Hydrological Processes in Urban Systems

Water quantity and quality in urban systems face numerous threats such as heavy precipitation and flooding, droughts, heat, various contaminants, combined sewer overflows and algae blooms (Gessner et al. 2014). Climate and demographic trends, ongoing urbanisation and changes in water consumption will further increase these pressures. Urban water systems are also characterised by tight linkages to natural aquatic ecosystems, which have been severely modified. The alterations encompass the morphology, fluxes and storage of water and matter as well as effects on aquatic and terrestrial communities. These are influenced by large-scale technical systems of water withdrawal, treatment, storage and distribution for potable consumption, wastewater and runoff collection, purification and subsequent discharge to receiving water bodies. Further multiple uses of water gain importance such as keeping more water (and matter) in a cycle or storm water harvesting. Ensuring sustained functioning of urban water systems under both current and future conditions urgently requires management based on solid system and process understanding through the collaboration among engineers, natural scientists and stakeholders. Future solutions to urban water issues need consideration of an increasing number of compartments and subsystems and have to involve aquatic communities more strongly as their interactions may considerably affect the overall system behaviour. Here, urban water interfaces play a key role and require a more integrative approach than what is currently practised. Such urban water interfaces are found, for example, in (i) urban watersheds, (ii) urban aquatic ecosystems, (iii) urban hyporheic zones and aquifers and (iv) sewer systems. The overall aims of this special issue are to advance the understanding of hydrological processes in urban systems, to predict future changes through various modelling methods and techniques, and to detect vulnerable and resilient conditions in urban water systems to derive adaptations for improved water management. The areas (i–iv) mentioned above are connected by common approaches in methods, techniques, and applications addressing urban water (re)cycling, microbiology & nutrient cycle, water composition & pollutants, and numerical flow & transport modelling. This special issue was motivated and initiated from discussions and findings from the Research Training Group Urban Water Interfaces (RTG UWI) funded by the DFG (Deutsche Forschungsgemeinschaft/German Research Foundation) from 2015 to 2024 (Hinkelmann et al. 2025). Further, it includes contributions of participants from the 2nd International Conference on Urban Water Interfaces held in Berlin, Germany from April 22 to 24, 2024 (www.tu.berlin/uwi) and of other researchers working on this Special Issue topic. The special issue starts with eight papers on urban watersheds. Through a comparison of urban sites in Germany and Scotland, Stevenson et al. (2025) found out that vegetation type has the largest impact on urban ecohydrological partitioning in both a water-limited site in Germany and an energy-limited site in Scotland. In Germany, trees increased transpiration, while hydrological functioning was more strongly influenced by hydroclimatic variability and low soil water content. In Scotland, shrubs enlarged transpiration, while high soil water content provided a buffer against low rainfall. Tams et al. (2025) assessed the age-dependent hydrological functioning of street trees in urban areas, investigating factors, which determine the water availability within the crown and in the close surroundings. They concluded that larger pits and fast-infiltrating substrates were beneficial for younger trees, while increased sealing and suboptimal catchment conditions led to problems for older trees. A research gap between leaf-level stress detection and large-scale stress monitoring systems for urban trees was elaborated by Hörmann et al. (2025). Most important was the detection of early stress indicators for the individual trees. Today, no stress detection system is in operation, although this could ensure tree health in the medium and long term. Time-frequency analysis was applied by López Moreira Mazacotte et al. (2025) to determine urban hydrological connectivity in a catchment located in Berlin, Germany. Their results pointed out that groundwater was the main driver for stream flow, revealing urban responses to extreme climate scenarios. They recommended sustainable water management to ensure groundwater resilience, specifically in the view of a more variable future climate. In terms of water quality in urban catchments, the impact of urban sanitary point sources such as nutrients from wastewater treatment plants or combined sewer overflows on the water quality during events in a headwater catchment of an urban system was analysed by Spill et al. (2025). With a novel