The partitioning of throughfall under urban tree canopies: a case study for birch (Betula pendula Roth.) and pine (Pinus nigra Arnold)

Throughfall is a critical component of the hydrological cycle, representing the majority of precipitation that reaches the ground contributing to soil water fluxes under vegetation canopies. It occurs through several mechanisms: as free throughfall (FR), splash throughfall, and canopy drip (CD) (Levia et al., 2019). Important progress has been made in investigating throughfall dynamics and drop size distribution (DSD) using disdrometers to understand the sub-canopy hydrologic and erosional processes. In the present study, we seek to quantify the relative proportions of throughfall components beneath isolated birch and pine trees using drop size data from OTT Parsivel disdrometers. Simultaneous measurements of drop size data for open rainfall and throughfall were conducted from July 2022 to July 2024 at an experimental urban park in Ljubljana, Slovenia. The partitioning of throughfall drops into FR, SP, and CD was carried out according to the protocol outlined by Levia et al. (2019). Analysis of drop counts indicates that throughfall drops originating from CD represent a significantly smaller fraction (<2-7%) of the total throughfall drop number compared to SP (60-69%) and FR (20-30%) for both tree species regardless of phenoseasons. However, in terms of drop volume, CD has the largest proportion for birch trees during the leafed period (40%) and for pine trees during both periods (70%). Due to the deciduous nature of birch trees, FR accounts for the largest volume percentage (42%) during the leafless period. Whereas the higher CD in pine trees is hypothesized to be attributed to the needle structures and waxy coating, which facilitates lateral flow of water that can lead to the formation of larger drops as smaller droplets coalesce before dripping to the ground. While the SP constitutes the largest proportion of the throughfall drop number, it represents a smaller percentage of throughfall volume due to its smaller drop diameters. The impact of larger drops hitting the foliage generates splash droplets, particularly during intense rainfall events and strong winds. This observation is reflected in the DSD of throughfall as the relative volume of drops >3.0 mm is higher under both trees than those in open rainfall across phenoseasons. The median drop diameter (D50) of throughfall is on average higher than the open rainfall, except for the leafless birch tree. Our study shed further insights into the rainfall partitioning process and serves as an initial step toward linking different types of TF inputs to water-mediated processes below the canopy. For instance, do areas with higher water inputs from CD exhibit variable and higher soil moisture? This may help improve our understanding of forest/tree canopy–water interactions.

Acknowledgment: This work was supported by the P2-0180 research program through the Ph.D. grant to the first author, which is financially supported by the Slovenian Research and Innovation Agency (ARIS). Moreover, this study was also carried out within the scope of the ongoing research projects J6-4628, J2-4489, and N2-0313 supported by the ARIS and SpongeScapes project (Grant Agreement ID No. 101112738), which is supported by the European Union’s Horizon Europe research and innovation programme.