Received: 31 January 2020 Accepted: 10 June 2020 DOI: 10.1002/hyp.13850 RESEARCH ARTICLE Urban water systems under climate stress: An isotopic perspective from Berlin, Germany Lena-Marie Kuhlemann1,2 | Doerthe Tetzlaff1,2,3 | Chris Soulsby3,1,4 1Department of Ecohydrology, Leibniz Institute of Freshwater Ecology and Inland Abstract Fisheries (IGB), Berlin, Germany Large urban areas are typically characterized by a mosaic of different land uses, 2 Department of Geography, Humboldt- with contrasting mixes of impermeable and permeable surfaces that alter “green” University Berlin, Berlin, Germany “ ” 3Northern Rivers Institute, School of and blue water flux partitioning. Understanding water partitioning in such hetero- Geosciences, University of Aberdeen, geneous environments is challenging but crucial for maintaining a sustainable water Aberdeen, UK management during future challenges of increasing urbanization and climate 4Chair of Water Resources Management and Modeling of Hydrosystems, Technical warming. Stable isotopes in water have outstanding potential to trace the par- University Berlin, Berlin, Germany titioning of rainfall along different flow paths and identify surface water sources. Correspondence While isotope studies are an established method in many experimental catchments, Lena-Marie Kuhlemann, Leibniz Institute of surprisingly few studies have been conducted in urban environments. Here, we per- Freshwater Ecology and Inland Fisheries (IGB), Berlin, Germany. formed synoptic sampling of isotopes in precipitation, surface water and groundwa- Email: [email protected] ter across the complex city landscape of Berlin, Germany, for a large -scale Funding information overview of the spatio-temporal dynamics of urban water cycling. By integrating Deutsche Forschungsgemeinschaft, Grant/ stable isotopes of water with other hydrogeochemical tracers we were able to iden- Award Number: GRK 2032/2 tify contributions of groundwater, surface runoff during storm events and effluent discharge on streams with variable degrees of urbanization. We could also assess the influence of summer evaporation on the larger Spree and Havel rivers and local wetlands during the exceptionally warm and dry summers of 2018 and 2019. Our results demonstrate that using stable isotopes and hydrogeochemical data in urban areas has great potential to improve our understanding of water partitioning in complex, anthropogenically-affected landscapes. This can help to address research priorities needed to tackle future challenges in cities, including the deterioration of water quality and increasing water scarcity driven by climate warming, by improving the understanding of time-variant rainfall-runoff behaviour of urban streams, incor- porating field data into ecohydrological models, and better quantifying urban evapotranspiration and groundwater recharge. KEYWORDS ecohydrology, hydrogeochemistry, isotopes, tracers, urban green spaces, urban hydrology This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. © 2020 The Authors. Hydrological Processes published by John Wiley & Sons Ltd. 3758 wileyonlinelibrary.com/journal/hyp Hydrological Processes. 2020;34:3758–3776. KUHLEMANN ET AL. 3759 1 | INTRODUCTION potential to elucidate spatio-temporal patterns has recently been highlighted as a means for understanding and managing anthropo- Today, more than half of the world's population and more than 70% genic impacts on the urban hydrological cycle (Ehleringer, Barnette, of people in Europe live in urban areas (United Nations, 2019). Con- Jameel, Tipple, & Bowen, 2016). sequently, many metropolitan areas face considerable challenges in Previous isotope studies are surprisingly scarce in urban areas, covering their increasing water demand, which often leads to the but have provided insights into stormwater-stream dynamics overexploitation of local water resources and the need to import (Jefferson, Bell, Clinton, & McMillan, 2015) and a basis for estimating water from remote areas (e.g. Good et al., 2014; Jago-On transit time distributions (Parajulee, Wania, & Mitchell, 2019; Soulsby, et al., 2009). By 2050, it is expected that around 27% of the cities Birkel, Geris, & Tetzlaff, 2015; Soulsby, Birkel, & Tetzlaff, 2014). Com- worldwide could be facing a water demand that exceeds their sur- bined with hydrochemical tracers and modelling, they have been used face water availability while another 19% could face competing to quantify the role of groundwater in urban water systems. For interests between urban and agricultural water demands (Floerke, example, Houhou et al. (2010) detected (ground-) water seepage into Schneider, & McDonald, 2018). To secure a sustainable water supply an