measurement setup, they could connect internal point sources and catchment processes. They found out that the point sources had a strong impact on streams with low dilution potential. Spill and Gassmann (2025) applied a hydrological model at high spatial and temporal resolution to simulate nitrate and ammonium transport processes from point and diffusive sources in the same headwater catchment mentioned above. The combined consideration of agricultural and urban sources pointed out the importance of accounting for the urban sources from wastewater treatment plants and combined sewer overflows to determine the right results. Angkanasirikul et al. (2026) identified key factors influencing tropical cyclone-driven flooding in Nothern Vietnam including the capital Ha Noi and the Red River Delta. Using fuzzy logic, they determined regional flood susceptibility maps, compared them with local reports and satellite data and finally offered a method applicable to similar regions. Stream flashiness in about 200 sites located in different physiographic regions of the Mid-Atlantic United States was investigated by Hurley et al. (2024) who identified several drivers and buffers of flashiness improving the understanding of the importance of land cover, wetlands and watershed compactness. They also determined a threshold of land development on the flashiness, which was similar in suburban and rural watersheds. There are three contributions on urban aquatic ecosystems. Reith et al. (2025) carried out high-resolution measurements of specific electrical conductivity in the River Spree, Berlin, Germany, which served as a tracer to detect seasonal reverse flows. These flows may transport treated wastewater, which can enter Lake Müggelsee that is used for drinking water supply. The transport and transformation of carbon and nitrogen in engineered rivers and canals in Western India was investigated by Sarkar and Kumar (2025). They observed that water stagnancy led to high biomass and that wastewater inputs increased the carbon concentrations in the rivers. Nakayama (2025) improved the understanding of plastic cycles in Japanese urban regions applying a process-based ecohydrological model, which served as a tool for adaptation measures. The author found out that micro- and macro-plastics dynamics in rivers had significantly changed in recent years. Urban hyporheic zones and aquifers were considered in five papers. Finegan and Hasenmueller (2025b) investigated the transport of municipal water through the floodplain to an urban stream system. While they found that the quantity was small, they suggested that a better understanding of the related processes in urban hyporheic zones is essential for protecting human and ecosystem health, especially in view of growing cities. An integral modelling approach was applied by Amann et al. (2025) for the first time to determine the hyporheic exchange caused by a porous log jam. Model results agreed well with an experiment, and in sensitivity studies the relationships between hyporheic exchange fluxes and the Frounde number as well the porosity of the log jam were elaborated. Alkandari and Alsumaiei (2025) calibrated a groundwater flow model for the shallow urban aquifer below Kuwait City. They investigated impacts of unplanned dewatering and incidental recharge on the groundwater levels. Their findings contributed to sustainable water resources management and can be transferred to other arid regions. Hydrogeochemical characteristics of hard-rock aquifers were analysed by Thanooja et al. (2025) in Kerala, India. Their studies included groundwater sampling, multivariate statistical analyses and geochemical modelling to better understand hydrochemical reactions and processes driving groundwater dynamics in that region. Kaminsky et al. (2025) observed increasing temperatures in the shallow urban aquifer below Vienna, Austria. The trend was non-linear and became larger in the period from 2011 to 2020 when compared to the previous decade. Drivers were discussed, novel methods were compared and recommendations for groundwater monitoring were provided. Finally, one contribution deals with sewer systems. Finegan and Hasenmueller (2025a) investigated the impacts of sewer renovation projects on surface water systems in St. Louis, Missouri, United States by weekly sampling over a period of about 8 years. On the short term during the construction phase, the water quality degraded; however, on the long term, water quality improved. In summary, this Special Issue provides an overview on a wide range of research on hydrological processes in urban systems. We have enjoyed the experience to serve as guest editors of this Special Issue and we hope that you find this collection of papers interesting and stimulating for future research. The authors would like to thank the German Research Foundation (DFG) which funded the Research Training Group Urban Water Interfaces (GRK 2032).