Looking north-eastwards from the south-west, the consequences of climate change reveal themselves from yet another angle: Berlin has been experiencing a water crisis for some time. According to the Berlin research institute Kompetenzzentrum Wasser, the region has ranked among the lowest in Germany in terms of local rainfall in recent years. Caused in part by summer heatwaves of over 38 °C, this has repeatedly led to prolonged periods of drought and the associated drying up of rivers, ponds, moors and wetlands. A vicious circle, as the drying out of moors leads, among other things, to an increased release of CO2, which in turn fuels climate change.
However, periods of drought with falling groundwater levels are no longer isolated phenomena in Germany: The Dürremonitor (drought monitor) of the „Helmholtz Zentrum für Umweltforschung“ shows that the moisture content of the topsoil (5-40 cm depth) as well as the total soil (50-150 cm depth) repeatedly ranges from „unusual dryness“ to „exceptional drought conditions“ over the course of the year – particularly in eastern and southern Germany. An interactive map of the cross-border collaborative data project „Under the surface“, which is based on data from the EU member states, illustrates the currently critical quantitative and qualitative state of groundwater in Germany and Europe.
2. The „sponge city“ as a water-sensitive model of urban planning
The aim of the „water-sensitive city“ or „sponge city“ is to make urban areas more sustainable in terms of water resources with the help of customised natural or technical solutions, thus making them more resilient to climate change. The effects of extreme weather events on people, nature and infrastructure are to be mitigated with special urban planning concepts. At the same time, the aim is to improve the general quality of life.
Specifically, the sponge city concept aims to restore the natural water cycle in cities as far as possible. By temporarily retaining or storing locally occurring precipitation, urban areas are to be „soaked up“ like a sponge. The planning measures address a fundamental problem: the sealing of surfaces. In Germany, the overall sealing rate is 50 to 60 %. A large proportion of rainwater also ends up (unused) in the sewage system, and the natural water cycle (infiltration, evaporation, cloud formation, rain), which also ensures a healthy microclimate, is disrupted. In addition, heavy rainfall events are increasingly overloading the sewerage system, leading to more frequent flooding with danger to life and limb and costly consequential damage. At the same time, there is often not enough water available for cooling (microclimate) or for watering green spaces during long periods of drought.
3. Solution approach: implementation of a „sustainable water cycle“
Measures for unsealing and greening
Depending on the location, various urban development measures can be considered to bring the natural water cycle back into harmony. The greening of building façades and the creation of roof gardens and retention green roofs not only ensures natural evaporation and cooling on hot days, but also promotes biodiversity. Removing or replacing sealed airtight/waterproof surfaces – such as asphalt, paving stones, etc. – can increase infiltration rates and groundwater recharge, as well as relieve the burden on the sewerage system, especially during heavy rainfall.
