Urban influences on rainfall interception of different tree species

The rapid expansion of impermeable surfaces in cities has a major impact on hydrology and meteorology. The infiltration of rainwater is reduced resulting in more overland flow with higher peak flows, but also reduced evapotranspiration and hence higher sensible heat flux. Urban trees are becoming more important as stormwater management or climate adaptation tools. The rainfall interception of trees already reduces overland flow generation and increases latent heat flux. An in-situ field experiment to measure throughfall on the common urban trees Acer platanoides (Norway maple) and Tilia cordata (small-leaved lime) was conducted to determine the interception of solitary trees on urban sites with different degrees of surface sealing and shading from surrounding buildings in the city of Freiburg, Germany. The influence of rainfall characteristics and tree morphological traits on interception behaviour was investigated with eight trees per species. 76 recorded rainfall events were evaluated from April to September 2021. The recorded interception rates were much higher compared to typical values in forests. Average interception rates were higher for T. cordata (70.29 ± 6.56%) than for A. platanoides (54.76 ± 10.29%). The average interception loss of the recorded events per tree was 2.58 ± 0.60 mm and 3.73 ± 0.29 mm for A. platanoides and T. cordata, respectively. For both tree species, significant linear correlations were found between the relative interception values and other factors like rainfall characteristics, the leaf area index (LAI), and the plant area index (PAI) (adj.R2 > 0.45). Compared to A. platanoides, T. cordata showed significant relationships between several tree morphological parameters (CR, CPA, CC, CV, LAD, PAD) and the relative interception values (adj.R2 > 0.43). The lowest LAI of both tree species were observed at sites with highest degree of surface sealing (tree pits), which also impacts the interception process. Our results provide a better understanding of the interception process of solitary trees for different urban settings. However, further field experiments with various tree species need to be conducted in order to obtain a larger database for simplified applications in modelling approaches and to support urban planners in managing stormwater runoff and adapt to climate change.