Contrasts in ecohydrological partitioning of heterogeneous urban green spaces in energy-limited versus water-limited hydroclimates

Urban green spaces (UGS) provide essential ecosystem services (ES), for example, precipitation infiltration for flood mitigation, transpiration (Tr) for local atmosphere cooling and groundwater recharge (Gr) for drinking water provision. However, vegetation type impacts the ecohydrological partitioning of incoming precipitation and therefore ES provision, whilst flux rate potential is different in disparate hydroclimates. Consequently, paired studies in different hydroclimates are useful to understand similarities and differences in vegetation controlled ecohydrological partitioning to effectively guide UGS management. We simultaneously undertook sub-daily soil moisture measurements beneath three contrasting urban vegetation types (grass, shrub, mature tree) between 01/01/2021 and 31/12/2023 for an inter-comparison of an energy-limited Scottish and a moisture-limited region of Germany. These data were integrated with hydroclimatic and sapflux data in the EcoHydroPlot model to constrain estimates of ecohydrological fluxes. Soil moisture data showed clear effects of the contrasting hydroclimates, with high and low VWC values in Scotland and Germany, respectively, whilst evapotranspiration potential was ~50% greater in Germany. Consequently, ecohydrological functioning and flux rates were fundamentally different, with Tr dominant in Germany and Gr dominant in Scotland. However, vegetation cover was shown in both countries to be a key control on urban ecohydrological partitioning with grass encouraging Gr, contrasting to evergreen shrubs in Scotland and mature trees in Germany elevating Tr. In Germany, impacts to hydrological functioning due to low soil VWC were marked with the mature trees high Tr rate shutting down Gr for the majority of the study period. The German site also showed greater hydrological functioning susceptibility to inter-annual hydroclimatic variability with all fluxes heavily suppressed during the 2022 drought. In contrast, the high VWC in Scotland provided some buffer against ongoing negative rainfall anomalies. Overall, the study indicated the importance of diverse UGS vegetation cover to encourage contrasting ecohydrological fluxes.