Contributions of different throughfall types to the soil moisture responses under the trees

Throughfall represents the primary pathway by which rainfall reaches the ground beneath any vegetation canopies through free throughfall (FR), splash throughfall (SP), and canopy drip (CD). This partitioning of throughfall fundamentally influences subsurface hydrological processes, particularly soil moisture responses under trees. This study examines the contributions of FR, SP, and CD to soil moisture responses under the birch (Betula pendula Roth.) and pine (Pinus nigra Arnold) trees. To quantify the relative proportions of FR, SP, and CD under each tree, simultaneous drop size data of gross rainfall and throughfall were measured using an OTT Parsivel disdrometer. The volumetric soil water content (VWC) under both trees was monitored using TEROS 10 sensors installed at three different depth profiles (16–20, 51–54, 74–76 cm). Findings demonstrate that a higher fraction of FR delivers unimpeded, rapid water inputs below the birch, which elicit faster upper soil moisture responses. Whereas, CD dominates throughfall volume under the pine, which provides a more gradual delivery of water inputs, resulting in a more delayed soil moisture response compared to birch. Statistical analysis further reveals a significant positive trend in Spearman correlation coefficients between throughfall types and lagged soil moisture at 16 cm depth under both trees. Correlations of SP and FR with soil moisture were consistently higher under the birch than the pine, suggesting more direct and rapid responses of VWC to these components beneath the birch. Under the pine, responses were more delayed, reflecting lower FR frequency due to higher interception and CD dominance in throughfall delivery. This implies the slower release of water from pine's needle-like foliage, resulting in a prolonged moisture response. Event-based analysis also shows that the increase in VWC under the birch corresponds more closely with periods of increased FR contribution. Although SP contributes to overall throughfall, the more direct, less diffuse nature of FR may be more effective in delivering water to the soil surface, triggering rapid soil moisture responses at 16 cm. Conversely, the VWC under the pine doesn't strongly correspond with peaks in FR or SP alone. Instead, gradual VWC increases at 16 cm correspond to sustained CD, punctuated by minor increments during concurrent FR and SP inputs. These findings elucidate how the different components of throughfall differentially drive event‑scale soil moisture responses beneath tree canopies, thus improving the understanding of small‑scale pathways by which canopy rainfall redistribution governs infiltration, storage, and percolation.

 

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.