We used stable isotopes to help assess ecohydrological partitioning in different types of urban green space in the city of Berlin. We focused particularly on the role of the near-surface of soils as a crucial interface that determines shallow subsurface (“non-Hortonian”) flows and water cycling in cities. Grassland soils tended to be wetter than soils under continuous urban tree cover, and areas around individual trees in parks or on streets. This is consistent with greater interception losses and transpiration from trees. Soil water isotopes showed distinct seasonality in response to precipitation inputs, mixing with resident soil water and the effects of evaporative fractionation. Effects of fractionation were most marked under individual trees and grasslands, where the effects of evaporation on the isotopic composition of percolating water from the upper 0.1 m of the soil was also most pronounced. Isotope dynamics showed that under all land covers, the upper soils had rapid water turnover and were dominated by younger water (<2 months old). At depths of 0.4 m, more damping was evident with water being ∼ 6 months old. Isotopes in plant water also show seasonal variations which were more marked in grasses than trees. Mixing models revealed that grass most likely recycled shallow, younger soil water in transpiration, whilst trees were more dependent on deeper, older sub-soil and groundwater sources. These preliminary results highlight the urgent need for a much greater understanding of water movement and cycling in the shallow critical zone of urban green spaces. This is essential to manage future resilience to climate change and understand the trade-offs between partitioning urban water between evapotranspiration and groundwater recharge.
Bibliographical noteFunding Information:
CM and CS are funded by the Einstein Stiftung Berlin, Grant/Award Number: EVF-2018-425; CS is also funded by the ISOLAND Project of the Leverhulme Trust, Grant Number: (RPG-2018-425). CM is kollegiate of the Research Training Group Urban Water Interfaces financed by Deutsche Forschungsgemeinschaft, Grant/Award Number: (GRK2032/2). Further, DT’s contribution was funded through the Einstein Research Unit “Climate and Water under Change“ (CLIWAC) from the Einstein Foundation Berlin and Berlin University Alliance.
We want to thank Lukas Kleine for the support and inspiration of the fieldwork programme, Aaron Smith for consulting regarding evaporative fraction, Lena-Marie Kuhlemann & Hauke Dämpfling for setting up the SUEO, David Dubbert for measuring the liquid xylem and precipitation samples. We are grateful to Dr. Matthias Beyer for lending us the cryogenic extraction equipment. IGB provided the lab facilities to measure the fast amount of samples. A particular thanks goes to the park and greenspace departments of Berlin-Mitte and Pankow allowing us to sample within their parks and especially to their field staff who keep the parks beautiful as they are in times of droughts, heavy storms and COVID-19!
- Green space
- Soil water
- Urban critical zone
- Urban hydrology