spells and droughts. By the same token, by capturing and retaining heavy precipitation and high river flows, dams and reservoirs help shield cities from flooding in upstream catchments. Equally importantly, stormwater drainage and storage systems protect downstream regions and cities themselves from the flooding hazards generated by urban areas. Urbanization typically increases the extent of impervious surfaces, from roads to rooftops. Consequently, urban landscapes are estimated to absorb and store five times less water than natural ecosystems. Absent adequate urban stormwater drainage and collection, excess runoff can inundate cities and wash into waterways to flood adjacent areas.
Improving urban water resilience will require forging holistic solutions adapted to cities’ hydrological, geographical, and ecological contexts and specific water security challenges. Nearly half of global urban water supply, for example, relies upon the natural storage furnished by groundwater aquifers. For many cities, though, increasing groundwater withdrawals to keep pace with rising demands will be unsustainable. Overexploiting underground sources risks exhausting vital groundwater reserves faster than natural replenishment and threatens to exacerbate urban land subsidence as overlying land compacts and sinks above depleted aquifers. Nearly one quarter of the world’s major cities lie in potential subsidence zones, with over half of those cities also located in flood prone areas. Likewise, efforts to augment urban water supplies by building dams and reservoir storage can also carry other, maladaptive repercussions. Because such infrastructure can alter the quantity, quality, and timing of water, nutrient, and sediment flows downstream, constructing these installations can ultimately transfer water security risks onto other water users, communities, and ecosystems.
Cities interrupt natural water cycles, establishing intermediating infrastructure systems for water withdrawals, storage, treatment, and drainage to meet human needs. Yet urban water services often design and manage water supply, wastewater, and stormwater functions separately. More integrated approaches can capitalize on systemic synergies to bolster urban water security. Practices such as rainwater harvesting with local storage, stormwater recovery, wastewater recycling for potable use and groundwater recharge, and distributed on-site collection and treatment can augment available water supplies, increase water productivity, and strengthen resilience while diminishing the need for large-scale centralized infrastructure.
Urban water systems are likewise characterized by complex interrelationships among interconnected water users and levels of governance. As cities’ water needs claim a swelling share of global water use, assuring urban water supply will increasingly contend with the demands of other consumers. One recent study found urban water demand colliding with agricultural needs in 41 percent of all river basins by 2050. Yet different municipal departments often manage different water-related issues, in turn interacting with additional policies, processes, and decisionmakers at regional and national levels. Effectively recognizing and navigating these intersections will require heightened attention to multilevel governance and systematic coordination across sectors, stakeholders, and scales. To meet the urban water security challenges of the twenty-first century, no city can be an island.
David Michel is the senior fellow for water security with the Global Food and Water Security Program at the Center for Strategic and International Studies in Washington, D.C.