In the years 2018 and 2019, the region experienced exceptionally warm and dry conditions, resulting in a significant decrease in surface water flows and groundwater levels. While the drinking water supply remained secure, these conditions raised concerns about the sustainability of urban water resource management in the face of climate change.

Berlin’s primary water sources are the Spree and Havel rivers. The Spree, encompassing a catchment area of approximately 10,000 km², including the Dahme catchment, serves purposes ranging from irrigation and industry to bank filtration. Historically, the river has also received pumped groundwater from the Lusatia (Lausitz) region to support lignite mining operations upstream, although these inputs will cease with the planned phase-out of coal mining in Germany by 2038. The Havel, converging with the Spree, drains a catchment area of about 3,500 km². Both rivers experience low discharge rates in summer, exacerbated by significant evaporation from alternating lake-stream sequences. The Spree’s tributaries vary in urbanization, featuring urban areas, green spaces, forests, wetlands and agricultural land (Kuhlemann et al., 2022).

Increasing pressure on Berlin’s urban water resources stems from population growth, urbanization, industrial expansion and the impending end of coal mining in Lusatia. For more than a century, open-pit mining in Lusatia has artificially bolstered the Spree River’s flow by injecting groundwater, a practice integral to Berlin’s drinking water supply. However, the cessation of lignite mining will profoundly alter the region’s water balance.

Berlin operates a semi-closed water supply system comprising nine waterworks that predominantly draw from local groundwater resources. Approximately 60% of this water is sourced through river bank filtration from nearby surface waters. Well galleries located along the riverbanks further extract water via bank filtration. However, diminishing surface water availability poses a risk to this filtration process, jeopardizing the city’s water supply.

As river flows are projected to decline, the proportion of effluents from wastewater treatment plants is expected to rise, leading to a deterioration in overall water quality. Challenges are compounded by the presence of combined sewage systems in the city center and occurrences of overflow events. Annually, around 7 million m³ of water flow into the Spree River, constituting up to 10% of its total volume during summer low-flow periods (Kuhlemann et al., 2020).

Comprehensive understanding of the urban water cycle, including quantitative and qualitative tipping points, is essential for effective risk management by water utilities and authorities tasked with securing the city’s drinking water supply. Identifying and assessing risks enables the identification of critical control points and the implementation of mitigation measures to ensure stable water management under evolving conditions. Another critical aspect is understanding the infiltration capacity of urban green spaces. While these areas can enhance groundwater recharge and mitigate urban heat island effects, they may also require irrigation during hot periods to sustain their ecological functions, potentially creating conflicts over water use. Thoughtful species selection and planting schemes, such as alternating trees, grasslands, and shrubs, are pivotal to maximizing the benefits of green areas while conserving water resources (Kuhlemann et al., 2022).