Urban tree drought stress: Sap flow measurements, model validation, and water management simulations.

Urban street trees face increasing drought stress due to climate change and continuous urban development, making effective water management strategies essential. This study monitored the transpiration and soil moisture dynamics of five urban Tilia cordata trees in Berlin over two consecutive years to understand their transpiration responses under varying urban conditions. The collected data were used to validate the URbanTRee model, which was then applied to simulate different passive irrigation scenarios (system-to-catchment ratios ≤ 1:3) and assess their effectiveness in mitigating drought stress. The URbanTRee model successfully captured seasonal variations in transpiration and soil moisture, identifying all major drought stress periods in 2022, although underestimations were observed towards the end of the season. At the hourly scale, the model reasonably depicted reductions in transpiration during shaded hours on clear-sky days (measured by 55-66 %; modelled by 35-60 %), but overestimations of modelled ETa during hours with partial shading or air temperatures above 30 °C suggest room for improvement. The scenario analysis further demonstrated that, depending on catchment type and tree water demand, system-to-catchment ratios of 1:1-1:2 can substantially decrease, but not fully eliminate drought stress for young urban trees in dry years. These findings highlight the importance of considering site-specific conditions and the limitations of passive irrigation when planning sustainable water management strategies for young urban trees.