The imprint of hydroclimate, urbanization and catchment connectivity on the stable isotope dynamics of a large river in Berlin, Germany

Sustainably meeting the growing water demand of cities during a changing climate will be a major challenge in the near future. Adjusting water management strategies towards preserving urban water resources while best utilizing urban “blue” and “green” infrastructure will require precise knowledge of urban water cycling, storage and losses across large built-up areas. To gain this understanding, using stable isotopes of water as tracers in the hydrological cycle can provide valuable insights, but their application remains challenging in complex and heterogeneous metropolitan areas. This study used stable isotopes, combined with hydrometric and climatic data, to assess the imprint of the urban area and climate warming on a large urbanised river system, the River Spree in Berlin, Germany, during the exceptionally warm and dry 2018 – 2020 period. Isotope dynamics in the Spree reflected gradually varying seasonal patterns with more enriched values in summer and more depleted values in winter. Large-scale climate variability and evaporative losses in the upstream catchment, along with anthropogenic activities, impacted isotope and discharge dynamics in the Spree, especially during warm and dry summer and autumn periods. In local urban sub-catchments, variability of isotope dynamics and subsequent water ages and transit times were largely affected by engineered system components, i.e. catchment imperviousness, connectivity to storm drains and effluent discharge. Berlin's green spaces were largely disconnected from surface waters. Urban trees in particular had high water demands during dry periods that may be hard to sustain, as trees showed slower replenishment of soil moisture during winter, resulting in shorter recharge periods under trees. More research will be needed on: (a) how planting design with alternations of urban grassland, shrub and trees can enhance infiltration and maintain water demands for trees during drought; and (b) on better quantifying open-water evaporation in the upstream catchment to fully account for catchment water losses. To sustainably manage and preserve not just Berlin's but also similar cities’ water resources in the future, our results indicate that both integrated city- and catchment-scale measures will be necessary.