According to the World Water Development Report, global water consumption has seen a six-fold rise over the past 100 years (UN Water 2020). Above all, it’s towns and cities that are faced with growing demand for water due to vast population growth, increasing urbanisation and economic development. At the same time, climate change is altering the quality and quantity of water available for basic human needs.
Water as a valuable resource
Water is the basis of life for all living organisms and underpins the quality of life in today’s towns and cities. In view of global warming, the importance of this resource is growing. The current rise in urban populations and the squeeze on local water resources require sustainable solutions.
Even standard green roofs are a key way of providing sustainable, urban water usage. These roofs retain water, increase evaporation, and reduce runoff. Retention roofs go one step further by boosting the natural water cycle.
Urban water requirements are rising
As economic development rises, the water demand per capita worldwide is growing. In addition to the increasing demand per capita, population growth is another reason. For instance, the global population has tripled since 1930, while water consumption has shot up sixfold.
Agriculture accounts for 70% of water consumption while urban areas come second at 30%, although they only occupy about 2% of the Earth’s land area. Industry, the service sector, and homes are the main consumers (UN Water 2022).
At the moment, urban water consumption is increasing faster than that of agriculture. The current urbanisation and associated concentration of space are putting ever more pressure on water as a resource. As a result, water consumption in towns and cities will have to compete even more fiercely with other sectors.
Fig. 1: Global water demand measured by water used for artificial irrigation, industry or domestic use; adapted based on data from figure 5.4 in OECD 2012.
Disruption of the natural water cycle in urban areas
In urban areas, dense soil sealing disrupts the natural water cycle. Rainwater can no longer evaporate or infiltrate naturally, or only does so to a limited extent. During heavy rainfall, sealed surfaces mean sewers overflow while a lack of cooling through evaporation intensifies the urban summer heat.
The higher temperatures in towns and cities encourage local thunderstorms. Heat islands develop which, in combination with specific weather conditions such as high levels of solar radiation and unstable stratified air masses, also trigger local storm cells or heavy rainfall. Climate change amplifies these effects. Rising temperatures in towns and cities increase the saturation vapour pressure and encourage more precipitation to occur. As a result, heavy rainfall can be more frequent and intensive in urban areas.
What’s more, the underground water infrastructure in towns and cities is designed to drain rainwater into the sewers quickly. During heavy rainfall, these are quickly overwhelmed, which leads to flooding.
While 75% of rainwater in nature evaporates into the natural water cycle, this proportion drops to just 5% in developed urban areas.
Fig. 2: Urban and natural water supply; own graphic 2022
With this in mind, it’s clear that sustainable rainwater management in urban areas is vital to combat extreme weather conditions such as heatwaves and heavy rain.
A new direction for urban rainwater management
Over the past few years, urban planners have focused on draining off rainwater. However, a new direction of travel is transpiring where infiltration is seen as an urban water drainage option. The focus is on feeding rainwater into the natural water cycle.
Until now, rainwater in towns and cities was viewed as a problem that needed to be eliminated quickly. During heavy rainfall, rainwater was directed into the sewers as swiftly as possible. Water retention on the surface, evaporation via vegetation and the associated cooling effect during evaporation were not considered. Limited infiltration prevented groundwater recharge.
Sustainable rainwater management sees rainwater as an important resource. Local retention, infiltration, evaporation, and storage of rainwater are important objectives in responding to more frequent and extreme storms, drier summers and increasing urbanisation. On buildings, it’s green roofs that do these jobs.
Bibliography
UN Water (2020): Water and climate change. Paris: UNESCO (The United Nations world water development report, 2020). Retrieved here: Link
UN Water (2022): Groundwater making the invisible visible. Paris: UNESCO (The United Nations world water development report, 2022). Retrieved here: Link
List of figures
Fig. 1: OECD (2012): OECD Environmental Outlook to 2050, THE CONSEQUENCES OF INACTION, OECD Publishing, p. 217 (output from IMAGE). Retrieved here: Link
