Investigating the reduction of rainfall intensity beneath an urban deciduous tree canopy

Trees have an indispensable role to play in the hydrological cycle. The process of interception by tree canopies alters the magnitude, pathway, and intensity of rainfall reaching the ground. This study investigates the rainfall intensity-attenuating effects of canopy interception by open-grown birch trees (Betula pendula Roth.) in an urban environment and the influence of atmospheric variables. Rainfall partitioning was measured in a research plot in the city of Ljubljana, Slovenia, from August 2021 to August 2022. Simultaneously, optical disdrometers above and below the birch tree canopy measured microstructures of rainfall and throughfall, from which the intensities were calculated. During the measurement period, the birch tree intercepted on average 25.6 % of gross rainfall, with the interception being twice as high during the leafed season than in the leafless season. Consequently, the total number and volume of drops under the canopy were reduced on average by 16.4 % and 48.7 %, respectively, indicating the interception and fragmentation of raindrops by the canopy. Owing to these processes, the leafed and leafless states of the birch tree canopy attenuate the average intensities of rainfall by 50.2 % and 41.6 %, respectively. Canopy interception also moderates the maximum 10-minute rainfall intensities by 11.6–83.8 % and 13.1–74.2 % during the leafed and leafless periods, respectively. This percentage of reduction in the rainfall intensities below the canopy decreases with rainfall amount and in the absence of foliage. Aside from phenoseasons, we also found that vapor pressure deficit and air temperature were among the atmospheric variables that exert the highest influence on the intensities of throughfall. Furthermore, the regression analysis between the maximum throughfall intensity and peak water level for each rainfall event indicates that the reduction of rainfall intensity by the canopy has a significant effect on runoff peak water level (R2 = 0.76, p < 0.001)