urban sewer system, showing infiltration “hot spots” in relation to in the future, as well as to mitigate flood hazards from urban runoff wastewater flow rates. Through spatio-temporal surveys in the urban- (e.g. Konrad, 2013) and to maintain dry weather flows to sustain ized Jordan River Basin in Utah, Follstad Shah et al. (2019) demon- aquatic ecosystems and safely dilute waste water effluents strated the important role of groundwater as a contributor to urban (e.g. Englert, Zubrod, Schulz, & Bundschuh, 2013; Gücker, Brauns, & surface waters, despite the influence of engineered water inputs like Pusch, 2006), integrated management of urban water resources is wastewater effluents. Further, it was revealed that land use and infil- crucial. This requires process-based understanding of water sources tration rate can impact shallow aquifer systems affected by the urban in urban water bodies and insight into the interactions between heat island (UHI) effect (Salem, Taniguchi, & Sakura, 2004). Ground- engineered hydrosystems in built-up areas and more natural hydro- water as an active component of the urban water cycle with rapid systems in urban green spaces (Gessner et al., 2014; recharge and significant discharge into urban streams has been identi- McGrane, 2016). It is well-known that urbanization has a significant fied by Meriano, Howard, and Eyles (2011) in the 75% urbanized Fre- impact on catchment hydrology. Urbanization tends to increase nchman's Bay study area in Canada. In central Ethiopia, spatial direct runoff from impervious areas and artificial storm drains during isotopic variations have been successfully utilized to distinguish rainfall events; reduce infiltration, groundwater recharge and base- groundwater systems with different lithologies, residence times and flow; and increase the influence of waste water effluents anthropogenic influences (Demlie, Wohnlich, & Ayenew, 2008). Spa- (Endreny, 2005; Fletcher, Andrieu, & Hamel, 2013). However, other tially distributed isotopic landscape maps (isoscapes) have shown how studies show that imperviousness and reduced evaporation can isotopic variations can be used in urban hydrology to investigate dom- increase localized infiltration and, along with gains from underground inant mechanisms of water transport and effects of evaporation/con- pipe leakage, increase groundwater recharge (Lerner, 2002; densation (Bowen, 2010; Bowen et al., 2009). Such maps have helped Schirmer, Leschik, & Musolff, 2013). Although specific impacts of constrain drinking water sources in complex urban distribution net- urbanization on streams can differ between regions and climates works, depending on seasonality, population density or water system (Brown et al., 2009; Hale, Scoggins, Smucker, & Such, 2016), the structure, both locally (e.g. Jameel et al., 2016; Tipple et al., 2017) and general patterns of flashier hydrographs, deterioration of water qual- at large scales (e.g. Bowen, Ehleringer, Chesson, Stange, & ity, increased nutrient loads, and degraded channel morphology – Cerling, 2007; Landwehr, Coplen, & Stewart, 2014). which together lead to a loss of biotic richness – has been termed Whilst such studies demonstrate the potential of isotopes in the “urban stream syndrome” andobservedincitiesaroundthe urban hydrology, there remain considerable research gaps. In particu- world (Booth, Roy, Smith, & Capps, 2016; Walsh et al., 2005). lar, conducting field studies across the landscape of larger cities The heterogeneity of urban areas and their hinterlands, together remains challenging. This is because constraining water fluxes in com- with a long history of engineering and environmental management, plex urban areas, with heterogeneity in land cover distribution, sub- often leads to evolving, complex hydrological systems that are frag- surface flow paths, recharge conditions and contaminant patterns mented, sparsely monitored and poorly understood. Useful, integrat- (Fletcher et al., 2013; Schirmer et al., 2013) requires monitoring exten- ing tools to investigate water sources and fluxes in the hydrological sive spatio-temporal scales, which is hard to maintain by local authori- cycle are environmental tracers. Stable isotopes in water, for exam- ties or researchers. Furthermore, river flow generated upstream of the ple, are affected by meteorological inputs and provide characteristic built-up area often interacts with local streams and waste water dis- “fingerprints” of water's origin and flow paths (Clark & Fritz, 1997). charged from domestic and industrial users, but also with local Unlike most chemical tracers, isotopes behave conservatively and groundwater resources and sewer systems draining cities.